Heavy Duty Trucking

SEP 2014

The Fleet Business Authority

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big brakes on the front axle, but they are 'underutilized' in normal conditions," explains Frank Gilboy, product man- ager of aftermarket brake shoes at Bendix Spicer Founda- tion Brake. "If there's insufficient heat to condition the brake properly, they will glaze and they will develop some noise and chatter." It's nothing to worry about from a performance perspec- tive, they say, but an unintended and perhaps irritating consequence of the rule change. "Fleets are seeing linings glaze up because they aren't being worked hard enough," says Jim Reis, vice president and general manager of Stemco's Brake Products division. "That's causing noise and vibration in as little as three months after the truck goes into service. Typically front brakes ran the life of the vehicle, if not for rust-jacking. Now we find they need attention because of the glaz- ing problem. It's not so much a matter of [truck owners] wearing the brakes out, but just being dissatisfied with the day-to-day performance." We have also heard reports of faster-than-normal brake drum wear, possibly related to the more aggressive brake linings used on steer axles. Chad Plank, president of Webb Wheel Products, says he began getting calls from customers about a year ago wondering why their brake drums were wearing so quickly. Since the steer-axle drums bore the symptoms of the problem, he got the calls. "Our investigation revealed no issues with our drums," he says. "Nothing had changed on our end, same material, same production process, same everything. We concluded it was due to the more aggressive linings OEs were using to meet RSD (reduced stopping distance rules)." According to Reis, before the new rules, it wasn't uncommon for a brake drum to outlast the linings two or three to one. That may not be the case going forward on steer axles. "Something usually has to give," he says. "You're changing energy into heat, and with the more aggressive friction, I can see where a full cast drum potentially could wear out faster." The balance of power Steer axle brakes are now doing proportionally more work than before. The physical changes required to meet the full-pressure application stopping distance require- ments left the front brakes in a position that even in low-pressure applications, they take some of the workload from the drive-axle brakes. Joe Kay, Meritor's director of engineering for brakes, North America, says different OEMs use different air system philosophies that can make a difference in wear characteristics. "Valve crack pressures, timing, and in some cases the plumbing of the system itself can create differences in wear rates," he says. "Most stops are at 15-20 psi, which means 15-20% of the brake system's capacity. If there is a 3-4 psi difference in application valve crack pressures from axle to axle, we can see some differences in wear based on timing and valve crack pressure." With the greater emphasis placed on the steer axle, it's now the drive-axle brakes that are just going along for the ride, so to speak. It used to be that steer-axle brakes would last 600,000-700,000 miles because they were so lightly used. "The rear linings may have actually been 'dumbed down' a little to prevent ABS events in the panic stop situation," observes Jeff Geist, director of engineering at Stemco. "I've seen them take some performance out of the rears so that they don't lock up and add to the stopping distance through ABS brake modulation. ABS will give you a controlled stop, but it won't be the shortest stop. It's going to increase the stopping distance." Kay says Meritor did make some changes to its drive- axle brake friction to account for the lower demand. "Pre-RSD, drive-axle brakes were capable of locking up the wheels," he says. "That's significant because once the wheel is locked, you can't add any more torque to it no matter what. So, because those brakes would be doing less work, we came up with some new friction formulas 104 HDT • SEPTEMBER 2014 www.truckinginfo.com Circle 201 on Reader Action Card Tires&Wheels

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