This is Part II of a three-part series on reduced stopping distance brake products. To read Part I, please CLICK HERE.
The story of today’s RSD regulations began in 2005, when NHTSA first expressed an interest in shortening the braking distances for commercial vehicles.
Citing a braking distance then nearly double most passenger cars, NHTSA suggested an amendment to FMVSS 121 “to reduce the required stopping distances for the loaded and unloaded service brake distances and emergency brake distances for truck tractors by 20 to 30 percent.”
The proposal was followed by more than three years of extensive testing and industry research.
It was during this time that the Heavy Duty Brake Manufacturer’s Council became involved, says Randy Petresh, vice president, technical services at Haldex.
“[HDBMC] had regularly scheduled meetings with NHTSA while the standard was being developed,” he says. “A lot of time went in to determining how much of a reduction [in stopping distance] they really wanted.”
In July 2009 NHTSA announced its new stopping distance requirement. This reduced the maximum stopping distance for loaded three-axle tractors from 355 ft. to 250 ft., and unloaded tractors to 235 ft.
Given lead time before Phase I of the FMVSS 121 amendment would take affect (Aug. 1, 2011), brake system and friction manufacturers immediately began testing concepts that would meet the RSD requirements.
The two sides worked together to hit the 2011 deadline.
“There was cooperation between the brake manufacturers and friction suppliers from the initial concepts all the way to the fleets,” says John Thompson, sales manager, commercial vehicles, NAFTA at TMD Friction.
Higher friction brake linings were identified as early on as 2007 by the brake manufacturers as part of a package (also including larger diameter and wider front brakes, as well as brake chamber size increases) that would meet RSD requirements, says Tom Rogers, senior applications engineer, Abex commercial vehicle friction at Federal-Mogul Motorparts.
Previous to RSD, most friction materials were classified as NAO or non-asbestos organics. In addition to new NAO materials, Rogers says Federal Mogul also developed semi-metallic friction solutions, as well as combination NAO/ semi-met solutions.
Each solution was capable of producing the torque required to stop the vehicle in their particular applications, but the semi-metallic and combo solutions also were better able to resist heat and fade at the higher loadings – helping to reduce stopping distance, Rogers says.
“RSD drive axle brake linings show improved speed spread effectiveness by reducing the in-stop fade characteristic, seen in regular friction materials,” add the Fras-le duo of Alexandre Casaril, development engineer, and Bernard de Jong, project engineer. “RSD drive axle brake linings are considerably less susceptible to thermal conditioning in order to obtain the nominal designed performance level, resulting in high stable, stopping power since first brake applications.”
Brake manufacturers also expanded conventional steer axle drum brake sizes from 15 in. x 4 in. to 16.5 in. x 5 in. and rear axle brake widths from 7 in. to 8 in. for most OEM applications.
Bauer says the different sizes were required as the amount of braking done by each axle changed.
“A larger brake was required to handle the amount of torque that would be required for such a dynamic stop,” says Petresh. “The 15 in. x 4 in. brake had been the primary steer axle for more than 20 years, and it had always been overloaded.”
The smaller brake’s preference over the 16.5 in. x 5 in. brakes was the result of its lighter weight and maneuverability, says Gary Ganaway, director of marketing and global customer solutions at Bendix Spicer Foundation Brake, LLC. “Before RSD, [16.5 in. x 5 in.] brakes were purely optional and were only adopted by fleets looking for longer brake life,” he says.
Larger brakes also helped meet RSD regulations by extending system life and aiding in heat transfer.
“RSD materials have to be more aggressive to reach their stopping distance requirements, and that causes the entire braking system to run a little bit hotter,” says Johnathon Capps, vice president, engineering at Webb Wheel Aftermarket. “Without any changes, that excess heat would cause the system to wear at a much faster rate.”
Rear axle brakes also were slightly tweaked to reduce fade and extend useful life, says Thompson.