Brake linings are among the components that heavy trucks consume on a constant basis. So why not standardize and quantify their performance characteristics? Turns out, it’s not that simple.
The Technology and Maintenance Council (TMC) and the Society of Automotive Engineers (SAE) have provided a litany of additions and subtractions to Recommended Practice (RP) 628A, which contains a list of acceptable torque ratings for aftermarket linings.
The National Highway Traffic Safety Administration (NHTSA) requires that OE linings comply with Federal Motor Vehicle Safety Standard (FMVSS) 121. But aftermarket linings remain totally exempt from NHTSA’s performance-based testing requirements. That’s why there is RP 628A.
TMC and SAE decided there should be a standard for aftermarket linings. After all, they are constantly called upon to do the same job as their OE counterparts. So RP 628 (now 628A) was created to help select replacement linings with the proper frictional characteristics needed to maintain some semblance of original performance. Participation by lining manufacturers is still voluntary. But the RP carries considerable weight in the industry, and manufacturers fear losing sales if their linings aren’t listed.
Much Ado About Codes
A discussion of aftermarket linings naturally starts with edge codes.
“Edge codes are based on ‘Chase machine’ testing conducted in accordance with the SAE J661 procedure for rating the effectiveness of brake block for commercial vehicles,” says Brent Armentrout, formerly of Haldex. “The test uses a 1- by 1-inch friction specimen mounted in a holder, which presses the specimen against the inner diameter of an externally heated ring or drum. The friction range corresponding to the letters used in the edge codes are listed in SAE J866.”
In the past, edge codes provided maintenance people a means to identify and select linings with similar characteristics. Or, so they thought. The old impression was that if linings displaying the same edge code (such as FF, EE, FE or EF) as their originals, braking force would be identical. Today, however, most technicians are aware that linings displaying the same edge code can have torque ratings that vary by as much as 40 percent.
Some industry sources say that samples from one batch of lining material – whether tested on a Chase machine or a large brake dynamometer – can vary in torque by 10 to 12 percent. Even testing different surfaces of a 1-inch-square sample may produce up to a 6 percent variation because of the not-so-perfect science of mixing brake lining ingredients.
Even NHTSA says lining performance can be wildly diverse. In 1991, the agency compared the performance of 21 aftermarket linings with their OE counterparts on a 16.5-by-7-inch air brake, with a 6-inch slack adjuster and type-30 air brake chamber. At 100 psi, torque output ranged from 60,000 lb-in. to more than 150,000 lb-in.
So when shopping for suitable aftermarket linings, it’s more useful to know torque output than edge codes.
So What’s In ‘Em?
A lining’s ingredients may contain a grab bag of materials, such as:
- fibers (fiberglass, steel, carbon, synthetics);
- abrasives (aluminum oxide, magnesium oxide, zinc oxide, silicon carbide);
- friction modifiers;
- binders (phenolic resins and rubber);
- and carbonaceous components (coke, carbon and graphite).
These recipes are computer-blended and, typically, pressure-cured in long slabs, cooked, cut, shaped, drilled and affixed to shoes.
Most quality-conscious aftermarket lining manufacturers claim to sample and lab-test their raw materials on a routine basis. Checks include acidity, moisture, fiber size and ash content, among others.
Where’s J1802?
An SAE standard, J1802, was intended to provide data for a numerical friction rating, to be encoded on a lining’s edge by a series of long-lasting grooves.
Basically, J1802 involves using a brake dynamometer to determine a lining’s average torque at application pressures ranging from 20 psi to 80 psi, in 10-psi increments. In turn, those torque figures are used to calculate what’s called a “brake factor” that’s useful for obtaining a comparative rating.
“Although SAE J1802 was originally intended to replace J661, there have been no recent changes,” notes Armentrout. “There was a series of ’round robin’ tests conducted in the late ’90s to ascertain the repeatability of SAE J1802 tests but, for whatever reason, the SAE committee participants lost interest in further pursuing 1802.”
To date, J1802 struggles to provide consistent results when one set of linings is mounted on one test fixture (i.e., a custom-made, full-size, 16.5-by-7-inch air brake) and evaluated on nine different brake dynos. This is problematic because when labs use their own test fixtures, accuracy is reduced. Apparently, minor differences between test fixtures and identical lining samples have caused the results of tests to vary, by upwards of 30 percent.
“Enduring problems with J1802 certainly aren’t the fault of NHTSA’s lab,” says Larry Strawhorn, former ATA vice president of engineering. “In fact, the guys at NHTSA ran so many tests that their dyno broke.”
Strawhorn’s concern is that the industry preoccupation with RP 628A has diminished all residual interest in J1802.
“I’m very concerned that the glowing ember of SAE J1802 is about to wink out forever,” he laments.
Industry sources say that, at this time, J1802 is pretty much stuck in the mud. So, in the interim, the industry’s eyes are focused, intently, on RP 628A.
Passing the baton
Because of the slowness of J1802, a TMC task force organized by the S.6 Chassis Study Group, decided to join forces with SAE and fill the lining information gap with its own test program.
Essentially, an FMVSS-121 test was to be run on aftermarket linings. This was intended to document full compliance with OE torque, fade and recovery requirements. However, this was only for aftermarket lining suppliers that were willing to pay for the privilege.
So after three years of incubation, RP 628 was hatched in March of 1995. According to the task force, a dyno test would be run on three sets of each lining formulation to determine its compliance with FMVSS-121 and its average torque output at 40 psi braking pressure (the mid-point of an FMVSS-121 test) at 50 mph.
