[RBW] Re: AR front brake shudder and fork flex
Mike, Under hard braking most of the braking is done by the front brake. Bicycles have a relatively high center of gravity (due to the rider) and get a lot of weight transfer onto the front wheel during braking. With disks or V-brakes on a MTB it is quite easy to lift the rear wheel off the ground by using the front brake. Angus On Feb 19, 6:33 pm, Michael_S mikeybi...@rocketmail.com wrote: I could never understand why you would put the stronger brake ( neo- retro) on the front. I know the rear installation sometimes has clearance problems but that is where you need the greater braking force closer to the center of gravity of bike/rider. Couple that with fork flex and the other associated issues and it's a no brainier to use the Touring version up front. Plus it adds some nice symmetry to the bike :^P ~Mike On Feb 19, 4:03 pm, William tapebu...@gmail.com wrote: Angus It doesn't surprise me a whole lot. I've run the geometry numbers, and straddle height makes essentially no difference on mafac shape cantilevers. The feel at the lever is almost independent of straddle height. Low profile cantilevers depend a TON on straddle height. You can set up the brakes with a really low straddle for power and a squishy feel at the lever, or set it up tall for pukka pukka at the lever with much less power. With a tall straddle set up, its really really to load up the front brake. Like try to do an endo, you probably can't do it. That means you've de-powered your brakes so you can't get the feedback started. Just a guess. I experienced that on my cross bike. Neo-retros were terrifying. Touring cantis were a little better, and ceramic rims/pads were another step better. I think your observations are consistent. On Feb 19, 2:37 pm, Angus angusle...@sbcglobal.net wrote: I guess what I struggle with is that I didn't change the cable, or hanger, or ferrule, or the fork...only the brakes themselves; and the problem stopped...completely...even with the same brake pads. One way to reduce braking performance with the same force is to change the contact area between the brake pad and the rim. Which is what happens when the pads go into a toe-out situation. And why would my front tire lift off the ground? In free body diaphragm terms, the braking force (and the fork flexing backwards) would increase the vertical load on the front tire contact patch. Angus On Feb 19, 4:04 pm, William tapebu...@gmail.com wrote: Tim If you think about it some more, I think you'll see it. The tire lifting off the ground un-flexes the fork, relaxing the cable tension and loosens the clamping force of the brakes on the rim. It can't be otherwise. Like a bow-and-arrow in reverse I'll go ahead and make a statement and claim it as fact and see if anyone can even anecdotally dispute it. We'll see where that takes us. Virtually everyone has seen, experienced or heard about this violent fore-aft shuddering on a bicycle under hard front braking. My claim is that every single one of them was a bike with cantilever brakes or center pull brakes. It doesn't happen with V brakes and it doesnt happen with caliper brakes, or disk brakes for that matter. That's because brakes with all-housing are immune to any flex-induced tensioning and detensioning of the cable. Canti-bikes and centerpull bikes don't HAVE to have this problem, but V-brake, disk brake, and caliper brake bikes can't have it. If this had to do with toed in brake pads micro gripping and releasing, it would be equally common on all rim brake types. Furthermore, there is no free-body diagram one could draw to claim that a brake caliper of any kind squeezing harder on a rim will result in the brake pad squeezing LESS hard on the rim and allow it to release. That's just not physically possible. The sliding rim sort of shrugging the brakepad off of it, like some little wrestling move doesn't hold up. On Feb 19, 1:33 pm, Tim McNamara tim...@bitstream.net wrote: On Feb 19, 2011, at 12:51 PM, Ray Shine wrote: Excellent explanation. Even I could make sense of it! Thank you! From: William tapebu...@gmail.com To: RBW Owners Bunch rbw-owners-bunch@googlegroups.com Sent: Sat, February 19, 2011 9:36:14 AM Subject: [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend
[RBW] Re: AR front brake shudder and fork flex
+2 on the fork crown hanger. The Tektro one works for me (Haven't tried the Specialized). Hhm, seems like all my braking hardware is Tektro these days. However, on a bike that had no through-hole through the fork crown (silly people at Indy Fab), I couldn't add a fork crown hanger and resorted to extreme toe OUT on the brakes (Shimano BR-550). Solved the problem, perhaps by reducing braking power. Cheers, Gernot On Feb 20, 1:04 am, Peter Pesce petepe...@gmail.com wrote: +1 for the fork crown hanger. Simplest fix. On Feb 19, 12:36 pm, William tapebu...@gmail.com wrote: This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. This is the process, and it's not as well known as it should be. Forks with more flex and grabbier brakes exacerbate this. Extreme toe in techniques work because they make the brakes less grabby. Others have success with other brake pad compounds. I ran ceramic rims on a cross bike for just this reason, since ceramics and their associated green brake pads offer a very smoothly modulating brake surface. They almost never grab. A brake booster would only help to the extent that it keeps toed in pads from flattening out. In that way, the booster kind of acts as a de-booster, since it keeps the brakes from being too powerful. The thing that is common to most of our Rivendells is an extremely tall head tube and consequently a really long cable run from hanger to brake. The other very common technique to address this is to make that run of cable as short as possible by using a fork crown hanger. Now most of that cable run down to the hanger is housing, which flexes along with the fork and doesn't tighten the cable. I put a crown hanger on the Bombadil for exactly this reason. Mounting the hanger here takes any flex of the steerer and the crown out of the equation. It's now only flexing of the blades from the crown down to the brake posts that will feed into the tightening/flexing/tightening feedback. One of the old sages wrote on this on the internet. I don't remember if it was Jobst or Qvale or another one of the masters. That's where I learned about it. Here's a photo of that hanger setup: http://www.flickr.com/photos/45758191@N04/5236889932/ On Feb 19, 6:27 am, Ray Shine r.sh...@sbcglobal.net wrote: Jim -- Disregard prior request for follow-up comment (unless you don't mind). I think I understand now after reading the Shelson piece several times. Thanks for the link. From: CycloFiend cyclofi...@earthlink.net To: rbw-owners-bunch@googlegroups.com Sent: Fri, February 18, 2011 11:56:27 PM Subject: Re: [RBW] AR front brake shudder and fork flex on 2/18/11 7:09 PM, rw1911 at rw1...@gmail.com wrote: I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? This has cropped up on CX boards (and maybe iBob - too late to skim the archives there). The working theory (which seems pretty salient) is that flex in the hanger tends causing the shuddering. There's
[RBW] Re: AR front brake shudder and fork flex
