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The job of any brake, remember, is to convert motion into heat and then shed that heat away from the vehicle. Increasing the area of the lining that contacts the drum gives you the ability to generate more braking torque with less in-stop fade, and the most logical place for a bigger brake is up front. NHTSA set the current stoppingdistance standard (mirrored in Canadian regulations) using 15-x-4-in S-cam brakes and Type 20 air chambers on the front axle. In writing its proposed new rule, NHTSA tested two beefier steer-axle brake packages you could spec today if you wanted to: 16.5-x-6-in and 16.5-x-5-in sizes. In concert with standard S-cam brakes on the drive axles, the bigger steer-axle brakes helped produce an average stopping distance of 269 feet, a 25-percent improvement on the existing standard. It’s important to keep in mind that these bigger brakes not only are wider, they’re larger in diameter than the 15-in brakes they would replace. “Wide brake packages tend to shed heat better and wear longer; today’s wider shoes give you almost a 25-percent increase in the wearable lining life,” says Paul Johnston, senior director of the North American foundation brake business unit at ArvinMeritor. “But wider brakes alone won’t necessarily put out enough torque to stop the vehicle in a shorter distance.” For that, you need a shoe and lining with a greater diameter. A 16.5-x-5-in drum brake provides 12 percent more wearable lining volume than a 15-x-4-in brake. There’s more lining for the drum to grab, giving a boost to the brake’s coefficient of friction. “No one is certain what the final stopping distance requirement will be,” Johnston says, “but I’m confident that when all is said and done a larger-diameter front brake with a more aggressive lining and larger brake chamber will allow most three-axle tractors to comply using an all-drum solution.” HEAVY CHALLENGES The shorter stopping distance requirements in NHTSA’s proposal would not be a snap to achieve for everyone. They raise important questions for brake engineers, vehicle manufacturers, and certain segments of the trucking industry to tackle in the coming months: In Gear 1What about trucks that already use 16.5-x-5-in brakes on the steer axle? Roughly 10 percent of air-braked tractors made today have high-output drum brakes on the steer axle, most of them for heavy hauling, refuse fleets, and other severe-service applications. “A 4x2 tractor or a tractor involved in pulling very heavy weights probably already has a larger drum brake on the steer axle,” Johnston says. “Vehicles in these types of service may need disc brakes either at the steer position or at all positions to meet the standards in a final rulemaking.” 2Will larger brakes require a more robust suspension and other components? “When the steer axle has to accommodate more brake torque during a hard stop, what you can get is tremendous wrapup on what is typically a light-duty spring,” says Clark. “To prevent that, what you’ll see is an anti-wrap up type of suspension— something that’s better equipped to manage the stresses that may come with a more aggressive brake.” 3Should truck owners expect longer service life from larger brakes? That depends. All the basic principles of brake maintenance apply to bigger drum brakes on the steer axle, says Tom Golden, manager of technical services for BrakePro, which supplies a broad range of both OE and aftermarket linings for a range of applications. “The effectiveness and longevity of any drum brake depends on one thing: lining contact with the drum when the brakes are applied,” Golden says. “A bigger, wider brake still has to be properly maintained in order to ensure adequate contact, and the friction material still should meet the load and axle requirements and the application or use of the truck.” If one wheel is doing too much work, or not enough, the result will be premature wear and poor performance no matter what size brake you use. 4Will NHTSA put forth a firm stopping distance benchmark, instead of the range of improved braking performance in its proposal? Johnston says yes. “I think it’s clear there will be one minimum performance standard, one stopping-distance number instead of a range,” Johnston says. Imagine a market where MARCH 2006 59
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Page 57: In Gear EQUIPMENT NEWS, REVIEWS, AN
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Page 67 and 68: THE AIR IS FREE: Several companies
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