Aftermarket linings that fail to meet FMVSS-121 remain unlisted by TMC. Linings providing compliant test results can:
- Have their torque ratings and FMVSS-121 compliance formally accepted by SAE’s Brake Lining Performance Review Committee, which meets quarterly to review test data that’s presented anonymously. The performance committee reviews results of FMVSS-121 dynamometer performance tests conducted by qualified laboratories, in accordance with the test conditions described in RP 628A.
- Be publicly listed by TMC, in terms of their individual torque rating at 40 psi.
- Be retested every five years to remain listed. This takes into consideration some original torque rating variations, assuming batch-related variables.
This has enabled fleet owners to use TMC’s list to select a suitably rated replacement for OE linings. However, fleets must find out ratings on their own about equivalent, 40-psi torque ratings for their OE linings.
Over the years, the RP has been expanded to multiple axle loads and air chamber sizes. The next step is to include different size brakes, like 15- and 12-inch diameter brakes and even air disc brakes, says Jim Clark, CoC director, Bendix Spicer Foundation Brake. Clark heads the committee working on the lining test program. Clark was also the chairman of TMC’s original task force.
Bumpy Road
Initially, TMC proposed that lining makers submit three samples of each aftermarket product for testing on a single, precisely calibrated brake dyno and test fixture, to be monitored by SAE’s Performance Review Institute.
Using a single dyno, says Clark, would have leveled the playing field and produced good, comparative data, but lining manufacturers insisted on selecting their own brake dynos and fixtures.
It’s thought that lining makers made that decision in order to:
- Avoid a monumental log jam at one dyno;
- Avoid discrepancies between previously generated FMVSS-121 test data (distributed to fleets) and data to be generated by an alien dyno;
- Avoid generating such precise torque ratings that the secret origins of private-labeled linings would be revealed.
As damage control, TMC requires that every participant’s dyno periodically be certified, to SAE’s satisfaction, by an approved, independent engineering lab.
Among the certified dynos are: A unit operated by a brake manufacturer; two units that conduct FMVSS-121 tests for numerous brake-lining marketers; one unit at NHTSA’s Vehicle Research and Test Center; and two units operated by lining makers.
It was also agreed that a participating lining maker would provide for every batch of samples a notarized certification that performance was typical for that lining formulation. (That formulation, in turn, would not be changed without notification to SAE).
Armed with the game plan, most lining makers rushed home to initiate testing of their favorite products. However, amid the ceaseless whir of brake dynos, trouble developed.
At first, nearly 10 percent of the aftermarket linings that were submitted to SAE either failed to meet FMVSS-121 standards or were rejected due to unacceptable documentation, says Clark.
For instance, the committee won’t approve an average torque rating for a product where there’s too much variation between the results of the three tests. And all three tests must pass FMVSS-121.
The committee found several incidents where the test data submitted to the engineering committee wasn’t properly presented or the samples clearly failed FMVSS-121.
To stress accurate accounting, the committee raised the fee by $200 and used the extra revenue to have independent labs randomly select and retest some of the originally listed linings using an off-the-shelf sample purchased from a parts distributor.
In addition, TMC contributes significant funding to ensure that RP 628A’s data continues to be validated by tests of off-the-shelf samples. If the retested lining flunks, its maker is invited to “respond to the discrepancy” and is offered an opportunity to pay for testing another off-the-shelf sample. If the second sample also fails to measure up, the lining is subject to being de-listed. Or, assuming that it passes FMVSS-121, it can be re-listed with a revised torque rating.
The goal of the committee was to re-test close to 10 percent of the originally listed linings. Ultimately, however, all linings are subject to re-testing, because their certifications expire after five years.
Some Happy, Some Not
Some lining manufacturers would prefer to rewrite RP 628A from the beginning. The reason is that the RP was initially used as a powerful marketing tool by listed lining manufacturers. This, in turn, led some manufacturers to boldly promote their products as “approved” or “certified” or “quality assured” by TMC and SAE.
RP 628A’s intent frequently is misinterpreted. “A false sense of security” results from equating inclusion in TMC’s list with a formal SAE/TMC endorsement of superior overall performance, cautions Prakash Jain, ArvinMeritor’s director of joint venture business development.
Robert Braswell, TMC’s technical director, supports that view. “Guys actually were calling me for the Top Ten, highest torque linings, like RP 628A is some sort of Consumer Reports listing,” he says. “Misapply an aggressive lining, and you’ve got problems.”
It seems there has been much confusion about what the RP really provides. Some manufacturers claim that buying the most aggressive lining automatically provides the best lining. But in some cases, customers run the risk of over-braking their vehicles. So, we’re back to the assumption that maintaining compatibility with OE linings is the way to go. Also, inclusion in TMC’s list doesn’t guarantee that critical performance variables such as a lining’s wear rate, growth, effect on drum life, flexibility or structural integrity is good or bad.
In response to such concerns, the following cautions were added to RP 628A:
- The torque rating of an aftermarket lining should approximately match that of the original-equipment lining;
- By itself, a high torque rating doesn’t necessarily denote high quality or superior overall performance;
- Listed torque ratings only are approximate, due to test variables.
In 1987, ATA petitioned NHTSA to initiate rulemaking that would ensure accurate ratings and consistent performance. But when NHTSA missed its target of December 2001 to finish its work, Congress provided the administration funding to proceed. The Department of Transportation also has set aside money in its budget appropriations package for research into brake lining friction.
Now, Oak Ridge National Laboratories, Oak Ridge, Tenn., has partnered with NHTSA to conduct research into standards for aftermarket brake linings. ORNL will prepare a summary report and submit it to NHTSA. The goal is to establish a simple, useful rating system agreeable to all.
Ultimately, a new rating scheme will be introduced for industry review. A determination will be made as to whether the current testing procedures need to be modified, or if a completely new testing procedure is needed.