...@gmail.com To: RBW Owners Bunch rbw-owners-bunch@googlegroups.com Sent: Sat, February 19, 2011 9:36:14 AM Subject: [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. So far so good. Except that you'd have to be flexing the fork between the brake pad contact point and the brake cable hanger on the top of the headset; this also requires flexing the steerer and possibly the head tube. That's not impossible, I suppose. I have read that steerers can flex in the lower part, near the lower headset race. Maybe that can flex enough. Or maybe there's enough flex in the fork legs between the braze-on and the bottom headset cups; you'd only need a little bit of stretch, maybe a mm or so, to significantly tighten the brake. The alternative is the fork legs twisting as the brake pads are dragged forward. Oval tubing is poorly resistant to being twisted (which is why ovalized down tubes don't stiffen the BB- they are twisted rather than loaded laterally. And why Ritchey ovalizes the seat tube, which is loaded laterally). My thought is that the pads are dragged forward until the front edge lifts enough that friction is reduced and the rim can slip; as the pads snap back they grab again and the cycle is repeated. This is why a brake booster works, it prevents the fork legs from being twisted by constraining the ends of the braze-ons from swinging away from the centerline. Even simpler is if there's a bump at the rim joint or a bump in the rim from an impact; that can cause this sort of thing. The visible process is the wagging of the forks as a symptom of the stick-slip cycle. It can be very dramatic- my friend Steve's S-works looked like the front end was going to fly apart. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. Here's where we run into problems with this explanation IMHO. Since you're decelerating, you're loading the front tire more heavily and pushing it against the ground. This makes it harder for the tire to skip. And, if this happened in a turn, you'd just crash. Besides, lifting the tire off the ground wouldn't loosen the pads by any mechanism I can think of right now. I could be quite wrong, of course. Wouldn't be the first time...- Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text - -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
Re: [RBW] Re: AR front brake shudder and fork flex
on 2/20/11 7:35 AM, Michael_S at mikeybi...@rocketmail.com wrote: Angus, That's exactly my point. If you've ever ridden steep technical terrain off road, you quickly learn never to apply significant pressure to the front brake or you'll go right over the bars. In fact you frequently use only the rear brake and shift your weight back wards on really steep descents to compensate for that weigh transfer. On pavement it's less critical due to increased friction and the fact that roads aren't as steep. It just seems common sense to design your bike with less powerful front brakes for just that reason. I do see your point: the front brake is essentially made more effecient by its placement on the front of a bicycle. But, I've always been of the you are safer with a sharp knife/axe state of mind. I want my front brake to work well, consistently and effeciently. Then I'll make compensations with force and timing (and body position). We used to ride trails with our rear brakes unhooked (and some would just unhook your rear brake for fun when you weren't looking), just to force ourselves to get a feel for it. Yes, we stacked hard more than a few times (which is why I don't recommend it), but we were young and indestructable then. ;^) On trails, the rear brake is pretty useless. As soon as you start skidding, your speed increases. There's absolutely an angle of descent which precludes the use of front braking, but that's pretty close to free-fall, IME. Judicious balancing of momentum and front braking has been the trick for me in descending. - J -- Jim Edgar cyclofi...@earthlink.net Cyclofiend Bicycle Photo Galleries - http://www.cyclofiend.com Current Classics - Cross Bikes Singlespeed - Working Bikes Gallery updates now appear here - http://cyclofiend.blogspot.com That which is overdesigned, too highly specific, anticipates outcome; the anticipation of outcome guarantees, if not failure, the absence of grace. William Gibson - All Tomorrow's Parties -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
[RBW] Re: AR front brake shudder and fork flex
Gernot Funny you mention it. My most epic battles with shudder was on an Indy Fab also. At the time I hadn't learned about the crown hanger solution, but knowing it wasn't drilled excuses my ignorance (at least in my mind). I ended up just selling it, around the time the Hillborne came in to the family. On Feb 20, 7:33 am, Earl Grey earlg...@gmail.com wrote: +2 on the fork crown hanger. The Tektro one works for me (Haven't tried the Specialized). Hhm, seems like all my braking hardware is Tektro these days. However, on a bike that had no through-hole through the fork crown (silly people at Indy Fab), I couldn't add a fork crown hanger and resorted to extreme toe OUT on the brakes (Shimano BR-550). Solved the problem, perhaps by reducing braking power. Cheers, Gernot On Feb 20, 1:04 am, Peter Pesce petepe...@gmail.com wrote: +1 for the fork crown hanger. Simplest fix. On Feb 19, 12:36 pm, William tapebu...@gmail.com wrote: This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. This is the process, and it's not as well known as it should be. Forks with more flex and grabbier brakes exacerbate this. Extreme toe in techniques work because they make the brakes less grabby. Others have success with other brake pad compounds. I ran ceramic rims on a cross bike for just this reason, since ceramics and their associated green brake pads offer a very smoothly modulating brake surface. They almost never grab. A brake booster would only help to the extent that it keeps toed in pads from flattening out. In that way, the booster kind of acts as a de-booster, since it keeps the brakes from being too powerful. The thing that is common to most of our Rivendells is an extremely tall head tube and consequently a really long cable run from hanger to brake. The other very common technique to address this is to make that run of cable as short as possible by using a fork crown hanger. Now most of that cable run down to the hanger is housing, which flexes along with the fork and doesn't tighten the cable. I put a crown hanger on the Bombadil for exactly this reason. Mounting the hanger here takes any flex of the steerer and the crown out of the equation. It's now only flexing of the blades from the crown down to the brake posts that will feed into the tightening/flexing/tightening feedback. One of the old sages wrote on this on the internet. I don't remember if it was Jobst or Qvale or another one of the masters. That's where I learned about it. Here's a photo of that hanger setup: http://www.flickr.com/photos/45758191@N04/5236889932/ On Feb 19, 6:27 am, Ray Shine r.sh...@sbcglobal.net wrote: Jim -- Disregard prior request for follow-up comment (unless you don't mind). I think I understand now after reading the Shelson piece several times. Thanks for the link. From: CycloFiend cyclofi...@earthlink.net To: rbw-owners-bunch@googlegroups.com Sent: Fri, February 18, 2011 11:56:27 PM Subject: Re: [RBW] AR front brake shudder and fork flex on 2/18/11 7:09 PM, rw1911 at rw1...@gmail.com wrote: I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder
[RBW] Re: AR front brake shudder and fork flex
I'm glad this came up, as i've been fighting slop in my front wheel for weeks and nearly destroyed my headset in the process. Now i think it could be what's described here (having not used cantis in a long time). For the fork crown hanger, i already have lots of stuff through my front fork crown (marks rack, fender), is there enough clearance with a front rack to use a fork crown hanger? On Feb 20, 1:08 pm, William tapebu...@gmail.com wrote: Gernot Funny you mention it. My most epic battles with shudder was on an Indy Fab also. At the time I hadn't learned about the crown hanger solution, but knowing it wasn't drilled excuses my ignorance (at least in my mind). I ended up just selling it, around the time the Hillborne came in to the family. -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
[RBW] Re: AR front brake shudder and fork flex
