Type of brakes - Drum brakes Video
 
Drum brakes have brake drums that rotate with the wheels. Brake shoes fitted with brake linings (friction material) which press against the drums from the inside to generate braking force (decelerate and stop) are set inside of the drums.

With this system, friction is generated by pressing the brake linings against the inside surfaces of the drums. This friction converts kinetic energy into thermal energy. Drum rotation helps to press the shoes and the lining against the drum with more force, offering superior braking force in comparison with disc brakes. On the other hand, it is very important to design the components so that the heat from the thermal energy is dissipated efficiently into the atmosphere.

There are three types of drum brakes depending on how the brake shoes are pressed on to the drums; leading/trailing shoe type, twin leading shoe type and duo-servo type.
 
How drum brakes work
When the driver steps on the brake pedal, the power is amplified by the brake booster (servo system) and changed into hydraulic pressure (oil-pressure) by the master cylinder. The pressure reaches the brakes on the wheels via tubing filled with brake oil (brake fluid). The delivered pressure pushes the pistons on the brakes of the four wheels. The pistons press the brake linings, which are friction materials, against the inside surfaces of the brake drums which rotate with the wheels. The linings are pressed on the rotating drums, which in turn decelerate the wheels, thereby slowing down and stopping the vehicle.
 

 
Advantages of drum brakes Disadvantages of drum brakes
  • Drum brakes can provide more braking force than an equal diameter disc brake.
  • Drum brakes last longer because drum brakes have increased friction contact area than a disc.
  • Drum brakes are cheaper to manufacture than disc brakes.
  • Rear drum brakes generate lower heat.
  • Drum brakes have a built-in self energizing effect that requires less input force (such as hydraulic pressure
  • Wheel cylinders are simpler to recondition than with disc brake callipers.
  • Brake shoes can be remanufactured for future use.
  • Drums have slightly lower frequency of maintenance due to better corrosion resistance.

 

 

 
  • Excessive heating can happen due to heavy braking, which then can cause the drum to distort, and thus cause vibration under braking.
  • Under hard braking, the diameter of the drum increases slightly due to thermal expansion, the driver must press the brake pedal farther.
  • Brake shoes can overheat to the point where they become glazed.
  • Excessive brake drum heating can cause the brake fluid to vaporize.
  • 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 is enough, the brake will stay engaged due to self-application, even when the external application force is released.
  • Another disadvantage of drum brakes is their relative complexity.
  • Maintenance of drum brakes is more time-consuming, compared to disc brakes.