Friday, November 5, 2010

Cyclocross Brake Chatter/Shudder

Lennard Zinn wrote a great article (HERE) about the common problem of chattering front cantilever brakes on cyclocross bikes. Basically, when the cable housing stops above the top of the steerer tube, as it does on most 'cross bikes, fork and frame flex caused by braking forces can create sharp spikes in cable tension. These spikes lead to increased braking force, to the point where one of the following must happen:

1. The front wheel locks up and skids
2. The rider is thrown over the handlebars
OR 3. The brake pad finally breaks free from the rim surface

The path of least resistance is option number three, and that's precisely what happens. After the pad breaks free however, the cycle begins all over again. This constant sticking and slipping is what causes the shudder that can be truly terrifying.

This Trek X02 had the worst chattering front brake I had ever seen when equipped with its stock cable hanger (above the head tube). Simply switching the hanger position completely eliminated the chatter. No change in brake pad material or toe-in angle was required.

A very stiff frame and fork should reduce the problem, but I have seen really nasty chattering on newer 'cross bikes with 1.5" oversized lower head tubes that are supposed to be ultra stiff. It seems that any frame and fork, oversized or not, will flex to some degree under hard front braking. If the cable hanger is placed above the head tube, this flex is going to translate into cable tension spikes to some degree. On some bikes, it is minor enough that there's not much of a problem, but a crown-mounted hanger could be the solution if your cyclocross bike shudders badly.

As Zinn states, many carbon fork crowns are not equipped with a bolt hole for mounting a hanger there. Luckily the XO2 above did have a hole. If you're stuck without a bolt hole in your fork crown, you could try Cane Creek's Drop-V lever and using linear pull front brakes instead of cantilevers. The Drop-V is a brake lever only and not a shifter, so you would only be able to run a single chainring on the front.

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