with cantilever brakes or center pull brakes. It doesn't happen with V brakes and it doesnt happen with caliper brakes, or disk brakes for that matter. That's because brakes with all-housing are immune to any flex-induced tensioning and detensioning of the cable. Canti-bikes and centerpull bikes don't HAVE to have this problem, but V-brake, disk brake, and caliper brake bikes can't have it. If this had to do with toed in brake pads micro gripping and releasing, it would be equally common on all rim brake types. Furthermore, there is no free-body diagram one could draw to claim that a brake caliper of any kind squeezing harder on a rim will result in the brake pad squeezing LESS hard on the rim and allow it to release. That's just not physically possible. The sliding rim sort of shrugging the brakepad off of it, like some little wrestling move doesn't hold up. On Feb 19, 1:33 pm, Tim McNamara tim...@bitstream.net wrote: On Feb 19, 2011, at 12:51 PM, Ray Shine wrote: Excellent explanation. Even I could make sense of it! Thank you! From: William tapebu...@gmail.com To: RBW Owners Bunch rbw-owners-bunch@googlegroups.com Sent: Sat, February 19, 2011 9:36:14 AM Subject: [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. So far so good. Except that you'd have to be flexing the fork between the brake pad contact point and the brake cable hanger on the top of the headset; this also requires flexing the steerer and possibly the head tube. That's not impossible, I suppose. I have read that steerers can flex in the lower part, near the lower headset race. Maybe that can flex enough. Or maybe there's enough flex in the fork legs between the braze-on and the bottom headset cups; you'd only need a little bit of stretch, maybe a mm or so, to significantly tighten the brake. The alternative is the fork legs twisting as the brake pads are dragged forward. Oval tubing is poorly resistant to being twisted (which is why ovalized down tubes don't stiffen the BB- they are twisted rather than loaded laterally. And why Ritchey ovalizes the seat tube, which is loaded laterally). My thought is that the pads are dragged forward until the front edge lifts enough that friction is reduced and the rim can slip; as the pads snap back they grab again and the cycle is repeated. This is why a brake booster works, it prevents the fork legs from being twisted by constraining the ends of the braze-ons from swinging away from the centerline. Even simpler is if there's a bump at the rim joint or a bump in the rim from an impact; that can cause this sort of thing. The visible process is the wagging of the forks as a symptom of the stick-slip cycle. It can be very dramatic- my friend Steve's S-works looked like the front end was going to fly apart. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. Here's where we run into problems with this explanation IMHO. Since you're decelerating, you're loading the front tire more heavily and pushing it against the ground. This makes it harder
[RBW] Re: AR front brake shudder and fork flex
Not just angle, but also conditions. Today went for a 10 mile ride in a pretty serious winter storm. On downhills the front brake was not just useless, but dangerous. Got serious washout/lockup just touching it. Packed snow/ice will do that. First time in many that the downhills were slower than climbing. One major street ridden down at 5mph. FWIW, the bike in question had V brakes. As of now, none of my bikes have cantilevers. Having swapped the Hillborne over to V brakes over the winter. (Ah, for spring when I can get back outside on the Sam.) Eric Platt St. Paul, MN On Feb 20, 11:59 am, CycloFiend cyclofi...@earthlink.net wrote: on 2/20/11 7:35 AM, Michael_S at mikeybi...@rocketmail.com wrote: Angus, That's exactly my point. If you've ever ridden steep technical terrain off road, you quickly learn never to apply significant (snip) -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
[RBW] Re: AR front brake shudder and fork flex
Incredible response with a lot of great information and ideas for me to try. Thanks everyone. On Feb 18, 10:09 pm, rw1911 rw1...@gmail.com wrote: I'm assuming (hoping) this is a simple setup issue... I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? One thing I noticed during installation is that the Paul brakes sit a good bit higher on the studs than the Shimano's they replaced. Could this be contributing to the flex in the mounting studs and/or fork? Any experience/advice is appreciated. -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
[RBW] Re: AR front brake shudder and fork flex
I had a set or Ribbit (typical Cyclocross brakes, Mafac copies) on my Atlantis that did a very similar thing. I am not familiar with Paul's brakes so this may not apply (some brakes have an internal bushing and don't have a bushing riding directly on the cantilever post on the frame). Also Jim may be entirely correct in his explanation; without researching on the internet my mind took me down a different path. After fiddling around with the usual things, I noticed the Ribbits had a lot of clearance between the cantilever post on the fork and the inside diameter of the bushing in the brakes (a loose fit on the posts). This allowed the toe to change; as the brakes were applied the the force on the pads took up the clearance between the posts and the brakes tending to rotate the brakes to a toe-out position. I changed to Shimano XT High Profile cantilevers (which have very little slop) and the problem stopped completely. Sometimes more toe than seems reasonable can help shuddering brakes; some things flex more than we would like to believe. I tend to run the straddle cables quite high, the Shimanos on the Atlantis I have the straddle cable about 1/2 above the lower headset cup. The Mafacs on the Quickbeam the straddle cable is a few inches above the lower headset cup. Makes for a very firm feel at the brake lever. Angus On Feb 18, 9:09 pm, rw1911 rw1...@gmail.com wrote: I'm assuming (hoping) this is a simple setup issue... I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? One thing I noticed during installation is that the Paul brakes sit a good bit higher on the studs than the Shimano's they replaced. Could this be contributing to the flex in the mounting studs and/or fork? Any experience/advice is appreciated. -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
[RBW] Re: AR front brake shudder and fork flex
My experience is the same as Angus's. My CR720's fit very loosely on the studs and when the pads contact the rim they pivot on the studs enough to go toe out if you do not set up the pads to be toe-in under contact and rotational load. Cured the violent low-speed shudder for me. -Mike C On Feb 19, 7:55 am, Angus angusle...@sbcglobal.net wrote: I had a set or Ribbit (typical Cyclocross brakes, Mafac copies) on my Atlantis that did a very similar thing. I am not familiar with Paul's brakes so this may not apply (some brakes have an internal bushing and don't have a bushing riding directly on the cantilever post on the frame). Also Jim may be entirely correct in his explanation; without researching on the internet my mind took me down a different path. After fiddling around with the usual things, I noticed the Ribbits had a lot of clearance between the cantilever post on the fork and the inside diameter of the bushing in the brakes (a loose fit on the posts). This allowed the toe to change; as the brakes were applied the the force on the pads took up the clearance between the posts and the brakes tending to rotate the brakes to a toe-out position. I changed to Shimano XT High Profile cantilevers (which have very little slop) and the problem stopped completely. Sometimes more toe than seems reasonable can help shuddering brakes; some things flex more than we would like to believe. I tend to run the straddle cables quite high, the Shimanos on the Atlantis I have the straddle cable about 1/2 above the lower headset cup. The Mafacs on the Quickbeam the straddle cable is a few inches above the lower headset cup. Makes for a very firm feel at the brake lever. Angus On Feb 18, 9:09 pm, rw1911 rw1...@gmail.com wrote: I'm assuming (hoping) this is a simple setup issue... I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? One thing I noticed during installation is that the Paul brakes sit a good bit higher on the studs than the Shimano's they replaced. Could this be contributing to the flex in the mounting studs and/or fork? Any experience/advice is appreciated.- Hide quoted text - - Show quoted text - -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
