Drum Brakes vs. Disc Brakes: What’s the Difference?

Did you know that the braking system is one of the most important components to decide on when spec’ing a commercial truck? Because heavy-duty vehicles have longer stopping distances — more than the length of two football fields when traveling at 65 mph — the brakes play a vital role in ensuring the safety of the driver and those around them.

Commercial vehicles have air brake systems equipped with drum brakes or air disc brakes (also referred to as ADBs or just disc brakes), or sometimes a combination of the two. Drum brakes have historically come standard on commercial trucks and are the most widely used braking system in the truck industry due to their lower cost and ease of manufacturing. However, the adoption of disc brakes has been steadily growing over the past decade, mostly due to some manufacturers now offering disc brakes as standard on heavy-duty and vocational models because of their superior stopping power and longer life before replacement.

However, there are vocation-specific circumstances in which a specific type of brake may be a better fit. Drum brakes and disc brakes function similarly in that they both turn kinetic energy (the wheel’s movement) into thermal energy (heat) to slow down a vehicle. What differs is the way that heat builds up when friction is applied within the braking system. It’s important to understand the difference between drum brakes and disc brakes and their pros and cons that will impact drivers in different situations.

How Drum Brakes Work

Drum brakes consist of a brake chamber, push rod, slack adjuster, camshaft (S-cam), rollers, a pair of brake shoes, the drum and a return spring. When a driver presses the brake pedal, pressurized air travels to the brake chamber and forces the push rod to actuate the slack adjuster, which in turn rotates the S-cam. As the S-cam rotates, rollers spread the brake shoes apart and force them against the walls of the drum. The friction created by the brake shoes rubbing against the drum slows down the vehicle. Once the driver releases the brake pedal, the return spring pulls the brake shoes away from the drum to allow the wheel to rotate.

Pros and Cons of Drum Brakes

Pros

• Cost – Drum brakes have been in production since the early 1900s and have historically been very popular. Their larger economy of scale typically makes replacement parts cheaper than disc brakes. Drum brakes are also cheaper to manufacture than disc brakes.
• Durability – Drum brakes are generally more durable than disc brakes and require fewer lining replacements because their larger friction contact area helps them to last longer.
• Self-Energizing System – Drum braking systems are self-energizing, meaning the rotation of the drum can forcefully drag one or both brake shoes against the inside of the drum. This increases the vehicle’s stopping power without the driver having to apply extra pressure on the brake pedal.
• Low Maintenance – Because all the components of a drum brake are enclosed within the drum, the parts are more resistant to corrosion and maintenance is simplified.
• Parking – Drum brakes can be engaged while parking without the worry of the brake shoes contracting and the brakes failing.

Cons

• Overheating – One of the biggest downsides to drum brakes is their tendency to overheat quickly in conditions where a driver has to brake frequently. This occurs because the components are housed within the metal drum with limited air circulation. When heat builds up, it can cause the brake shoes to glaze or smooth out and produce less friction, which ultimately increases stopping time. This is referred to as brake fade.
• Wet Weather Performance Issues – The enclosed design of drum brakes causes them to take longer to dry off in wet weather. This exposes the brake shoes to moisture longer.
• Grabbing – Grabbing is a condition in which the drum brake becomes overly sensitive and applies more braking force than required. This can lead to the tires skidding and a loss of control of the vehicle. Grabbing can be caused when the brake shoes are wet or rusty.
• Weight – Drum brakes are generally heavier than disc brakes. The lighter the overall weight of a vehicle, the easier it is to brake.
• Complex Construction – A drum brake has many components that require an expert mechanic for repairs. Inspections are also made more difficult because components are less accessible inside the drum.
• Thermal Expansion – When a driver has to hit the brakes hard, the diameter of the drum expands due to a phenomenon called thermal expansion. This reduces the braking response and the driver must press the brake pedal harder to stop the vehicle.
• More Prone to Failure – Drum brakes are also more prone to failure than disc brakes and cannot be easily fixed by resurfacing the brake shoes. Most of the time the only option is to replace the drum, which can be costly.

How Disc Brakes Work

Instead of using brake shoes and a circular drum to create friction, air disc brakes create friction using a caliper, a disc-shaped rotor and a pair of brake pads. As a wheel spins on a vehicle, the disc-shaped rotor spins with it. When the brake pedal is engaged, compressed air forces a metal piston inside the caliper to clamp down on the brake pads on either side of the spinning rotor with great force. This pressure creates friction between the pads and rotor to quickly slow down or stop the rotation of the wheel. When the brake pedal is released, the caliper releases pressure on the brake pads to allow the rotor to continue spinning.

