A friend of mine says buying brake linings is like buying pantyhose. She’s speaking of the range of choice in size, color, fit, material, style and finally, it seems, function. If any of my male readers have ever found themselves in that section of a department store, they’ll appreciate the analogy.
There is no shortage of options with brake friction. Purchasing priorities range from price to performance, and somewhere in between is application, vehicle configuration, and the reason for the purchase. Reasons range from mid-life relines to pre-trade-in relines, with the odd repair thrown in to mix things up a little.
All that is to say there are as many parameters to consider as there are choices of friction products in the market. Making the best choice is worth some consideration.
Cost is obviously high on the list of considerations, and we’ll get to that shortly, but performance is something we know some fleets seem willing to trade away in pursuit of lower cost. In 2011 regulators made changes to the stopping distance requirements for heavy trucks, effectively cutting about 80 feet from the maximum allowable stopping distance. Brake makers and OEMs met the requirements with disc brakes and beefed up drum brakes, and they consumed a lot of engineering resources to make that happen. But those requirements don’t technically apply to trucks once they are in service.
The only requirement that exists to hold a fleet’s feet to the fire on stopping distance is 49CFR 393.40, which states, “Buses, trucks and truck-tractors equipped with air brake systems and manufactured on or after March 1, 1975, and trailers manufactured on or after January 1, 1975, must, at a minimum, have a service brake system that meets the requirements of FMVSS No. 121 in effect on the date of manufacture.”
There aren’t many ways to interpret that except that trucks must be maintained in such a way so that they remain compliant with the standards in effect the date they were built — which includes the reduced-stopping-distance requirements. But much of 393.40 is practically unenforceable, so some truck owners buy cheap brake linings and feel good about the money they saved.
But how about all the money you spent up front on the advanced driver assistance systems, collision mitigation systems, vehicle stability systems, etc.?
“A lot of fleets are utilizing these systems to help improve or enhance their safety. If you change out that original friction to something that performs worse in stopping distance, it will certainly impact the electronics of the collision mitigation system,” says Keith McComsey, director of marketing and customer solutions for wheel-ends at Bendix Spicer Foundation Brake. “To save a few dollars on friction material, you’re effectively hindering all those investments you made in making the vehicle much safer.”
Those systems are designed to reduce the closing speed as much as possible to hopefully prevent a collision, or at worst, slow the truck sufficiently to minimize the impact of a collision. By changing the stopping characteristics of the foundation brakes, the collision mitigation system cannot perform as it was intended. That shouldn’t come as a surprise, and it’s a fact widely understood by plaintiff’s attorneys. Imagine yourself in the witness chair defending your choice of aftermarket friction in a fatal collision case. The friction material may not have made a functional difference in the outcome of the collision, but no attorney worth his or her salt is going pass up asking why you chose a cheap and unproven aftermarket brake lining rather than the more expensive OE lining or high-quality aftermarket product.
Brake friction material has to accomplish several things to be effective. It has to stop the vehicle within a reasonable or prescribed distance. It has to wear at a reasonable rate so it is cost-effective. And it has to do all this without adding undue stress to the brake drum or rotor.
For example, an aggressive lining might offer a shorter stopping distance, but it could chew up the drum or rotor in the process. Since drums and rotors, especially rotors, are much more expensive than linings or pads, going for the aggressive lining wouldn’t be cost-effective at all.
Linings that offer long life might be characterized as hard and therefore potentially damaging to the drum or rotor. Linings that claim to be quiet might be relatively soft, and therefore short-lived.
“Anybody can make a pad to last long, so they’ll tout the wear performance, and/or stopping distance if they’ve tested it, but no one ever talks about what impact that has to the rotor,” McComsey points out. “In a lot of cases that we’ve tested, those pads add a lot of heat to the rotor, so the rotor — in our testing — has demonstrated that it would stress crack. Those stress cracks would propagate to the point where it would exceed what CSA allows, so you could potentially be taken out of service, and/or it would rupture the rotor to the point where it would fail.”
And linings and pads are usually formulated to suit specific applications. In long-haul over-the-road operations, braking demands and heat dissipation requirements are lower than those of a concrete mixer or a logging truck.
“You need the correct friction formula for your application,” says John Thompson, OE sales manager for TMD Friction. “Each formula is specifically designed for different purposes such as: carrying heavier loads, on/off highway driving, temperature ranges or frequent stops, etc.”
How do you evaluate what you’re buying?
Lining and pad material are marked with letters and numbers called edge codes, but they really don’t tell you much beyond who made it, what it’s made of, and its coefficient of friction. According to Abex, they are in no way an indication of lining quality. The letters (EE, FF, GG etc.) describe a range of normal and hot friction values measured when a 1-inch square piece of material is subjected to varying conditions of temperature, pressure, and rubbing speed on a test machine.
“It’s important to know who you’re buying from; who is actually making the product,” says Dennis Griffin, product manager for Abex friction. “There are certain recognized names at the top of the aftermarket friction supplier list, and we have come to know and trust them. But with the white-box and private label products you have no idea what you’re getting, how it was made or how it was tested.”
Some supplier test results can be misleading if you don’t know what you’re looking at. The marketing material or the data sheets might indicate the lining or pad is RSD-compliant, but is it really? An enormous amount of testing went into meeting reduced stopping distance regulations when they were first implemented, including dynamometer testing and actual track testing. You might see dyno test results indicate a stopping distance of say, 160 feet while RSD requires 250 feet. Sounds good right?
“Based on our track testing experience with RSD, tire traction and ABS activity make that physically impossible,” says Joe Kay, Meritor’s chief engineer for braking systems. “It’s all about physics. In meeting the RSD requirement, everything on the vehicle became important, from the suspension, the tire selection, brake system timing, etc. To meet the 250-foot requirement, the brakes had to perform pretty high and very consistently. The dynamometer can help predict how the brakes will perform even under different axle loads, but it can’t provide actual stopping distance measurements.”
What’s at stake
When buying friction material, the first hard encounter is the cost. Good, well-tested and documented product from a well-known supplier will almost always cost more. And while it might be tempting to downgrade the spec to save a few bucks, the results can come back to bite you. Good or bad, you won’t see the difference immediately, and the fleet buyer may never notice a difference. Drivers might experience the difference in performance, but realistically, how much of a difference would they be able to detect on a truck with 300,000 or 400,000 miles on it? The maintenance shop might see a difference over time when the inferior product starts coming apart or damaging drums and rotors.
Since very few fleets have the resources to do their own performance tests for factors such as stopping distance, lining swell, heat dissipation, and rotor or drum wear, the rest of us have to rely on experience or the word of the supplier. Since replacement disc brake pads are relatively new to the market (compared to drum-brake lining materials), fleets are presently inclined to stick pretty close to the OE suppliers to replacement material, says Greg Cooper, Stemco’s brakes products manager.
“We see a very high percentage of customers going back to original equipment, but that will change as more markets open up to aftermarket suppliers,” Cooper says. “I’ve noticed that fleets will open up to aftermarket if they trust where the product comes from, if they are familiar with it, if there’s some brand recognition and they are convinced there has been adequate testing for fit, form, function and it passes FMVSS 121 requirements or if the supplier can provide sufficient background information about the testing.”
Choosing the right friction product can be a daunting task. There’s a lot product on the market, and pressure to reduce costs is very real. However, like with pantyhose, I’m told, there’s a big difference in quality between the best and the worst on the shelves, but not so much of a difference in the top tier products. Buy the best product you can at the best possible price, and you’ll likely come away happy with your decision.