[RBW] Re: AR front brake shudder and fork flex
This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. This is the process, and it's not as well known as it should be. Forks with more flex and grabbier brakes exacerbate this. Extreme toe in techniques work because they make the brakes less grabby. Others have success with other brake pad compounds. I ran ceramic rims on a cross bike for just this reason, since ceramics and their associated green brake pads offer a very smoothly modulating brake surface. They almost never grab. A brake booster would only help to the extent that it keeps toed in pads from flattening out. In that way, the booster kind of acts as a de-booster, since it keeps the brakes from being too powerful. The thing that is common to most of our Rivendells is an extremely tall head tube and consequently a really long cable run from hanger to brake. The other very common technique to address this is to make that run of cable as short as possible by using a fork crown hanger. Now most of that cable run down to the hanger is housing, which flexes along with the fork and doesn't tighten the cable. I put a crown hanger on the Bombadil for exactly this reason. Mounting the hanger here takes any flex of the steerer and the crown out of the equation. It's now only flexing of the blades from the crown down to the brake posts that will feed into the tightening/flexing/tightening feedback. One of the old sages wrote on this on the internet. I don't remember if it was Jobst or Qvale or another one of the masters. That's where I learned about it. Here's a photo of that hanger setup: http://www.flickr.com/photos/45758191@N04/5236889932/ On Feb 19, 6:27 am, Ray Shine r.sh...@sbcglobal.net wrote: Jim -- Disregard prior request for follow-up comment (unless you don't mind). I think I understand now after reading the Shelson piece several times. Thanks for the link. From: CycloFiend cyclofi...@earthlink.net To: rbw-owners-bunch@googlegroups.com Sent: Fri, February 18, 2011 11:56:27 PM Subject: Re: [RBW] AR front brake shudder and fork flex on 2/18/11 7:09 PM, rw1911 at rw1...@gmail.com wrote: I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? This has cropped up on CX boards (and maybe iBob - too late to skim the archives there). The working theory (which seems pretty salient) is that flex in the hanger tends causing the shuddering. There's really two separate issues going on - the fork flex you are seeing and the shuddering of the brakes. With averagely strong brakes, you will generally see some flexing of the blades. More than likely, you don't notice it until the shuddering starts, but the two aren't necessarily linked. Take a look at the thickness of the hanger - if it's a less expensive stamped piece, you might try a thicker part. Also, pay attention to the fit of the ferrule on the cable end. If there's movement there, that will tend to exacerbate it. As you clamp down and the pads clench, if the hanger flexes, it will lessen the pressure on the brake pads. Less pressure on the pad causes the hanger to straighten applying more pressure
[RBW] Re: AR front brake shudder and fork flex
On Feb 19, 2011, at 1:56 AM, CycloFiend wrote: on 2/18/11 7:09 PM, rw1911 at rw1...@gmail.com wrote: I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? This has cropped up on CX boards (and maybe iBob - too late to skim the archives there). The working theory (which seems pretty salient) is that flex in the hanger tends causing the shuddering. Flex in what hanger? Do you mean the one that connects the brake cable to the straddle wire? If so that's very likely to be a red herring IMHO. It's too small compared to the thickness of the metal to flex significantly. This is more or less typical in bikes with cantilevers and light road forks. It's one of the reasons that old MTBs with rigid forks had such big fork legs. The worst fork judder I ever saw was my friend Steve's Specialized S-Works from the early 90s; it had a really light road type fork and shook to beat the band on any braking at all. The bike weighed about 20 lbs but that was the tradeoff. My StumpJumper with suspension fork had no fork judder, but the fork was massive and there was a brake booster. Steve put a brake booster on his S-Works and it stopped the problem. The shudder is caused by stick-slip as the rim passes through the pads and the main issue there is fork flex (so long as the rim is clean and the pads are in good shape). Using a brake booster constrains the lateral motion of the cantilever studs as the fork flexes and stops the judder. On my 26 A/R I use a Nitto Mini front rack and that seems to stiffen things up enough that there is no fork judder under any kind of braking. I don't remember if there was any problem with this before I installed the rack, it's been on there about 10 years. My CX bike has some of this but it doesn't bother me enough to put a brake booster on it. The geometry of the brake itself, how much it flexes, how much play there is on the post, etc. are also likely to be contributors. To sum up my recommendations: 1. Clean the rim to make sure there are no sticky or slipper spots on the braking surface (the OP said that he did this). Also check the rim joint to make sure there is not a bump there which initiates the problem- if so, file the joint smooth. 2. Clean the surface of the brake pads with a file or sandpaper. Check toe-in to make sure this is correct. 3. Check to make sure the retaining bolts are properly tightened and that there is minimal play of the brake on the braze-on post (putting a thin washer between the bolt and the brake may be necessary- do not put it behind the brake, though- you don't want to move the brake away from the fork). 4. If those don't fix it, stiffen the front end with a brake booster or something like a Nitto Mini (if it fits) or a Mark's Rack. The Nitto Mini will be stiffer and more likely to be useful in this regard because of the way it mounts to the fork crown, but it may not fit the bike. Rivendell can advise. -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
[RBW] Re: AR front brake shudder and fork flex
+1 for the fork crown hanger. Simplest fix. On Feb 19, 12:36 pm, William tapebu...@gmail.com wrote: This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. This is the process, and it's not as well known as it should be. Forks with more flex and grabbier brakes exacerbate this. Extreme toe in techniques work because they make the brakes less grabby. Others have success with other brake pad compounds. I ran ceramic rims on a cross bike for just this reason, since ceramics and their associated green brake pads offer a very smoothly modulating brake surface. They almost never grab. A brake booster would only help to the extent that it keeps toed in pads from flattening out. In that way, the booster kind of acts as a de-booster, since it keeps the brakes from being too powerful. The thing that is common to most of our Rivendells is an extremely tall head tube and consequently a really long cable run from hanger to brake. The other very common technique to address this is to make that run of cable as short as possible by using a fork crown hanger. Now most of that cable run down to the hanger is housing, which flexes along with the fork and doesn't tighten the cable. I put a crown hanger on the Bombadil for exactly this reason. Mounting the hanger here takes any flex of the steerer and the crown out of the equation. It's now only flexing of the blades from the crown down to the brake posts that will feed into the tightening/flexing/tightening feedback. One of the old sages wrote on this on the internet. I don't remember if it was Jobst or Qvale or another one of the masters. That's where I learned about it. Here's a photo of that hanger setup: http://www.flickr.com/photos/45758191@N04/5236889932/ On Feb 19, 6:27 am, Ray Shine r.sh...