When it comes to replacing friction, today’s market is crowded with options – whether you’ve got air disc or drum brakes on your tractor or trailer. That also means there’s no shortage of questions about brake pads and friction selection, which can have direct and long-lasting impacts on vehicle safety, performance, and total cost of operation. With a quick dive into some details about discs, drums, and friction, we can help answer some of those questions.

Friction Differences

Let’s start with differences between air disc and drum brake friction, which stem from both the designs of the systems themselves and the effects of federal Reduced Stopping Distance (RSD) mandates that began in 2011.

In drum brake friction offerings, there’s a high degree of variation: Selecting the ideal friction formulation depends heavily upon things like the vehicle’s application, duty cycle, gross axle weight rating, and operating environment. Furthermore, RSD regulations (starting in 2011) necessitated changes in drum friction formulation. In order to improve brake performance with reductions in stopping distance,  some compositions became semi-metallic at all axle positions, and some were engineered to provide a more aggressive brake performance at the front steer axle when compared to the drive axles. Conversely, air disc brake (ADB) friction has remained consistently semi-metallic and maintained the same formulation across all axle positions.

Additionally, due to the wide range of drum brake system choices that meet or exceed federal requirements, the specific friction couple between the brake shoe/friction and the drum isn’t engineered for optimal wear, simply because there are so many brake and drum combinations (sizes, friction formulations, drum metallurgy, etc.) to choose from. Conversely, on the air disc brake side, Bendix engineers its ADB systems with the complete friction couple in mind. This means we take into account both the pad and the rotor, maximizing the overall brake’s stopping performance as well as the wear life of both the pad friction and the rotor components.

Finally, there’s the difference in the friction attachment. Drum brake friction is riveted to a shoe, an assembly method that carries the risk of moisture or contaminants getting between the friction and shoe, which can lead to rust-jacking. Air disc brake pad construction bonds the friction to a backing plate, virtually eliminating the risk of rust-jacking.

Options and Knowledge

The aftermarket provides many friction options for both discs and drums. Not all of them, however, will meet FMVSS-121 requirements in effect at the time of a vehicle’s manufacture. This could therefore impact your vehicle’s performance, including stopping distance and parking capability. When replacing your vehicle’s friction (whether drum brake or air disc brake), Bendix recommends that fleets and owner-operators replace friction with like-for-like components in order to maintain the original manufacturer’s braking performance levels —helping keep roads safer and reducing your risk of liability.  Since some AM frictions have been shown to increase stopping distances (when compared to OEM friction), accident investigators may find that an accident could have been preventable had the friction been replaced with the original OEM friction.

Some thoughts on friction in the context of the increasing adoption of air disc brakes: As ADBs have gained market acceptance, pad friction has evolved to account for varying fleet factors, including application, tractor/trailer brake combinations, and duty cycle. At Bendix, we created new frictions to meet the wear rate performance needs based on these factors. One of those frictions (BX283) helps manage high temperatures in applications with repeated, frequent brake usage such as refuse, school bus, and pickup/delivery. Another Bendix brake friction (BX276) provides a longer life friction for line-haul and trailers through more wearable volume, as well as a revised formulation that improved the wear rate by up to 40 percent. More fleets are adopting ADB, and at the same time brake technology continues to evolve to accommodate application differences while providing the desired safety and wear performance benefits expected from this high-performance brake system.

Mix-and-Match: Yes or No?

Because we’re often asked questions about mixing air disc and drum brakes on tractors and trailers – we even did a Truck Talk with Bendix podcast episode on the topic – it’s worth discussing a similar issue at the friction level, too.  Any time you start to mix friction brands or move away from the original OEM spec’ed friction, you run the risk of significantly impacting a vehicle’s stopping distance performance, parking capability, and friction wear performance. Additionally, TMC Recommended Practice (RP-607B) outlines changing friction equally across the axle and not mixing: Installing different brands or types of friction on a single axle could create an imbalance between right and left sides, potentially leading to a vehicle pulling to one side during braking (increasing your risk for an accident), uneven wear, and other problems.

Time for a Change

So, when is it time to replace your friction?

On air disc brakes, the best way to know is through routine preventative maintenance and visual inspection. If you’ve got Bendix® ADB22X brakes, there are a couple of ways to check your friction’s remaining life.  Start with a quick, easy visual check, comparing the edge of the friction to see whether it’s in line with the edge of the blue pad retention spring. But if it is close and/or if a more accurate indication is needed, compare the friction to the casted wear indicators on the caliper/carrier assembly, or use a Bendix pad/rotor wear gauge (PN K109114). If you’re conducting a more thorough examination, make sure to check the friction for cracks or abnormal wear.

Visual inspections can also be conducted on drum brakes, removing dust shields if necessary. Measure the shoe friction thickness between the friction blocks with the proper tool (Part number  K109114). You should also replace the friction if you find missing chunks, radial cracks along the edge of the friction, or cracks that come over the edge of the friction and extend down to the shoe table.

If you want to dig further into air disc and drum brake maintenance, check out the Bendix Tech Talks on our YouTube channel, or browse the Bendix Tech Tips online archive.

Bendix Blog

Technical and industry insight from OUR experts.

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