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 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 > athttp://groups.google.com/group/rbw-owners-bunch?hl=en. -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. 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