Now I’ve often wondered about this, and even though about asking, but you generally get called something like Dumbass for asking silly questions on here.
Why is a rim brake, which effectively uses a maximum sized rotor, less powerful than a tiny disc at the hub? Fair enough the rubber brake block is going to be less efficient that disc pads as you don’t won’t to wear your rims away too quickly, and cables may be less effective than an hydraulic system, but surely that can’t account for all the difference.
It’s down to how much mechanical advantage you can put in the system.
Pulling the levers on a disc brake moves the calipers a tiny distance – probably less than a mm. Pulling the lever on any rim brake moves it several mm. This flip side of this is that the same force on the levers exerts a much greater force at the calipers of disc brakes than on rim calipers.
You can use the high mechanical advantage of a disc caliper on a disc but not a rim for two reasons:
1. A disc can be made sufficiently true and with sufficiently consistent thickness to work with calipers with tiny amounts of travel. It would be pretty much impossible to true a rim to the tolerances required for a disc-style caliper.
2. A disc can withstand the forces exerted by such a caliper – a rim would probably collapse under that kind of force.
The much higher mechanical advantage of a disc brake lever/caliper outweighs the lower mechanical advantage resulting from the smaller diameter of the disc compared to the rim.
In fact, a well set up rim brake isn’t that different from a well set up disc brake… until you cover it in mud and water. The higher forces in a disc brake do a much better job of clearing the braking surface.
