There are some things you don't want to stop, such as the celebration after you've won the feature, and then there are things you want to stop, such as your race car at the end of the straight. But why use disc brakes when you already have good working drum brakes? Disc brakes can be used hard without fading, a good quality to have when racing around a track that requires a lot of braking. But drums don't always fade out, so why change?
First you need to know how each type of brake works, so let's start with the drum brake. The proper name for this brake is an internal expanding drum brake. The drum is shaped somewhat like a pie pan, and inside on the pan's lip is a machined friction surface. There are also two shoes attached to the rear-end housing that carry a friction surface. These shoes, with the aid of hydraulic cylinder pressure, expand out until they rub against the drum. This creates the friction needed to stop.
There are several reasons drum brakes are less than desirable in a race car. As the drum heats up, like any metal, it expands. When this occurs the brake shoes must extend further out to stay in contact with the drum. When the drum absorbs too much friction-generated heat, it expands beyond the reach of the hydraulic cylinder that pushes the shoes out. With little pressure on the shoes there is little friction. This point is known as brake fade. It is indeed a scary feeling to be pushing on the brake pedal with a stiff leg while pulling the steering wheel four inches out of shape and still not stopping.
Another seldom realized fault of the drum brake is its self-energizing quality. This refers to the way the brake shoes are arranged inside the drum. The shoe is anchored at one end, and the other is attached to the hydraulic cylinder, often called the wheel cylinder. Due to the direction of rotation of the drum, one shoe, when touched to the drum, will be drawn tighter to the drum than the pressure being applied. This is the self-energizing effect. It made braking effort easier in the days before power brakes. It also has the effect of changing the braking feel to the driver, as more and more self-energizing effect comes into play under hard braking.
Disc BrakesThe disc system works on a different principle. The disc brake system uses a rotor attached to the axle. This rotor spins much like a pizza on the finger of a good chef. This rotor can be solid or have vanes cast inside to form a sort of air pump. This vented rotor pulls in air to cool the rotor. A solid rotor will warp with excessive heat. This can happen with a vented rotor too, but at a higher temperature.
The other part of this brake is the caliper. This is a hydraulic cylinder actuated by the master cylinder that is attached to the brake pedal. Imagine a large pair of pliers where the jaws are being closed together by a hydraulic cylinder. This is somewhat the way a brake caliper works. In a real brake caliper the jaws have pads. These pads are made of a material that produces friction but doesn't wear out quickly.
So in operation, the cylinder in the caliper forces the pads to grasp the rotor, which slows the car. Why is this so good? For one thing, until the limits of the material used in the brakes are reached, the disc brake generally works better as it gets hot. Unlike the drum, the rotor does not expand away from the friction surfaces.
The disc brake is much more consistent in its use than the drum. With the disc, the driver's feel is consistent when pressure is applied. The braking feel is closer to a straight line. Let's just say if 10 pounds were applied to the brake pedal, then 100 pounds would be applied to the caliper. If 20 pounds were applied, the result at the caliper would be 200. Then 30 pounds would equal 300 . . . well, you get the idea. This feedback to the driver would be far different with drum brakes.