Pros and Cons of Air Disc Brakes

Pros

• Improved Stopping Distance – With disc brakes, stopping distance can be 17–33% shorter than drum brakes, depending on the speed of the vehicle. Shorter stopping distances are important for keeping the vehicle compliant with the National Highway Traffic Safety Administration’s Reduced Stopping Distance (RSD) rules that require heavy-duty trucks to stop in no more than 250 feet when loaded to max capacity and within 235 feet when lightly loaded. The ability to stop more quickly can also reduce the risk of accidents or decrease their severity.
• Easier Maintenance – Disc brakes are easily accessed without removing the wheel and are built with fewer components, making maintenance and installation faster and simpler.
• Self-Centering and Self-Adjusting – The caliper found inside disc brakes slides from side to side and moves to the center of the two brake pads each time the brake is applied. This leads to more consistent wear of the brake pads and offers more consistent braking over the life of the pads.
• Fewer Compliance, Safety, Accountability (CSA) Violations – Brakes are the second-most penalized component on trucks during CSA inspections conducted by the Federal Motor Carrier Safety Administration, which can lead to unnecessary delays and downtime. Because disc brakes do not need to be adjusted, CSA citations for “brakes out-of-adjustment” are eliminated and drivers do not have to worry about them impacting their scores.
• Better Heat Dissipation – Since the rotor is fully exposed to outside air, any heat produced by the friction of the brake pads dissipates into the atmosphere. This prevents overheating of the brakes and improves braking power.
• Less Brake Fade – Because less heat is produced and contained within the braking system, brake pads are less likely to experience brake fade compared to the brake shoes found in drum brakes.
• Standard Performance in Wet Weather – The open-air design of disc brakes allows them to dry off more quickly when wet, reducing the risk of slipping or grabbing.

Cons

• Cost – Disc brakes are much more expensive than drum brakes up front. However, some newer disc brake manufacturers have focused on cutting weight and simplifying the design, which in turn is driving down costs.
• Less Effective as Parking Brakes – Brake pads and rotor discs can expand when heated and contract when cold, making them ineffective as parking brakes. When parked, the brake pads cool off, shrink, and lose contact with the rotor — rendering the brakes ineffective.
• Brake Pad Wear-Out – In some applications, brake pads may wear out more quickly than the brake shoes on drum brakes due to the powerful squeezing force the caliper applies on the pads and rotor. This pressure provides better stopping power but reduced brake pad life.
• Rotor Warping – There is a chance that the rotor may warp when the driver brakes at high speeds. This happens due to very high temperatures on the discs followed by immediate cooling.

Which Type of Brake Is Best for Your Vehicle?

As we’ve demonstrated above, there are many pros and cons to both drum brakes and air disc brakes, but at the end of the day, deciding which option is best for your commercial truck comes down to its intended use. For example, the fade resistance of disc brakes makes them a popular option for vocational trucks driving in environments such as work sites that require frequent braking. Drum brakes, however, are a good choice for long-haul trucks, due to their durability and longer service intervals.

Driving habits, brake duty cycle, operating conditions, compliance and stopping distance are all variables that must be taken into consideration, and the value truck owners put on each of these aspects will ultimately drive their decision on which type of brakes to spec for their vehicles.

A certified technician can inspect the brakes on your truck and recommend the best option based on your specific application and needs. If you’re in the market for a new vehicle, find out which type of brakes come standard on the make and model you’re considering so that you can be aware of the maintenance needs for your specific type of brakes.

Maintaining Your Truck’s Brake System

No matter which braking system you have on your truck, regular maintenance is crucial to ensuring maximum performance and safety and increasing the life of your brakes. Ignoring brake issues can lead to breakdowns and accidents that cause harm, downtime, loss of revenue and potential violations. In 2022, over 13% of vehicles inspected during the Commercial Vehicle Safety Alliance’s Brake Safety Week were placed out of service due to brake-related violations.

You can make sure your brakes remain in good condition by inspecting for worn-out, damaged or rusted components; listening for unusual noises when pressing the brake pedal; and watching for warning signals on the dashboard.

Rush Truck Centers offers same-day brake system inspections and service at our locations across the U.S. Our dedicated technicians will focus on your brake system and conduct a comprehensive inspection on brake pads, rotors, brake shoes, and drums and provide per-axle prices for parts and labor. Learn more about our brake inspection services to ensure your truck’s braking system is in peak condition, or contact us today to request a service appointment at one of our more than 140 locations nationwide.

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