Fred Seelig wondered in a comment if I could provide a scientifically derived explanation of the apparent greater stopping power in a disc brake over a rim brake. Fred’s comment got me thinking about it. How, physically, do disc brakes generate more stopping force? Today’s post is a rambling attempt to sort out in my mind the physical parameters involved in disc vs rim braking.
There is no doubt that disc brakes generate more stopping force at the tire-road (or tire-trail) interface for a given hand force and lever travel. The first time I rode a bike with disc brakes, I was surprised, and a bit scared by the raw stopping power at my fingertips. We could argue about modulation, temperature fade, performance when wet or muddy, cost, weight, rim wear, spoke forces, and on and on. But there are literally thousands of discussions on the topic out there already. Just Google “Rim vs Disc Brakes”.
Fred wondered how disc brakes can generate more stopping force from the same input. He noted that it is easier to stop a wheel with a tangential force near the outer diameter (a rim brake) than at some smaller diameter (a disc brake).
The laws of physics, unlike the laws of god and man, cannot be broken. Conservation of energy is one of those laws. I can’t get more work out of a system than the work I put in. All I can do is convert the energy from one form to another. In the case of a friction brake, I am converting energy from kinetic (rotation and forward motion, mostly forward motion energy) to heat. Something else is going on that enables the disc to generate so much stopping power. Continue reading “How Do Disc Brakes Generate Greater Stopping Power Than Rim Brakes?”