Understanding Drum Brakes and Disc Brakes

truck brakes

Heavy duty truck brakes

Did you know it usually takes the length of two football fields for a semi-truck traveling at 65 mph to stop moving when brakes are operating normally? Because big rigs have longer stopping distances, it’s important to understand the different types of brakes, how they function, and how to maintain them so you can drive and stop safely. Commercial vehicles have air brake systems that are equipped with drum or disc brakes (or both) — the information below can help you understand each type.

Comparing brake types

Drum brakes and disk brakes function similarly (turning kinetic energy into heat), but the way heat builds up when friction occurs is the primary difference. Learn how each type functions and their advantages and disadvantages. 

Drum brakes

How they function

Drum brakes are composed of the brake chamber, push rod, slack adjuster, S-cam (camshaft), rollers, shoes, lining, drum and return spring. When the brake pedal is pressed, pressurized air (stored in air reservoirs) travels to the brake chamber. This forces the push rod to actuate the slack adjuster, which rotates the S-cam. The rotation of the camshaft allows the rollers to spread the shoes apart and force the lining against the drum. As the lining rubs against the drum, kinetic energy (wheel movement) is converted into thermal energy (heat). The created friction causes the wheel to stop moving. Once the pedal is released, the return springs pull the shoes away from the drum back into resting position and allow the drum to resume rotating along with the wheel.


The benefits of drum brakes include:

  • Clean operation (free of dirt and other road debris since it’s enclosed)
  • Inexpensive construction and installation
  • Simple maintenance
  • Cheap part replacement


While drum brakes have many benefits, they do have one downside — overheating may cause increased stopping time, which is called brake fade. This happens because the components are housed within the metal drum with limited air circulation. When heat builds up, it causes the drum to expand and creates more pushrod travel, which can impact stopping ability.   

Disc brakes

How they function

The primary components of disc brakes include the brake chamber, eccentric lever, power screw, brake pads and rotor. When the wheel is in motion, the rotor spins with it. As the pedal is pushed, compressed air travels to the brake chamber where the eccentric lever activates the braking force. The power screw then clamps the brake pads down on both sides of the rotor, causing friction to build up and stop the vehicle.


The benefits of disc brakes include:

  • Easy inspection (wheel doesn’t have to be removed)
  • The rotor is fully exposed to outside air (this helps keep it cool)
  • Resistant to wet weather (because they are fully exposed, water and debris can get off the rotor to help with performance)


While disc brakes do have faster stopping capabilities, they’re more expensive than drum brakes. Even though performance is a priority for many carriers and drivers, the added cost can lead many to choose vehicles equipped with drum brakes instead.

To help offset costs and provide the benefits fleets are seeking, many manufacturers have found it ideal to use both brake types — discs on the front wheels and drums on the rear wheels. Manufacturing a vehicle like this is considered ideal because it provides the necessary stopping force in the front, but helps reduce costs in the back.

Tips for maintaining your brakes

Whether your vehicle has drum brakes, disc brakes or both, it’s important to maintain them on a regular basis. Ignoring brake issues (big or small) can lead to breakdowns and accidents — which may mean extended periods of downtime and loss of revenue. Make sure your brakes remain in good condition by:

  • Inspecting for worn-out, damaged or rusted components
  • Listening for unusual noises when pressing the pedal
  • Watching for warning signals on the dashboard  

Brake maintenance from FleetNet America®

Need help keeping your brakes working efficiently? FleetNet America offers a preventive maintenance (PM) program so that you can stay ahead on brake maintenance and reduce the likelihood of mechanical issues. If you’re interested in learning more about how we can care for your brakes with PM, call 877-559-9640.

If you have any questions about heavy-duty vehicle brakes, leave a comment below — we’re happy to help.