One of the most common questions we hear from fleets and drivers who are considering air disc brakes is “How long will my pads last?” And it’s understandable, since the cost of replacing brake friction is certainly one component of measuring the return on investment in ADB technology. The thing is, while it is true that air disc brake friction offers longer life than drum brake friction, it’s impossible to assign friction a mileage figure that will be accurate across all fleets, applications, and driving habits: There are just too many variables.
By taking a look at some of the factors affecting ADB friction life, trucking industry teams can gain helpful insight into making the best and safest equipment choices.
Two Types of Factors
First off, we can classify all the things that impact brake friction wear into two categories: controllable and uncontrollable.
In the latter category are factors at the fleet operational level. They’re just facts of life, or the rules of a given playing field. Operating environment and terrain, for instance, fit into this category – city vs. highway, mountains vs. plains. You don’t get to change these, but it’s clear that repeatedly traversing the Continental Divide is going to treat your brakes a bit differently than hauling across the Kansas flatlands. Same goes for the percentage of time you’re hauling fully loaded vs. empty, or your fleet’s typical number of brake applications in a given day. These factors can’t be controlled, but fleet equipment managers need to be keenly aware of the direct impact they have on brake pad life.
Controllable factors in brake friction life include decisions made at the vehicle spec’ing level and those related to driver habits behind the wheel. In addition, every fleet must continue to perform proper air disc brake maintenance. Let’s dig a bit into each of those.
What You Spec Matters
Anything that contributes to slowing a vehicle down will have an impact on brake wear, simply because if some other factor is doing a bit of the work it takes to lower a vehicle’s speed, that’s a bit less work the brakes have to handle. Aerodynamics at higher speeds, for example, lower a truck’s wind resistance – which is good for gas mileage, but it requires the brake to do more work. Tire rolling resistance is a similar factor.
One key – and often overlooked – consideration is an engine brake, or engine retarder. Under normal deceleration (5 ft per second squared, or about .15g), an 80,000-lb. vehicle uses about 1,000 horsepower to stop from 60 mph. An engine retarder will actually handle about 50 percent of that work, leaving just under half the work to the foundation brakes. (The remainder is absorbed by aerodynamic drag and rolling resistance.) This means that in the same situation, the foundation brakes of a vehicle without an engine retarder would have their workload nearly doubled.
At Bendix, we’ve seen fleets adopt air disc brakes, but then not spec the engine retarder, because they figured that the higher-performing ADBs would make it unnecessary. But while ADBs are more than capable of exceeding today’s stopping distance standards unassisted, that extra workload with every stop is going to have a long-term impact on the life of their friction. So the friction life of a drum brake paired with an engine retarder can’t be compared to an ADB without one – it’s just not apples to apples.
Behind the Wheel
Drivers can also be a key contributor to brake pad life, depending on their habits when it comes to using the engine brake, a trailer hand brake that only engages the trailer brakes, or their tendencies toward aggressive stops. For example, if a driver owns their tractor, but not their trailer, they may rely more heavily on using trailer hand brakes, since long-term trailer brake maintenance is not their responsibility.
How a driver regularly handles stops affects brake pad life in a few ways. Take that same 80,000-pound vehicle we mentioned earlier: Stopping it from 60 mph using hard deceleration (10 feet per second squared, or about .3g) requires double the horsepower. An engine retarder maintains the same level of assistance as under normal deceleration – about 500 hp worth – but since the overall need is now 2,000 hp, the foundation brakes on that system are taking about triple the load they’re handling under normal deceleration. And again – the more work they do, the faster those pads will wear. Just think what that would be without the engine retarder.
Harder stops also generate higher brake temperatures, and as things get hotter, brake friction wears faster. The degree to which all these factors affect brake pad life is dramatic. Within a single fleet, with identically spec’ed trucks, we’ve seen hundreds of thousands of miles of difference in pad life from vehicle to vehicle. What that means is that when fleets and owner-operators ask us what kind of life they’ll get from their air disc brake pads, we’re likely to respond with questions.
And that’s a good thing: Because the more you know about your vehicles and day-to-day operations, the better equipped you’ll be to keep them on the road and operating safely.
Learn more about Bendix air disc and foundation drum brake solutions at Bendix.com, or contact us at 1-800-AIR-BRAKE.
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