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HEY!

I'm not actively blogging here anymore. But if you got here because you were searching for something about bikes, you might want to check out my latest project, Vermont Goldsprints. In summer of 2014, I bought a used goldsprints racing setup and have made it a mission to get more bikes in more people's faces by putting on fun races in unexpected places. Come join me!

8.27.2009

The Cure for Cantilever Cross Fork Chatter?

My primary commuter ride is a Scattante XRL cross frameset that I built up with a mix of parts bin, Ebay and other stuff.  I've been riding it to work every day since April and have been generally very happy with it. One major issue I discovered as soon as I got it out on the road was that using the front brake with any amount of force resulted in an immediate and violent oscillation of the front wheel and fork.  Enough to shake me right off of the handlebars.   Since most of your braking force on a bicycle comes from the front brake, this was disconcerting to say the least. It's hard to consider a bike to be "all dialed in" with such a major issue left outstanding.  As a commuter, I go through a lot of hard braking and accelerations on every trip, and my weekly maintenance regime was starting to include having to remove copious amounts of brake dust fromt he rear rim and drivetrain.  No fun at all.     
Like many people who encounter this, I thought it was a problem with the design of the fork. But how could a fork designed for the rigors of 'cross use be such a noodle under braking? I was pretty much resigned to live with the problem until I could afford to replace the fork and pay the weight penalty of installing a steel fork.
Fortunately, before I did all of that, I found this article on Nippleworks, which explains the phenomenon of cross fork brake chatter in terms of the sequence of events that takes place when you engage the front brake: 
"1) The brake cable comes under tension
2) The brake pads apply braking force to the rim
3) The ground applies backwards force to the tire
4) The fork deflects backwards
5) Fork deflection causes brake cable tension to increase
6) Brake pads apply more force to the rim

At this point, something has to give way:

a) The brake pads stop the wheel cold and you go over the handlebars
b) The ground gives way and you have a front wheel skid
c) The brake pads slip on the rim and the vicious cycle of (1 to 6 + c) repeats"
and suggests a few solutions:


  • "Decrease free brake cable length by mounting the hanger on the fork (like older front suspension mountain bikes: http://www.sheldonbrown.com/harris/images/tektro1257.jpg

  • Install linear pull brakes which don't promote this behavior: http://nippleworks.blogspot.com/2008/12/linear-pull-brakes-with-drop-bar-levers.html

  • Monkey with toe-in and otherwise change the mechanical advantage of the brakes to minimize the effect

  • Install a really compliant cable hanger to deflect under cable tension?

  • Disc brakes?"






  • The fork isn't set up for disc brakes, so that was right out. I didn't want to buy linear pull brakes and the levers I would need to go with them, and while the really compliant cable hanger idea was tempting, I'm not really set up to make something like that.
    The photos above show that I went with the first bulleted suggestion by installing a Tektro Housing Stop on the fork, which reduced the amount of free cable from 4.2 inches down to about an inch. Since the original housing stop is an integral part of the headset, I drilled it out so the now-longer housing could run through it. In the last picture in the series, you can see that I applied some of the suggestions of the third bullet, by adding a really generous amount of toe-in to the brake pads.  That adjustment on its own hadn't worked before. 
    So, how does it work now?  Great.  I have been riding the bike every day for about a month since I set it up and have not made any adjustments.  The front brake is very powerful, and I can only introduce a minor shudder in the fork by braking very, very hard, as if I was making a panic stop using only the front brake. 
    I want to thank Nippleworks for taking this question on.  A search for "fork chatter" and "cross bike" brings up numerous message board discussions about this problem being prevalent in different cross bike models, but not a lot of solutions.  As Nippleworks discussed, although some bikes and forks might be more susceptible, this is going to be something of an issue whenever a flexible cross fork is combined with a lot of free cable. The more free cable, the more there is to stretch, release and contract in the endless cycle described above.  
    I'm sure my choice of components aggravated the situation.  The fork is pretty soft to begin with.  I can flex it a little bit just by leaning hard on the handlebars.  I picked retro Tektro CR720 cantilevers with a really wide profile, which means that in addition to the free cable coming from the cable stop to the straddle cable saddle, the straddle cable itself is quite long.  Something like an Avid Shorty would allow for a shorter straddle and would eliminate even more free cable from the system.  My rims, Velocity Fusions (which I love) have a machined brake track, which is a little more grippy than a non-machined track- that might contribute as well.  A harder rubber brake pad (people like Kool-Stop Salmons) would be a little slipperier.  
    But even with stock brake pads, wide profile cantilevers, and a grippy rim, the cable stop solved the problem for under ten dollars and the cost a piece of longer housing. With this problem resolved, I'm really happy with this bike.  It's a pleasure to ride, nimble, solid, comfortable on back roads as well as pavement at speed (45mph down Smugglers Notch does make those Panaracer Cinder X knobbies scream, though!) and upright enough to be comfortable without sacrificing too much speed to the wind.