Heavy Duty Trucking

SEP 2014

The Fleet Business Authority

Issue link: https://heavydutytrucking.epubxp.com/i/382090

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Circle 152 on Reader Action Card www.truckinginfo.com SEPTEMBER 2014 • HDT 105 T he reduced stopping distance requirements mandated by the National Highway Traffic Safety Administration forced truck manu- facturers to put steer-axle brakes back into the game after decades of sitting on the sidelines. Faced with the need to whack nearly 100 feet from the distance re- quired to stop a test truck (the test rig pulls an un-braked trailer loaded so the tractor's axles carry a total weight of 59,600 pounds), and drive- axle brakes that were already at their torque limit, truck makers had to power up the steer-axle brakes. Disc brakes were an option, but they remain unpopular with U.S. fleets. Upgrading the front brakes was the only other option. "They upsized the front brakes from a typical 15-by-4-inch brake to 16.5-by-5-inch brake, which provid- ed a lot of additional torque," says Jeff Geist, director of Engineering at Stemco. "Some OEs went to a brake lining with a higher coeffi- cient of friction, and in some cases, the air chambers on the steer-axle brakes were upsized from a typical 20-square-inch chamber to a 24 to put even more power to the brake." All that extra torque on the front axle did succeed in bringing the truck to a stop within a shorter distance – but remember, the test is conducted with a 100-psi brake application. In normal service, truck brakes seldom see much more than 15 psi to 20 psi application pressure. Complications arise when the brakes aren't used in the way they were tested for RSD. "Under normal driving condi- tions, the brake linings may not get hot enough to be operating in their 'sweet spot,'" says Frank Gilboy, product manager of aftermarket brake shoes at Bendix Spicer Foun- dation Brake. "Front brakes are seeing more of their share of the work, but it may not be enough to condition the lining the way it needs to be conditioned." How the RSD rules changed the wheel-end Wider brake blocks and larger brake chambers deliver more brake torque, but are underutilized in day-to-day conditions.

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