@sbcglobal.net wrote: Jim -- Disregard prior request for follow-up comment (unless you don't mind). I think I understand now after reading the Shelson piece several times. Thanks for the link. From: CycloFiend cyclofi...@earthlink.net To: rbw-owners-bunch@googlegroups.com Sent: Fri, February 18, 2011 11:56:27 PM Subject: Re: [RBW] AR front brake shudder and fork flex on 2/18/11 7:09 PM, rw1911 at rw1...@gmail.com wrote: I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? This has cropped up on CX boards (and maybe iBob - too late to skim the archives there). The working theory (which seems pretty salient) is that flex in the hanger tends causing the shuddering. There's really two separate issues going on - the fork flex you are seeing and the shuddering of the brakes. With averagely strong brakes, you will generally see some flexing of the blades. More than likely, you don't notice it until the shuddering starts, but the two aren't necessarily linked. Take a look at the thickness of the hanger - if it's a less expensive stamped piece, you might try a thicker part. Also, pay attention to the fit of the ferrule on the cable end. If there's movement there, that will tend to exacerbate it. As you
Re: [RBW] Re: AR front brake shudder and fork flex
Someone Anonymous posted: I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. To which I reply: I have Paul Touring brakes on my 26 version Allrounder. I don't have any shuddering issues. Maybe try putting the touring model on the front for a few rides and see if you still have the issue. If the shudder goes away or lessens, go with a matching set of Touring brakes. I'm sure you could find someone to take the spare Neo-Retro off your hands. Just a thought. -Mike in once again frigid So. Boston, Mass -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
Re: [RBW] Re: AR front brake shudder and fork flex
Excellent explanation. Even I could make sense of it! Thank you! From: William tapebu...@gmail.com To: RBW Owners Bunch rbw-owners-bunch@googlegroups.com Sent: Sat, February 19, 2011 9:36:14 AM Subject: [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. This is the process, and it's not as well known as it should be. Forks with more flex and grabbier brakes exacerbate this. Extreme toe in techniques work because they make the brakes less grabby. Others have success with other brake pad compounds. I ran ceramic rims on a cross bike for just this reason, since ceramics and their associated green brake pads offer a very smoothly modulating brake surface. They almost never grab. A brake booster would only help to the extent that it keeps toed in pads from flattening out. In that way, the booster kind of acts as a de-booster, since it keeps the brakes from being too powerful. The thing that is common to most of our Rivendells is an extremely tall head tube and consequently a really long cable run from hanger to brake. The other very common technique to address this is to make that run of cable as short as possible by using a fork crown hanger. Now most of that cable run down to the hanger is housing, which flexes along with the fork and doesn't tighten the cable. I put a crown hanger on the Bombadil for exactly this reason. Mounting the hanger here takes any flex of the steerer and the crown out of the equation. It's now only flexing of the blades from the crown down to the brake posts that will feed into the tightening/flexing/tightening feedback. One of the old sages wrote on this on the internet. I don't remember if it was Jobst or Qvale or another one of the masters. That's where I learned about it. Here's a photo of that hanger setup: http://www.flickr.com/photos/45758191@N04/5236889932/ On Feb 19, 6:27 am, Ray Shine r.sh...@sbcglobal.net wrote: Jim -- Disregard prior request for follow-up comment (unless you don't mind). I think I understand now after reading the Shelson piece several times. Thanks for the link. From: CycloFiend cyclofi...@earthlink.net To: rbw-owners-bunch@googlegroups.com Sent: Fri, February 18, 2011 11:56:27 PM Subject: Re: [RBW] AR front brake shudder and fork flex on 2/18/11 7:09 PM, rw1911 at rw1...@gmail.com wrote: I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? This has cropped up on CX boards (and maybe iBob - too late to skim the archives there). The working theory (which seems pretty salient) is that flex in the hanger tends causing the shuddering. There's really two separate issues going on - the fork flex you are seeing and the shuddering of the brakes. With averagely strong brakes, you will generally see some flexing of the blades. More than likely, you don't notice it until the shuddering starts, but the two aren't necessarily linked. Take a look at the thickness of the hanger - if it's a less expensive stamped piece, you might try a thicker part. Also, pay attention to the fit
Re: [RBW] Re: AR front brake shudder and fork flex
use a fork crown hanger. the tektros ones kinda suck, but the specialized ones are great. On Sat, Feb 19, 2011 at 12:51 PM, Ray Shine r.sh...@sbcglobal.net wrote: Excellent explanation. Even I could make sense of it! Thank you! -- *From:* William tapebu...@gmail.com *To:* RBW Owners Bunch rbw-owners-bunch@googlegroups.com *Sent:* Sat, February 19, 2011 9:36:14 AM *Subject:* [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. This is the process, and it's not as well known as it should be. Forks with more flex and grabbier brakes exacerbate this. Extreme toe in techniques work because they make the brakes less grabby. Others have success with other brake pad compounds. I ran ceramic rims on a cross bike for just this reason, since ceramics and their associated green brake pads offer a very smoothly modulating brake surface. They almost never grab. A brake booster would only help to the extent that it keeps toed in pads from flattening out. In that way, the booster kind of acts as a de-booster, since it keeps the brakes from being too powerful. The thing that is common to most of our Rivendells is an extremely tall head tube and consequently a really long cable run from hanger to brake. The other very common technique to address this is to make that run of cable as short as possible by using a fork crown hanger. Now most of that cable run down to the hanger is housing, which flexes along with the fork and doesn't tighten the cable. I put a crown hanger on the Bombadil for exactly this reason. Mounting the hanger here takes any flex of the steerer and the crown out of the equation. It's now only flexing of the blades from the crown down to the brake posts that will feed into the tightening/flexing/tightening feedback. One of the old sages wrote on this on the internet. I don't remember if it was Jobst or Qvale or another one of the masters. That's where I learned about it. Here's a photo of that hanger setup: http://www.flickr.com/photos/45758191@N04/5236889932/ On Feb 19, 6:27 am, Ray Shine r.sh...@sbcglobal.net wrote: Jim -- Disregard prior request for follow-up comment (unless you don't mind). I think I understand now after reading the Shelson piece several times. Thanks for the link. From: CycloFiend cyclofi...@earthlink.net To: rbw-owners-bunch@googlegroups.com Sent: Fri, February 18, 2011 11:56:27 PM Subject: Re: [RBW] AR front brake shudder and fork flex on 2/18/11 7:09 PM, rw1911 at rw1...@gmail.com wrote: I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? This has cropped up on CX boards (and maybe iBob - too late to skim the archives there). The working theory (which seems pretty salient) is that flex in the hanger tends causing the shuddering. There's really two separate issues going on - the fork flex you are seeing and the shuddering of the brakes. With averagely strong brakes, you will generally see some flexing of the blades. More
