I have wondered about that. One one hand, a 20" rim has a lot less mass with which to dissipate heat versus a 26" or 700c. But on the other hand, the ground braking thrust produces less wheel torque, so shouldn't need to squeeze the brake as hard. Makes sense, as the total "work", i.e., the braking energy, is force x displacement, so it should even out, but again, 20" is less mass to take the same energy.

I was told years ago at a bike shop that disc brakes not only save the sidewalls, they reduce rim spoke stress (counterintuitively, as the spokes have to pull hard in braking), because aluminum rims have a high coefficient of thermal expansion, and heating them up overstresses spokes. This is true; When my front 406 rim started to show more concavity on the sidewalls, I tried using just the rear brake, and at the bottom of a 200 foot descent, *ding*, a rear spoke popped, drive side of course. That reason, as well as sidewall wear, and better braking in the wet, is why I want discs. The only advantage for me of rim brakes, is that I had an opportunity to buy a lifetime supply of cartridge style pads cheap, and disc pads will most certainly cost me more. I like the cartridge pads for ease of removing them for cleaning, however they were designed for road calipers, so I just needed to buy bolts long enough to accommodate the convex/concave washers for V-brakes, to use these. I know rubbery stuff ages, so I keep the stock in a cool dry place.