Fleet Management News & Business Info | Commercial Carrier Journal
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56 COMMERCIAL CARRIER JOURNAL | JULY 2015 EQUIPMENT: AIR DISC BRAKES Air disc brake acceptance has been a gradual process, with OEMs ini- tially introducing them on the steer axle. "Fleets typically try them out before making the decision to con- vert to them," says Randy Petresh, vice president of technical services at Haldex. "As a result, air disc brake market share penetration increases every year." To give credit where it is due, design tweaks on drum brake systems have allowed them to deliver improved braking performance well within NHTSA's guidelines. And while air disc brakes do offer signifi cantly enhanced braking performance in all weather conditions, drum brake enhancements have made it easier for fl eets to put off switching to air disc systems and avoid paying the current acquisition cost upcharge they still command. Drum brakes today feature larger chambers and 24-square-inch dia- phragms with a long 3-inch stroke; this compares to a 20-square-inch diaphragm with a 2½-inch stroke on older brakes. Also, modern drum brakes have precision camshaft journals, an improvement of the brake's geom- etry that helps reduce variation and improve overall braking performance, as well as greater width for increased braking surface, improved heat dis- sipation, reduced fade, longer life and lower cost per mile. "All of these improvements are topped off by additional lining and improved friction materials for longer service life," Oreskovic says. In many cases, however, convention- al drum brakes alone may not be suf- fi cient to meet the new requirements. "Air disc brakes and reinforced drum brakes have been shown to be the best solution for meeting more aggressive stopping distance regula- tions, with air disc brakes generally recognized as being superior to drum brakes in terms of braking perfor- mance, safety and driver comfort," says Brent Fitch, director of sales for Wabco. Also, air discs offer drivers a braking feel that is more like that of a passenger car, which is one reason why many truck drivers appreciate them, he says. Driver satisfaction with air disc brakes is a natural result of their design simplicity, says Joseph Kay, Meritor North America's director of engineering for brakes. "When you compare the physics of stopping a vehicle with drum brakes compared to disc brakes, it's no wonder drivers feel safer driving a truck equipped with air disc brakes," Kay says. The inherent design and geometry of how air discs work – pads clamp- ing down on a disc, as opposed to brake shoes expanding outward to make contact with a drum – allow for much smoother brake application across all driving conditions, he says. "In addition, disc brakes apply torque more evenly and consistently than drum brakes do. This gives the driver better 'feel' on the wheel when they hit the brakes and a feeling of enhanced vehicle control and safety." Petresh says that although drum brakes remain the standard and will continue to hold the larger market share going forward, reduced stopping distance regulations have sped up the air disc brake acceptance process. "Clearly, the performance aspect is the biggest advantage to the air disc brakes," he says. "Because of their need for safety margins, that was the big reason bulk haulers were early adapt- ers of air disc brakes. But there are other air disc brake benefi ts that have The weight benefit afforded by air disc brakes depends upon the overall wheel-end components chosen as part of the package. The 2013 stopping distance regulations require unloaded heavy trucks to stop 30 percent sooner than under the old braking guidelines, meaning a Class 8 tractor must come to a full and com- plete stop from 60 mph in 235 feet. 1 Standard 6x4 tractors Less than or equal to 59,600 lbs. 250 feet 355 feet Aug. 1, 2011 2 2-axle and 6x4 Severe Service Tractors Above 59,600 lbs. and less than or equal to 70,000 lbs. 250 feet 355 feet Aug. 1, 2013 Severe Service 6x4 Tractors Above 70,000 lbs. 310 feet Tractors with 4 or more axles Less than or equal to 85,000 lbs. 250 feet Tractors with 4 or more axles Above 85,000 lbs. 310 feet