A 600-pound motorcycle-rider combination zipping along the road at 70 miles an hour represents a powerful force of kinetic energy (energy stored up in a moving vehicle). A great effort is required to halt this mass that is traveling at high speed. Fortunately, we are able to convert such kinetic energy to heat by means of the braking system. The main job of a braking system is to rapidly convert kinetic energy into heat through friction and dissipate that heat into the surrounding air. With leverage gained by mechanical linkage, or the power boost gained from a hydraulic system, the cycle rider is able to force a braking shoe or disc of friction material against a steel drum or brake disc rotor to stop his or her machine.
Stopping a motorcycle depends on the efficiency of the braking system as well as the traction available between tires and terrain. If you apply the brakes too hard for available traction, you will skid as your wheels lock up. The quickest stopping takes place just before "lock up" on any kind of terrain. Sliding a tire to a stop wastes time and rubber... not to mention how dangerous it can be.
Most motorcycles tend to "nose dive" as the brakes are applied. When the forks are compressed, the trail specification is reduced and the overall wheelbase of the bike is lessened. These changes mean quicker, less stable handling, so be prepared for a different reaction rate from your machine when the forks are compressed during and just after braking.
When brake linings become overheated they lose a lot of their frictional properties. When this occurs, even intense lever pressure will fail to produce a fast, even stop. Overheating also leads to brake linings becoming glazed, and this lessens brake effectiveness greatly.
In a drum brake, still found at the rear of many modern motorcycles, the interaction between the shoe and drum is very important to efficient braking. The softer, high-friction material of the shoe lining is forced against the smooth, hard surface of the drum, resulting in extremely high friction, then heat. Tight contact at the rotating drum quickly wears away some material from the stationary brake shoe. This worn material comes off as powder or dust. As this dust breaks off from the lining it forms tiny wedges between the shoe and drum, which create more friction and heat, and result in better braking.
Disc brakes are by far superior in virtually every possible application to drum brakes and it is a significant wonder why so many motorcycle manufacturers continue to insist on fitting these more primitive drum braking systems on the rear of their motorcycles. Granted, the disc brake may be more expensive to manufacture, but the difference is extremely minor when compared to the cost of a modern motorcycle. Although they may give the rear end of a motorcycle an old fashioned, retro quality look, I would much rather lose the looks and gain braking power, so you can certainly chalk up my vote in the disc brake category. Drum brakes should have gone extinct at least two decades ago.