In two key instances, FMVSS 121 testing does not line up with new RSD standards. The new RSD rules require brakes to achieve a deceleration force of .634 Gs – a significant increase from the previous force of .37 Gs. This higher requirement is not reflected in the FMVSS 121 dynamometer test.
In addition, the laboratory environment of the FMVSS 121 test is unable to account for the weight transfer experienced in a braking vehicle, particularly the addition of weight to the steer axle. This is a worrisome oversight, as the TMC's aftermarket brake lining classification report states: "Brakes relined with certain aftermarket materials can have reduced braking output, cause a shift of work to brakes on other axles, and reduce the overall stopping capability of the vehicle."
The same TMC report bluntly states that the SAE J661 testing of small lining samples "is not considered accurate in determining performance on a full-size brake," making it abundantly clear that this particular procedure is no longer a reliable barometer of how a brake should perform in today's RSD environment.
Comparing Linings – A 96' Difference
To demonstrate how roadway safety may be jeopardized by the choice of replacement friction, Bendix compared the stopping distance performance of various linings on high-performance drum brakes. The company measured the 60 mph stopping distance of an RSD-compliant vehicle with high performance brakes and linings. Bendix then replaced the friction with multiple non-high performance Original Equipment and aftermarket materials that had passed the FMVSS 121 dyno test, but were not suitable for RSD compliance.
With nothing else changed, the stopping distance increased from 215 feet using the OEM friction to 311 feet with the worst performing aftermarket replacement friction – a stunning 45 percent decrease in performance. That 96-foot difference in stopping distance – a total of five passenger car lengths – is a stark illustration of the roadway safety at stake.
Also of note, the stopping distance for the high performance friction is 35 feet shorter than the RSD requirement of 250 feet, demonstrating the ability of high performance friction to far exceed RSD standards.
Improving Road Safety
Engineering teams from Bendix Spicer Foundation Brake and other OEMs invest heavily in designing and specifying brake packages that factor in a wide array of variables, such as axle load rating, wheelbase, and tire size. To produce a comprehensive brake package that places safety top of mind, engineers also specify the most effective friction materials for use in those brakes. Low-cost aftermarket friction is not engineered to the same standard and thus not suitable for consideration as Original Equipment on heavy trucks and tractors built after August 2011.
The safety concerns associated with substandard aftermarket friction are real. When the time comes to replace brake linings, fleet owners and operators are best served by relining with the same friction originally spec'd on the vehicle. By doing so, they can ensure RSD-level braking performance and enhance highway safety.
Bendix Commercial Vehicle Systems, a member of the Knorr-Bremse Group, develops and supplies leading-edge active safety technologies, air brake charging, and control systems and components under the...
Conclusions drawn from Bendix stopping-distance testing and analysis.