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... -- 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.