[RBW] Re: AR front brake shudder and fork flex
Excellent explanation Jim. I've had the same problem with tektro 720's on my ti rando with a ti fork. I played with different hangers, longer stem cable stop, changed the height of the hanger to shorten the amount of exposed cable between the stem cable stop and the hanger, I've toed in my brake shoes, tried different brake shoes, made my brakes really weak and really strong. I still had shudder. I ended up switching to a very unattractive V-brake and dia compe 287 levers, the shudder disappeared completely. Like my friend Dave said, it is the exposed cable from the steerer tube cable stop to the hanger that is flexing, causing the pressure on the pads to the rim to change, increasing and decreasing pressure as the cable flexes. By eliminating that exposed cable, with the v-brake cable routing, the oscillating, pulsing shudder disappears. It worked for me anyways... -Ely On Feb 18, 11:56 pm, CycloFiend cyclofi...@earthlink.net wrote: on 2/18/11 7:09 PM, rw1911 at rw1...@gmail.com wrote: I've recently installed Paul brakes (neo-retro front, touring rear) on my relatively new to me 700c All-Rounder. The rears are wonderful, if not too powerful... I can skid at will. However, I'm experiencing extreme shudder and fork flex on the front. The headset is tight and the pads are toe'd to contact forward. Under low to medium speed braking, I can see/feel the fork flex (a lot!) and shudder. The straddle cable is set at about the top third of the lower headset cup. I've cleaned the rim and while it has gotten slightly better with use, is this a matter of adjustment or is the neo- retro too powerful? This has cropped up on CX boards (and maybe iBob - too late to skim the archives there). The working theory (which seems pretty salient) is that flex in the hanger tends causing the shuddering. There's really two separate issues going on - the fork flex you are seeing and the shuddering of the brakes. With averagely strong brakes, you will generally see some flexing of the blades. More than likely, you don't notice it until the shuddering starts, but the two aren't necessarily linked. Take a look at the thickness of the hanger - if it's a less expensive stamped piece, you might try a thicker part. Also, pay attention to the fit of the ferrule on the cable end. If there's movement there, that will tend to exacerbate it. As you clamp down and the pads clench, if the hanger flexes, it will lessen the pressure on the brake pads. Less pressure on the pad causes the hanger to straighten applying more pressure to the pad, which causes the hanger to flex again... kind of similar to the anti-lock brake shudder you get on an auto. The neo-retros are pretty powerful, so you are probably getting a bit more oomph from the system. You might try adjusting the brakes so you get a bit less leverage on them. Sheldon shows the variables - http://sheldonbrown.com/cantilever-adjustment.html hope that helps. -- Jim Edgar cyclofi...@earthlink.net ³Velvet pillows, safari parks, sunglasses: people have become woolly mice. They still have bodies that can walk for five days and four nights through a desert of snow, without food, but they accept praise for having taken a one-hour bicycle ride.² - Tim Krabbe, The Rider Cyclofiend Bicycle Photo Galleries -http://www.cyclofiend.com Current Classics - Cross Bikes Singlespeed - Working Bikes Send In Your Photos! - Here's how:http://www.cyclofiend.com/guidelines -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
Re: [RBW] Re: AR front brake shudder and fork flex
On Feb 19, 2011, at 12:51 PM, Ray Shine wrote: Excellent explanation. Even I could make sense of it! Thank you! From: William tapebu...@gmail.com To: RBW Owners Bunch rbw-owners-bunch@googlegroups.com Sent: Sat, February 19, 2011 9:36:14 AM Subject: [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. So far so good. Except that you'd have to be flexing the fork between the brake pad contact point and the brake cable hanger on the top of the headset; this also requires flexing the steerer and possibly the head tube. That's not impossible, I suppose. I have read that steerers can flex in the lower part, near the lower headset race. Maybe that can flex enough. Or maybe there's enough flex in the fork legs between the braze-on and the bottom headset cups; you'd only need a little bit of stretch, maybe a mm or so, to significantly tighten the brake. The alternative is the fork legs twisting as the brake pads are dragged forward. Oval tubing is poorly resistant to being twisted (which is why ovalized down tubes don't stiffen the BB- they are twisted rather than loaded laterally. And why Ritchey ovalizes the seat tube, which is loaded laterally). My thought is that the pads are dragged forward until the front edge lifts enough that friction is reduced and the rim can slip; as the pads snap back they grab again and the cycle is repeated. This is why a brake booster works, it prevents the fork legs from being twisted by constraining the ends of the braze-ons from swinging away from the centerline. Even simpler is if there's a bump at the rim joint or a bump in the rim from an impact; that can cause this sort of thing. The visible process is the wagging of the forks as a symptom of the stick-slip cycle. It can be very dramatic- my friend Steve's S-works looked like the front end was going to fly apart. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. Here's where we run into problems with this explanation IMHO. Since you're decelerating, you're loading the front tire more heavily and pushing it against the ground. This makes it harder for the tire to skip. And, if this happened in a turn, you'd just crash. Besides, lifting the tire off the ground wouldn't loosen the pads by any mechanism I can think of right now. I could be quite wrong, of course. Wouldn't be the first time... -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
Re: [RBW] Re: AR front brake shudder and fork flex
on 2/19/11 9:43 AM, Tim McNamara at tim...@bitstream.net wrote: This has cropped up on CX boards (and maybe iBob - too late to skim the archives there). The working theory (which seems pretty salient) is that flex in the hanger tends causing the shuddering. Flex in what hanger? Do you mean the one that connects the brake cable to the straddle wire? If so that's very likely to be a red herring IMHO. It's too small compared to the thickness of the metal to flex significantly. The hanging cable stop which is usually sandwiched at the top of the headset on a traditional threaded fork. I'm not saying that's the only variable. Pad effectiveness, brake slop on the canti studs and brake leverage all can play a part as well. But, the Paul's brakes don't tend to flex too much, and they have a pretty good leverage. - Jim -- Jim Edgar cyclofi...@earthlink.net Cyclofiend Bicycle Photo Galleries - http://www.cyclofiend.com Current Classics - Cross Bikes Singlespeed - Working Bikes Gallery updates now appear here - http://cyclofiend.blogspot.com Nigel did some work for some of the other riders at Allied, onces who still rode metal. He hadn't liked it when Chevette had gone for a paper frame. -- William Gibson, Virtual Light -- You received this message because you are subscribed to the Google Groups RBW Owners Bunch group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
[RBW] Re: AR front brake shudder and fork flex
