At the 2000 SAE Truck & Bus meeting in Portland, Ore., Larry Strawhorn, engineering vice president of the American Trucking Associations, Alexandria, Va. (ATA), revisited a perennial subject in an oral presentation, “The Mystery of Aftermarket Brake Lining Selection.”
The theme was simple: It’s still almost impossible to shop for brake lining that’s compatible with original equipment in the aftermarket. The lining an OEM used may not even be available, meaning that “fleet maintenance people may have to guess” what lining to use to do a good brake job. It irks Strawhorn that this kind of inconsistency no longer exists when shopping for relay valves, yet continues with linings.
Why does lining compatibility matter? “Brake balance is a factor in every single stop. Replacement linings can change the balance,” is Strawhorn’s answer. And he worries that, if a brake system is out of balance because of the use of different linings at different wheel positions, electronic controls like antilock (ABS) and electronic braking, or brake-by-wire (EBS), will mask the symptoms of uneven brake force, and the driver won’t even know there’s a problem until he really needs full braking force. “Lining output is important,” he says. “Fleets need such ratings.”
What Strawhorn is saying makes a lot of sense. Think of it this way. ABS, if it’s working right, will keep all the wheels rolling fast enough to allow the tread to create lots of side force, thereby keeping the tractor and trailer in lane. But, how does it do this? By cycling the brakes. But air is highly compressible, not to mention other response-time-lengthening factors, such as the inertia and friction of a diaphragm, slack adjuster, S-cam, and all the hardware inside the wheel.
So, cycling the brakes takes time, and each cycle results in a slight reduction in stopping power.
This means an increase in stopping distance as compared with a stop made with a well-balanced system and a highly skilled driver, where every wheel is constantly right near its adhesion limit.
Other friction-incompatibility issues include: excess wear to the ABS and brake actuation components; uneven brake torque, which makes some brakes work too hard and overheat or fail early; and the whole question of what happens if the ABS system, or even a single component, malfunctions.
Still another concern of Strawhorn’s is the probability that a truck with substandard linings might fail a roadside brake-torque effectiveness test, even though it’s just had a brake job.
He points out that, in spite of substantial effort, there still isn’t a foolproof test of lining characteristics, let alone a consistent system of markings. Many linings fail to meet standards, with some 51% regarded as unacceptable-32% of them due to low brake torque (some others offer too much torque). The range of torque output, says Strawhorn, may be as great as 22%. Imagine trying to stop a truck when one wheel is getting 7,667 lb-ft of torque and another only 5,972!
ATA petitioned the National Highway Traffic Safety Administration (NHTSA) to work out a consistent lining performance standard way back in 1987. Two divisions of ATA’s The Maintenance Council cooperated in developing the J-1802 standard, which came along in 1992, but Strawhorn says the measurements still are not consistent enough. So, you still can’t be sure whether or not a lining will actually keep your system in balance. He believes the resulting confusion takes pressure off the manufacturers to produce consistent performance.
In summing up this situation, Strawhorn asked, “Will we have advanced EBS, yet not be capable of replacing the linings we have on a vehicle with the right product?”