[240racer]

There's more to it then just heat disapation. IMO, they aren't that bad at heat disapation. The usually have heat sink fins on the outside and when they used to use them on race cars, the drum was made out of Al and had a steel (or something) liner. I think the main problem with them is weight, they have a lot of unnecessary weight, since the braking forces aren't cancelled out like they are with a caliper. The shoes push out on the drum, but nothing pushes back in, except the drum itself, so not only does it have to react to the torque produced by the brake shoes, but the radial force as well. With disk brakes, the "normal" force from each pad is counteracted by the opposing pad, so there is only a torque load on the rotor itself. Also, drums are very complicated and not reliable. One other benefit of drums is that they allow the braking surface to be at a much larger diameter then disk brakes, given the same size wheel. This means that the actual friction force can be less to achieve the same braking torque. Drums have more swept area in general, and while I don't know why it's a good thing, everybody pretty much agrees that it is. Also, drums work good for trailers, since for some reason they work worse in reverse then they do going forward (refer to misnomer's post) So when you back up a trailer, it isn't trying to stop as much.