Drum brakes can do plenty of stopping- - - -otherwise 18 wheelers wouldn't still use them. Getting 40 tons hauled down from highway speeds repeatedly isn't an easy task. Disc brakes are really a giant leap backwards from drums as far as stopping ability goes. They use a tiny contact patch of the pad on the rotor, and extremely high clamping force to create the friction necessary to stop the vehicle. Vacuum-boosted drum brakes worked for years before discs became popular. The only advantage to discs is the fact they're less prone to "fade"- - - -where the drum or disc absorbs so much heat it loses its ability to provide braking effect. Discs cool faster than drums, due to the hollow spaces between the swept surfaces, and the long sweep after leaving the pads before the friction is applied again. Bigger diameter and/or wider drum brakes with a vacuum booster on the master cylinder would be as good as discs, maybe better, up to the point where fade begins.
Jerry
