Automotive brakes work on the concept of friction. When brake pads press against the rotor (and brake shoes push against the drum), the drag between the three dry surfaces causes the rotor to slow down and eventually stop. This drag is known as friction.

By definition, friction is the physical resistance to relative motion at two or more surfaces in a state of mutual contact. The three surfaces in the state of mutual contact (in other words, touching each other) in the brake system are the rotor and two brake pads. The greater the force exerted on the rotor by the pads, the greater the friction.

In many applications, friction is undesirable. In the case of a bearing and journal for a connecting rod, friction is minimized by lubrication. A thin film of oil separates the two surfaces to prevent mutual contact. Without any lubrication the two surfaces would wear extremely fast. But in other cases, friction is beneficial. One example is dry sandpaper. Moving the sandpaper across a surface, such as wood, wears away the wood to create a desired finish.

While there is friction between the contacting surfaces in a brake application that causes wear, that wear is secondary to the friction that causes the rotor to stop rotating. The big concern is heat generated by this friction. The heat from friction is proportional to the friction force. Let's examine this heat and its effect on brakes.

FRICTION AND HEAT
The heat generated by friction is easy to demonstrate. Grind the edge of a chisel on a grinding wheel, then touch the chisel. It's hot. That heat is generated by the friction between the chisel and grinding wheel. The same heat is generated when brakes are applied in a car. Kinetic energy (the energy of motion) is being converted into thermal energy (heat). Often, this heat can be felt if you place your hand on one of the wheels. If the wheel is extremely hot, that usually indicates a problem. Under normal operating circumstances, though, heat is not a problem.

So why is this heat such a concern? Very simply, this heat, if it becomes excessive, can affect brake performance (reducing friction between the pads and rotors), cause brake fade (reduced braking) and distort rotors. It's important, though, to look at disc brakes and drum brakes separately, as heat can affect these two styles of brakes differently.

DRUM BRAKES
A drum brake system, as shown in Figure 1, consists of two curved shoes which are pushed out against a rotating iron drum. As the temperature of the drum increases, it expands. As a result, the brakes shoes must move out farther in order to provide the same braking force. Under heavy use, this problem can become magnified, causing the brakes to fade (brake fade is reduced braking force experienced when applying the brakes). Also, the extreme heat can reduce the friction between the shoes and the drum. This will also result in brake fade.

Back during the muscle car era, it was not uncommon for your typical showroom-bought hot rod to have four-wheel drum brakes. We think about the horsepower those cars had and it's hard to imagine that such cars were equipped with that type of braking system. A car such as the ‘68
Dodge Charger is a perfect example.

How did you stop such a car? Well, it took a bit more effort than a modern car, thanks, in part to the fact that it might not have had power brakes, either. After some spirited driving and braking, fade was a problem. That's the way it was. In those days, disc brakes were for race cars.

In many applications, the drums had exterior cooling fins to dissipate the heat. Also, the thickness of the drum would be increased to prevent distortion. And, in performance applications, everything was just bigger (larger diameter drums and larger shoes with thicker padding).

While four-wheel disc brakes are common today, there are many vehicles that are still equipped with drum rear brakes. Because the front brakes do a majority of the stopping, the less expensive drum brakes can be used in the rear in everyday family-type vehicles (such as minivans) without sacrificing braking performance.

DISC BRAKES
Disc brakes work differently than drum brakes. Instead of the friction material on a shoe pushing out against a rotating drum, the friction material on a pad is pushed against the rotating rotor. The first advantage to this design is the mechanical advantage of pinching a rotating object to stop it as opposed to pushing out against it. Disc brakes are, mechanically, more efficient. Secondly, when the components become hot, the rotor and pads move closer together, not farther apart. This results in an increase in braking efficiency. Finally, the friction surfaces are open, not enclosed inside a drum. This allows for more efficient release of heat, resulting in cooler operation. On the downside, although disc brakes are simpler (fewer parts), this design is more expensive than drum brakes (that's one of the reasons why drum brakes are still used).

Heat, however, can still be a big problem. Under normal operating conditions (300° – 600°f), heat is not a problem, but once the conditions change, the problems begin. Abnormal driving conditions include riding the brake pedal (always a brake force on the rotor), long downhill drives (in which the brakes are constantly, or continually, being applied) and just flat out hard driving (hard, fast stops). None of these circumstances are good.

As the heat buildups in disc brakes, it must be dissipated quicker. In a performance application, this can be accomplished by the use of different materials. For instance, semi-metallic pads will draw heat off the rotor quicker than some ceramic pads. Also, rotor design can help. Thicker rotors will not distort as easily as thinner ones and the use of cooling ribs can help. In fact, some aftermarket rotors might have fewer cooling ribs to reduce weight and cost. This is not a good thing as those rotors will run hotter than the original.

Finally, excessive heat can reduce the friction between the pads and the rotor, causing brake fade (it's a weird feeling when you press the brake pedal and it goes down farther than normal with less braking power—that's brake fade). What's happening is the resin that binds the pad material together is giving off gases that act like a lubricant. And as we said earlier, lubricants reduce friction. Reduced friction results in less braking. Also, the pad material can actually begin to melt, leaving a glaze on the pad. Such pads cannot perform as designed and must be replaced.
Excessive heat can damage rotors, too. Distorted, or warped, rotors reduce braking efficiency and cause pulsations when braking.

Just a quick note on rotor thickness. Replace the rotor if it is worn below the minimum thickness. Also, buy a new one if machining it will result in the thickness falling below the spec. The minimum thickness specification is marked on the rotor hub (Figure 3).

If heat damage is detected, replace the brake components with the same design and material as the original parts.