Tim If you think about it some more, I think you'll see it. The tire lifting off the ground un-flexes the fork, relaxing the cable tension and loosens the clamping force of the brakes on the rim. It can't be otherwise. Like a bow-and-arrow in reverse I'll go ahead and make a statement and claim it as fact and see if anyone can even anecdotally dispute it. We'll see where that takes us. Virtually everyone has seen, experienced or heard about this violent fore-aft shuddering on a bicycle under hard front braking. My claim is that every single one of them was a bike with cantilever brakes or center pull brakes. It doesn't happen with V brakes and it doesnt happen with caliper brakes, or disk brakes for that matter. That's because brakes with all-housing are immune to any flex-induced tensioning and detensioning of the cable. Canti-bikes and centerpull bikes don't HAVE to have this problem, but V-brake, disk brake, and caliper brake bikes can't have it. If this had to do with toed in brake pads micro gripping and releasing, it would be equally common on all rim brake types. Furthermore, there is no free-body diagram one could draw to claim that a brake caliper of any kind squeezing harder on a rim will result in the brake pad squeezing LESS hard on the rim and allow it to release. That's just not physically possible. The sliding rim sort of shrugging the brakepad off of it, like some little wrestling move doesn't hold up. On Feb 19, 1:33 pm, Tim McNamara tim...@bitstream.net wrote: On Feb 19, 2011, at 12:51 PM, Ray Shine wrote: Excellent explanation. Even I could make sense of it! Thank you! From: William tapebu...@gmail.com To: RBW Owners Bunch rbw-owners-bunch@googlegroups.com Sent: Sat, February 19, 2011 9:36:14 AM Subject: [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. So far so good. Except that you'd have to be flexing the fork between the brake pad contact point and the brake cable hanger on the top of the headset; this also requires flexing the steerer and possibly the head tube. That's not impossible, I suppose. I have read that steerers can flex in the lower part, near the lower headset race. Maybe that can flex enough. Or maybe there's enough flex in the fork legs between the braze-on and the bottom headset cups; you'd only need a little bit of stretch, maybe a mm or so, to significantly tighten the brake. The alternative is the fork legs twisting as the brake pads are dragged forward. Oval tubing is poorly resistant to being twisted (which is why ovalized down tubes don't stiffen the BB- they are twisted rather than loaded laterally. And why Ritchey ovalizes the seat tube, which is loaded laterally). My thought is that the pads are dragged forward until the front edge lifts enough that friction is reduced and the rim can slip; as the pads snap back they grab again and the cycle is repeated. This is why a brake booster works, it prevents the fork legs from being twisted by constraining the ends of the braze-ons from swinging away from the centerline. Even simpler is if there's a bump at the rim joint or a bump in the rim from an impact; that can cause this sort of thing. The visible process is the wagging of the forks as a symptom of the stick-slip cycle. It can be very dramatic- my friend Steve's S-works looked like the front end was going to fly apart. This is a positive feedback that only stops when something lets go, and on the road, the thing that lets go is the road/tire interface. The tire momentarily lets go of the road, and the fork springs back forward which loosens the brake. When the tire hits the ground again it starts up all over again. Here's where we run into problems with this explanation IMHO. Since you're decelerating, you're loading the front tire more heavily and pushing it against the ground. This makes it harder for the tire to skip
[RBW] Re: AR front brake shudder and fork flex
I guess what I struggle with is that I didn't change the cable, or hanger, or ferrule, or the fork...only the brakes themselves; and the problem stopped...completely...even with the same brake pads. One way to reduce braking performance with the same force is to change the contact area between the brake pad and the rim. Which is what happens when the pads go into a toe-out situation. And why would my front tire lift off the ground? In free body diaphragm terms, the braking force (and the fork flexing backwards) would increase the vertical load on the front tire contact patch. Angus On Feb 19, 4:04 pm, William tapebu...@gmail.com wrote: Tim If you think about it some more, I think you'll see it. The tire lifting off the ground un-flexes the fork, relaxing the cable tension and loosens the clamping force of the brakes on the rim. It can't be otherwise. Like a bow-and-arrow in reverse I'll go ahead and make a statement and claim it as fact and see if anyone can even anecdotally dispute it. We'll see where that takes us. Virtually everyone has seen, experienced or heard about this violent fore-aft shuddering on a bicycle under hard front braking. My claim is that every single one of them was a bike with cantilever brakes or center pull brakes. It doesn't happen with V brakes and it doesnt happen with caliper brakes, or disk brakes for that matter. That's because brakes with all-housing are immune to any flex-induced tensioning and detensioning of the cable. Canti-bikes and centerpull bikes don't HAVE to have this problem, but V-brake, disk brake, and caliper brake bikes can't have it. If this had to do with toed in brake pads micro gripping and releasing, it would be equally common on all rim brake types. Furthermore, there is no free-body diagram one could draw to claim that a brake caliper of any kind squeezing harder on a rim will result in the brake pad squeezing LESS hard on the rim and allow it to release. That's just not physically possible. The sliding rim sort of shrugging the brakepad off of it, like some little wrestling move doesn't hold up. On Feb 19, 1:33 pm, Tim McNamara tim...@bitstream.net wrote: On Feb 19, 2011, at 12:51 PM, Ray Shine wrote: Excellent explanation. Even I could make sense of it! Thank you! From: William tapebu...@gmail.com To: RBW Owners Bunch rbw-owners-bunch@googlegroups.com Sent: Sat, February 19, 2011 9:36:14 AM Subject: [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. So far so good. Except that you'd have to be flexing the fork between the brake pad contact point and the brake cable hanger on the top of the headset; this also requires flexing the steerer and possibly the head tube. That's not impossible, I suppose. I have read that steerers can flex in the lower part, near the lower headset race. Maybe that can flex enough. Or maybe there's enough flex in the fork legs between the braze-on and the bottom headset cups; you'd only need a little bit of stretch, maybe a mm or so, to significantly tighten the brake. The alternative is the fork legs twisting as the brake pads are dragged forward. Oval tubing is poorly resistant to being twisted (which is why ovalized down tubes don't stiffen the BB- they are twisted rather than loaded laterally. And why Ritchey ovalizes the seat tube, which is loaded laterally). My thought is that the pads are dragged forward until the front edge lifts enough that friction is reduced and the rim can slip; as the pads snap back they grab again and the cycle is repeated. This is why a brake booster works, it prevents the fork legs from being twisted by constraining the ends of the braze-ons from swinging away from the centerline. Even simpler is if there's a bump at the rim joint or a bump in the rim from an impact; that can cause this sort of thing
[RBW] Re: AR front brake shudder and fork flex
