+1 for the fork crown hanger. Simplest fix.
On Feb 19, 12:36 pm, William <tapebu...@gmail.com> wrote: > 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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