Drum brakes, like most other brakes, convert kinetic energy into heat by friction. This heat should dissipate into the surrounding air, but can just as easily transfer to other braking system components. Brake drums must be large to cope with the massive forces involved, and must be able to absorb and dissipate a lot of heat. Heat transfer to air can be aided by incorporating cooling fins onto the drum. However, excessive heating can occur due to heavy or repeated braking, which can cause the drum to distort, leading to vibration under braking.
The other consequence of overheating is brake fade. This is due to one of several processes or more usually an accumulation of all of them.
- When the drums are heated by hard braking, the diameter of the drum increases slightly due to thermal expansion, so the shoes must move farther and the driver must press the brake pedal farther.
- The properties of the friction material can change if heated, resulting in less friction. This can be a much larger problem with drum brakes than disc brakes, since the shoes are inside the drum and not exposed to cooling ambient air. The loss of friction is usually only temporary and the material regains its efficiency when cooled, but if the surface overheats to the point where it becomes glazed the reduction in braking efficiency is more permanent. Surface glazing can be worn away with further use of the brakes, but that takes time.
- Excessive brake drum heating can cause the brake fluid to vaporize, which reduces the hydraulic pressure applied to the brake shoes. Therefore, the brakes provide less deceleration for a given amount of pressure on the pedal. The effect is worsened by poor maintenance. Brake fluid that is old and has absorbed moisture has a lower boiling point, so brake fade occurs sooner.
Brake fade is not always due to overheating. Water between the friction surfaces and the drum can act as a lubricant and reduce braking efficiency. The water tends to stay until heated sufficiently to vaporize, at which point braking efficiency returns. All friction braking systems have a maximum theoretical rate of energy conversion. Once that rate is reached, applying greater pedal pressure doesn't change it—in fact, the effects mentioned can substantially reduce it. Ultimately, this is what brake fade is, regardless of the mechanisms of its causes. Disc brakes are not immune to any of these processes, but they deal with heat and water more effectively than drums.
Drum brakes can be grabby if the drum surface gets light rust or if the brake is cold and damp, giving the pad material greater friction. Grabbing can be so severe that the tires skid and continue to skid even when the pedal is released. Grab is the opposite of fade: when the pad friction goes up, the self-assisting nature of the brakes causes application force to go up. If the pad friction and self-amplification are high enough, the brake stay engaged due to self-application, even when the external application force is released.
While disc brake rotors can be machined to clean the friction surface (i.e., 'turning'), the same generally cannot be done with brake drums. Machining the friction surface of a brake drum increases the diameter, which would require oversized shoes to maintain proper contact with the drum. However, since oversized shoes are generally unavailable for most applications, worn or damaged drums generally must be replaced.
Another disadvantage of drum brakes is their relative complexity. A person must have a general understanding of how drum brakes work and take simple steps to ensure the brakes are reassembled correctly when doing work on drum brakes. And, as a result of this increased complexity (compared to disc brakes), maintenance of drum brakes is generally more time-consuming. Also, the greater number of parts results in a greater number of failure modes compared to disc brakes. Springs can break from fatigue if not replaced along with worn brake shoes. And the drum and shoes can become damaged from scoring if various components (such as broken springs or self-adjusters) break and become loose inside the drum.
Also, drum brakes do not apply immediately when the wheel cylinders are pressurized, because the force of the return springs must be overcome before the shoes start to move towards the drum. This means that the very common hybrid disc/drum systems only brake with the discs on light pedal pressure unless extra hardware is added. In practice, a "proportioning valve" added to such cars applies the drums slightly before the discs. If the proportioning valve is left out or functions improperly, the vehicle stops only with the front discs during careful stops, which are most of the stops in daily driving. Such a car shows "brake dive" (the car tilts forward during braking) and very fast wear of front disc brake pad linings. In motorcycle applications "linked brakes" are essentially the same type of system.
Read more about this topic: Drum Brake
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