dragonmg

There was a really good explanation posited on this a few months back, but I haven't been able to find it for you. I'll try and restate it here, but I could be off a little. To a certain degree, all fork blades flex back and forth when going over uneven terrain - be it road bikes or cross bikes. The theory (that I'm buying into until I hear a better one) on shutter is that on cyclocross forks with cantilever brakes, the flexing back of the fork blades actually increases the distance between the front brake housing stop and the brake cable fixing bolt on the brake, causing the brake to tighten down harder as the fork flexes back. As the brake tightens, the fork returns forward quickly. This is repeated during hard braking and you have your shutter.

Shutter seems to be more prevalent in the lighterweight cross forks paired with strong cantilever brakes. The stronger the braking force and the less stiff the fork, the greater the shutter. Toeing in your canti pads effectually weakens the initial stopping force of your brakes, and can reduce the shutter. You still will get it when you squeeze down hard on the brakes though. In my experience, brake shutter isn't that big a deal in cross races, but if you use your cross bike for road training, it can be downright scary on fast and twisty road descents.

So back to the source, this theory has pinpointed the relationship between the upper cable stop and the brake fixing bolt as the problem. If this theory were to hold true, then brakes that have the housing run straight into the brake arm, such as linear pull and disc brakes, would not be subject to the same type of shutter. I haven't personally witnessed any major shutter with these other two brakes, but if other folks have, then it might be back to the drawing board with shutter theory.