Drum brakes

In short..

A “drum brake” is an older form of brake mechanism used in vehicles for the purposes of slowing down and stopping. It consists of a few parts where the most important ones are the “drum” and the “shoes“. When the driver presses the brake pedal, it pushes the “shoes” against the drum which is coupled to the wheel and so the wheel slows down.

Origins

In 1902, Louis Renault, one of the automotive pioneers, patented the drum brake mechanism. The first drum brakes used woven asbestos linings for the drum brake lining, as there was no better alternatives for the dissipation of heat. Old drum brakes were operated mechanically using the levers, rods and cables. In 1930s, some of the manufacturers modernized the mechanism so that the oil pressure in a small wheel cylinder and pistons operated the brakes.

In operation..

When the driver presses the brake pedal, brake fluid is forced under pressure from the master cylinder into the wheel cylinder, which in turn pushes the brake shoes into contact with the surface on the inside of the drum. This rubbing action reduces the rotation of the brake drum, which is coupled to the wheel. Hence the wheel slows down it’s rotation. When the pressure from the brake pedal is released, return springs pull the shoes back to their rest position. Other springs help hold the brake shoes in place and return the adjuster arm after it actuates.

For the drum brakes to function properly, the brake shoes must remain near the drum without touching it. If they get too far away from the drum, the drum brake mechanism piston will require more fluid to travel that distance, and driver’s brake pedal will sink closer to the floor when you apply the brakes. This is why most drum brakes have an automatic adjuster.

Automatic adjustment

Each time the vehicle stops while moving in reverse, the shoe is pulled close against the brake’s drum. When the gap gets big enough, the adjusting lever rocks enough to advance the adjuster gear by one tooth. The adjuster has threads on it, like a bolt, so that it unscrews a little bit when it turns, lengthening to fill in the gap. When the brake shoes wear a little more, the adjuster can advance again, so it always keeps the shoes close to the drum.

Some cars have an adjuster that is actuated when the hand brake is applied. This type of adjuster can come out of adjustment if the hand brake is not used for long periods of time. So if the vehicle has this type of adjuster, the driver should apply the hand brake at least once a week.

Drum brakes vs the Disc brakes

A disc brake has a disc shaped metal rotor spinning within a wheel. When the driver applies the pressure to the brake pedal, a caliper will squeeze the brake pads against the disc. This will slow the wheel’s rotation down as more pressure is applied to the brake pedal, bringing the vehicle to a stop.

Usually, the disc brakes are more effective, less prone to failing and fading but also much more expensive.

When the vehicle is braking, the weight of a vehicle is going to shift forward. That’s why over two thirds of the braking effort occurs in the front of the vehicle. There is not quite the need to have disc brakes on all four wheels. This is one of the reasons why most of the low and mid-range vehicles get the disc brakes on the front and the drum brakes on the back.

Want more info?

If you would like to learn more, I’d suggest you to check out these articles:

  • https://en.wikipedia.org/wiki/Drum_brake
  • https://auto.howstuffworks.com/auto-parts/brakes/brake-types/drum-brake.htm
  • https://procarmechanics.com/differences-between-drum-brakes-and-disc-brakes/

Original picture taken from: http://altra9.com.au/web/wp-content/uploads/2014/09/DSCF5451.jpg

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