Sunday, July 5, 2009

Cross bike fork chatter

Every year as cross season dawns, this subject becomes a major mailing list chatter. Why does my very expensive, carbon fiber, heavily engineered, modern cross fork chatter under braking load? Answer - it's an inherent flaw in the design of traditional cross bikes with cantilever brakes. Although cross season is a few months away, I've seen some traffic on the question, so I'll discuss it.
The diagram above shows what's going on when your braking with 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

The rapid repetition of that cycle is 'chatter'.

How do you solve it?

Happy Remounting


Matt Boulanger said...

Thanks for this! I had visualized that what you explained in this post was happening with my fork, but hadn't gotten it into words so eloquently. I hate to give up my retro-look tektro cantilvers for LP, but I'm going to mess with a different hanger and see if that helps, along with further fine-tuning the pad adjustment. I can't wait to use my front brake with confidence again!

-p said...

If you try the fork mounted cable hanger, let us know how that works. I imagine it will greatly improve the situation since older cantilever equipped front shock mountain bikes had that setup and seemed to be better.