Angus It doesn't surprise me a whole lot. I've run the geometry numbers, and straddle height makes essentially no difference on mafac shape cantilevers. The feel at the lever is almost independent of straddle height. Low profile cantilevers depend a TON on straddle height. You can set up the brakes with a really low straddle for power and a squishy feel at the lever, or set it up tall for pukka pukka at the lever with much less power. With a tall straddle set up, its really really to load up the front brake. Like try to do an endo, you probably can't do it. That means you've de-powered your brakes so you can't get the feedback started. Just a guess. I experienced that on my cross bike. Neo-retros were terrifying. Touring cantis were a little better, and ceramic rims/pads were another step better. I think your observations are consistent. On Feb 19, 2:37 pm, Angus angusle...@sbcglobal.net wrote: I guess what I struggle with is that I didn't change the cable, or hanger, or ferrule, or the fork...only the brakes themselves; and the problem stopped...completely...even with the same brake pads. One way to reduce braking performance with the same force is to change the contact area between the brake pad and the rim. Which is what happens when the pads go into a toe-out situation. And why would my front tire lift off the ground? In free body diaphragm terms, the braking force (and the fork flexing backwards) would increase the vertical load on the front tire contact patch. Angus On Feb 19, 4:04 pm, William tapebu...@gmail.com wrote: Tim If you think about it some more, I think you'll see it. The tire lifting off the ground un-flexes the fork, relaxing the cable tension and loosens the clamping force of the brakes on the rim. It can't be otherwise. Like a bow-and-arrow in reverse I'll go ahead and make a statement and claim it as fact and see if anyone can even anecdotally dispute it. We'll see where that takes us. Virtually everyone has seen, experienced or heard about this violent fore-aft shuddering on a bicycle under hard front braking. My claim is that every single one of them was a bike with cantilever brakes or center pull brakes. It doesn't happen with V brakes and it doesnt happen with caliper brakes, or disk brakes for that matter. That's because brakes with all-housing are immune to any flex-induced tensioning and detensioning of the cable. Canti-bikes and centerpull bikes don't HAVE to have this problem, but V-brake, disk brake, and caliper brake bikes can't have it. If this had to do with toed in brake pads micro gripping and releasing, it would be equally common on all rim brake types. Furthermore, there is no free-body diagram one could draw to claim that a brake caliper of any kind squeezing harder on a rim will result in the brake pad squeezing LESS hard on the rim and allow it to release. That's just not physically possible. The sliding rim sort of shrugging the brakepad off of it, like some little wrestling move doesn't hold up. On Feb 19, 1:33 pm, Tim McNamara tim...@bitstream.net wrote: On Feb 19, 2011, at 12:51 PM, Ray Shine wrote: Excellent explanation. Even I could make sense of it! Thank you! From: William tapebu...@gmail.com To: RBW Owners Bunch rbw-owners-bunch@googlegroups.com Sent: Sat, February 19, 2011 9:36:14 AM Subject: [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening your grip would have. This makes the force at the fork greater, flexing it more, tightening the brake more, and so on. So far so good. Except that you'd have to be flexing the fork between the brake pad contact point and the brake cable hanger on the top of the headset; this also requires flexing the steerer and possibly the head tube. That's not impossible, I suppose. I have read that steerers can flex in the lower part, near the lower headset race. Maybe that can flex enough. Or maybe there's enough flex in the fork
[RBW] Re: AR front brake shudder and fork flex
I could never understand why you would put the stronger brake ( neo- retro) on the front. I know the rear installation sometimes has clearance problems but that is where you need the greater braking force closer to the center of gravity of bike/rider. Couple that with fork flex and the other associated issues and it's a no brainier to use the Touring version up front. Plus it adds some nice symmetry to the bike :^P ~Mike On Feb 19, 4:03 pm, William tapebu...@gmail.com wrote: Angus It doesn't surprise me a whole lot. I've run the geometry numbers, and straddle height makes essentially no difference on mafac shape cantilevers. The feel at the lever is almost independent of straddle height. Low profile cantilevers depend a TON on straddle height. You can set up the brakes with a really low straddle for power and a squishy feel at the lever, or set it up tall for pukka pukka at the lever with much less power. With a tall straddle set up, its really really to load up the front brake. Like try to do an endo, you probably can't do it. That means you've de-powered your brakes so you can't get the feedback started. Just a guess. I experienced that on my cross bike. Neo-retros were terrifying. Touring cantis were a little better, and ceramic rims/pads were another step better. I think your observations are consistent. On Feb 19, 2:37 pm, Angus angusle...@sbcglobal.net wrote: I guess what I struggle with is that I didn't change the cable, or hanger, or ferrule, or the fork...only the brakes themselves; and the problem stopped...completely...even with the same brake pads. One way to reduce braking performance with the same force is to change the contact area between the brake pad and the rim. Which is what happens when the pads go into a toe-out situation. And why would my front tire lift off the ground? In free body diaphragm terms, the braking force (and the fork flexing backwards) would increase the vertical load on the front tire contact patch. Angus On Feb 19, 4:04 pm, William tapebu...@gmail.com wrote: Tim If you think about it some more, I think you'll see it. The tire lifting off the ground un-flexes the fork, relaxing the cable tension and loosens the clamping force of the brakes on the rim. It can't be otherwise. Like a bow-and-arrow in reverse I'll go ahead and make a statement and claim it as fact and see if anyone can even anecdotally dispute it. We'll see where that takes us. Virtually everyone has seen, experienced or heard about this violent fore-aft shuddering on a bicycle under hard front braking. My claim is that every single one of them was a bike with cantilever brakes or center pull brakes. It doesn't happen with V brakes and it doesnt happen with caliper brakes, or disk brakes for that matter. That's because brakes with all-housing are immune to any flex-induced tensioning and detensioning of the cable. Canti-bikes and centerpull bikes don't HAVE to have this problem, but V-brake, disk brake, and caliper brake bikes can't have it. If this had to do with toed in brake pads micro gripping and releasing, it would be equally common on all rim brake types. Furthermore, there is no free-body diagram one could draw to claim that a brake caliper of any kind squeezing harder on a rim will result in the brake pad squeezing LESS hard on the rim and allow it to release. That's just not physically possible. The sliding rim sort of shrugging the brakepad off of it, like some little wrestling move doesn't hold up. On Feb 19, 1:33 pm, Tim McNamara tim...@bitstream.net wrote: On Feb 19, 2011, at 12:51 PM, Ray Shine wrote: Excellent explanation. Even I could make sense of it! Thank you! From: William tapebu...@gmail.com To: RBW Owners Bunch rbw-owners-bunch@googlegroups.com Sent: Sat, February 19, 2011 9:36:14 AM Subject: [RBW] Re: AR front brake shudder and fork flex This topic comes up repeatedly. The discussions typically focus on treatment, which is natural, because you just want the thing to go away. But understanding the cause is usually helpful in figuring out the treatment. The cause is as follows: You grab your front brake, which tries to stop the wheel rotating. The road is pushing back on your tire and your body's forward momentum is pushing forward on the front hub. This moment tries to bend back the front fork. You can do this part for yourself in the garage. Lock up the front brake and push forward on the bike. Everyone with me? Cool. Now look at the cable. The length of cable going from the hanger down to the brake is hanging in space in FRONT of the fork which is flexing BACK. The distance the cable spans is increasing, effectively making the cable shorter, which is going to tighten the front brake, the same way tightening
