drum brakes

Cars are often equipped with drum brakes in the rear and disc brakes in the front for cost reasons. Drum brakes are cheaper for the manufacturer to install.

Drum brakes work on the same principle as disc brakes. Shoes push against a moving surface and the friction lowers the rotational speed of the wheel. The basic construction of a drum brake is a cylinder (the drum) which contains two brake shoes and a piston. The drum rotates on the same axis as the wheel. The shoes face the inside of the cylinder and the piston sits between them. There are also a number of springs in place to pull the shoes back away from the drum when the brakes aren't active and some levers for the emergency brake.

When the brake pedal is pushed, brake fluid from the master cylinder enters the piston and forces the shoes apart, pushing them against the drum. The emergency brake system is much easier to implement in cars with drum brakes. The power for the braking comes from a cable which, when pulled tight, forces the brake shoes apart. This is important, since emergency brakes must operate even if the hydraulic system isn't functional.

The brakes are located on the inside of the wheels (where the tire is attached), and are attached to the hub. The brakes have many parts inside, the most obvious of which are the shoes. They are semicircular bits of metal with pads on the outside. At one end, they will be connected by the piston, and at the other by the adjuster mechanism.

Drum brakes lose effectiveness in sustained braking because they quickly overheat. Disc brakes run much cooler comparatively, and will be much more effective at slowing you while going down hills. The danger of overheating means that drum brakes must have less braking power than a disc brake. Disc brakes can put a lot more hydraulic force into stopping the car without damaging the brakes or fading.

Because friction is involved in this braking system, there will be wear. It is important to keep the gap between the drum and the shoes constant, so that there is no noticeable change in the action of the brake pedal. Modern drum brakes have a self-adjusting mechanism that automatically moves the shoes closer to the drum as they wear. As shoes wear out, a small metal bit touches the drum, making a squeaking noise while braking lightly. This is an indication that the shoes will need to be changed soon.

If the brake shoes are replaced at the proper time (before they are completely worn away), they should be the only thing that will need servicing. Generally, it is a good idea to replace the shoes when there is between 1/32" and 1/16" of material left. If the shoes are allowed to remain in poor shape for too long, rivets in the shoe may gouge the inside of the drum, producing unpleasant noises, and less effective brakes. Contamination due to oil or other fluids or erosion due to burning are also factors in brake wear. Sudden, heavy braking greatly increases wear and can burn the surface of the shoes or the inside of the drum.

Should you have to replace your brake shoes, it is an inexpensive repair. Cost should be in the range of $30 each (Canadian) plus installation.

• http://www.howstuffworks.com/drum-brake.htm
• http://www.raybestos.com/OnLineTraining/cpt-014.htm
• http://www.edmunds.com/ownership/techcenter/articles/43857/article.html

Drum brakes, n:

The bane of your DIY mechanic's existence. I would rather rebuild my entire engine than rebuild my drum brakes.

Unfortunately, drum brakes are ubiquitous. Unless you have a high end car, or opted for the "4 wheel disc brake" package, you have at least one set of drum brakes in your driveway. If you have an old car rusting to death somewhere, it could have 4 wheel drum brakes. Those tricky bastards are everywhere! Let's get some intel.

This ASCII art for this would be extremely complex, so Google is your friend. Get yourself a good head on shot, not an exploded view.

Drum Brake Components

• Drums may not be in your picture. If you take the tire off to examine the drum brake, you'll be staring at a flat surface. That's the drum. With both hands, pull it straight towards you off the wheel studs to reveal the actual braking mechanism. If you examine the inside of the drum, you should find an amount of black dust and a shining silver surface.
  Drums occasionally need machining, or "turning" in automotive phraseology. They must be replaced when subjected to abuse.


• Wheel cylinders should be the easiest thing to identify next. At the top of the brake apparatus you should see a cylinder oriented horizontally. The brake lines coming from the master cylinder and power booster connect to the wheel cylinders which transfer pressure from the hydraulic brake fluid outward to the brake shoes. The ends are covered with rubber caps and usually contain a spring with a separate set of caps.
  Wheel cylinders will need to be replaced or rebuilt over time.


• Brake shoes are the metal arcs pressing into the wheel cylinder's rubber caps. As the pressure builds in the wheel cylinders, the brake shoes are forced outward in relation to the axle. This movement brings the shoes into contact with the inner edge of the drum producing friction, heat, dust and stoppage. You should be able to see the layer of brake material that makes up the contact surface of the shoe. This is what wears away and eventually allows a metal tab to rub the inside of the drum and tell you to go to Midas.
  The shoes are one part the grease monkey will tell you is out of spec and needs to be replaced. Then you tell him that good people at e2 have you covered, and you'll do the work yourself.


• Adjuster Screw Assemblies differ from car to car. On my Ford, this between the bottom ends of the brake shoes. YMMV, but it will be pinched between the shoes. The most unique feature about it is the toothed gear. This allows brake adjustments to made when the shoes have become worn, but still usable. If the brakes are not periodically adjusted, they will take slightly longer to engage. On most modern cars with disc brakes in the front and drum brakes in the rear, a driver may think that their brakes are going out and apply excessive force to stop the car. However, the front brakes are fine but when the rear brakes suddenly engage with more force than the driver expects, results are indeterminate.
  Adjuster screw assemblies should not have to be replaced.


• Strut bar. You should know this is by process of elimination. These physically vary from car to car, but it's whatever piece of metal that prevents the shoes from collapsing inward under the pull of springs.
  If someone tries to sell you one of these, kick them in the face.


• Emergency brake cable connections are hiding somewhere. By definition, this is a cable that connects to the shoes allowing them to be engaged with no power from the engine. Note: the cable is behind the large plate that all the brake components mount to, so you probably can't see it in your diagram. While outside the scope of this node, your power brakes have no, um, power while the engine is off. Nothing can create the hydraulic pressure to actuate the brakes. An emergency brake, or hand brake, allows the rear brakes to be engaged by hand. Considering the number of people I see parking their cars incorrectly, the population at large has no idea how or why this works.
  Emergency brake anything should not have to be replaced.


• Springs account for about half of the parts in drum brakes. And deep in the springs lies my intense hatred of drum brakes. It is a pain in the ass to take them all off and put them on again. Trust me. There isn't much to say here; springs hold things down or serve to draw parts back to their starting locations. It sounds simple until you have to put two shoes, a strut bar and three springs on at once and you realize you don't have 8 arms.
  Old springs occasionally break (pun!) when you remove or install them, but they are available in brake rebuild kits.

Coming soon, a guide to changing the damn things!