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 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 - -- 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.
