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.
