Beyond the Braking Point
Part V of V: Drum Brakes

Statistics indicate that more than one in ten vehicles on the road are being operated with a brake system that will not pass inspection. As a result, collisions involving alleged brake failures are not uncommon. An experienced accident reconstruction engineer is able to perform tests on the vehicle in the field and in a laboratory that can determine whether a brake failure actually happened, and more importantly, demonstrate how it happened.

Over the next few months we will introduce e-zine articles that focus on different components of the brake system: master cylinder, vacuum booster, and an assortment of hydraulic fittings and hoses that carry fluid pressure to the brake calipers and rotors, or the brake shoes and drums. Each article will highlight a different component and discuss how it works, what can fail, and how it can be tested.

To read the complete article, please visit our website at http://www.warren-group.com/articles/beyondbrakingpoint.html.

Drum Brakes

How do they work?

Drum brakes were widely used on earlier model vehicles. Vehicles that are more modern have either front disk brakes with rear drum brakes, or four-wheel disk brakes. A typical drum brake assembly is shown in Fig. 9. In this type of system, a hydraulic wheel cylinder presses the shoes against the inside surface of a rotating drum. As you can see in the photo, the number of components makes this system much more complex.

What Can Fail?

A system of levers and springs was used to make the brakes "self-adjusting" in theory, however, the self-adjusting properties of disk brakes proved much more simple and reliable. Drum brakes are also much more susceptible to improper re-assembly during service, and must be carefully inspected for proper assembly following a collision where a brake failure is alleged.

Can it be tested?

As with disc brakes, drum brakes, the engineer will check drum brakes for evidence of leaking seals and trapped air. A functional test of the drum brake assembly is also necessary to determine if any components fail to move properly.

This is the concluding e-zine on brakes. We hope you found it useful and informative.

To view the entire brake series, please visit our e-zine archives at www.warren-group.com or view the complete article at http://www.warren-group.com/articles/beyondbrakingpoint.html.