Differences between symetrical and asymetrical brake pads for cantilever / v-brakes

Question

What are the functional differences of the 'long tail' brake pads (more pad on one rear facing side of the post than the front side) versus the symmetrical (same amount of pad on either side of the post) brake pads for cantilever and v-brakes. Why would I prefer one type over the other?

Answer 1

If you take a symmetrical pad with a given size, it is subject to two forces while braking:

1. The braking pressure caused by cable action, directed perpendicularly against the rim. In frame-boss mounted brakes (UX, V-Brake, Cantilever), this generates a torque around the bosses, forcing the bosses and the frame to "open" and twist. This creates a "toe in" or "toe out", depending of the brake being in the rear or front.
2. The tangential force transmitted by rim rotation, which twists the frame and fork tubes inwards in the rear, and outwards in the front.

The second force being much greater than the first, it is advantageous to mount an ASYMMETRICAL pad with the tail always rearwards, so that the asymmetrical reactive force (due to the shape of the pad) tends to neutralize the "toe-in" (or out?) effect created by the tangential force of the rim motion.

One caveat! In narrow forks, the tail can interfere with the fork itself, thus making it difficult to open the brake for wheel removal.

Hope this helps!