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 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. 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