By Paul Abelson
senior technical editor
Back in the day, front brakes on tractor-trailers were optional. Conventional wisdom was that the brakes would lock up, resulting in loss of steering control. With 16 other wheels being braked, stopping distances and control would be in balance.
Research later proved that false. Weight transfer during stopping put added load on steer tires, preventing smaller 15-inch brakes from locking. Conversely, the research found front brakes were effective and necessary in stopping trucks.
When Federal Motor Vehicle Safety Standard FMVSS 121 became law in 1975, it mandated front brakes along with the first (unsuccessful) generation of anti-lock brakes. It also mandated maximum stopping distances for tractors pulling test trailers. Currently, the required distance to stop from 60 mph is 355 feet. With front brakes mandated, virtually all of today’s tractors exceed the requirement by 15 percent or more, stopping within an average of 302 feet.
The new required stopping distance will be 250 feet. NHTSA wants trucks’ stopping ability to be as close to that of cars as possible. Typically, cars stop from 60 mph in less than 130 feet.
This past July, the National Highway Traffic Safety Administration issued its long-awaited revisions to FMVSS 121, to become effective August 2011 on 2012 model year tractors. Straight trucks and specialty vehicles will be included in subsequent years.
The advance notice of proposed rulemaking first appeared in 2002, and the notice of proposed rulemaking was issued in December 2005. It indicated stopping distances would be reduced by 20 percent to 30 percent, but the exact figure was not specified pending industry input. The final rule calls for a
30 percent reduction.
Test indicates that today’s brakes, 15-by-4-inch drums on steer axles and 16 1/2-by-7-inch drums on drive axles, cannot achieve the new target. The best stopping distances today are as high as 292 feet, which varies with truck configuration.
Industry tests conducted by brake system suppliers and fleets, as reported at TMC meetings, indicate that higher steer axle torque will need to be larger to achieve the 250-ft. requirement. Larger front drum brakes (16 1/2-by-5-1/2-inch) coupled with normal (16 1/2-by-7-inch) drums on drive axles can just meet the requirement, for at least a few stops.
Even more consistent stopping distances can be achieved using wider 16 1/2-by-8 5/8-inch drums on drive wheels with the larger 16 1/2-by-5 1/2-inch wide steer axle brakes. Air disc brakes on the steer axle coupled with standard drive axle drum brakes easily stop in less than 250 feet. ADBs all around will stop trucks in just 234 feet, even allowing for a large margin of error.
Because about half the states allow trucks to operate at 70 mph or more, and only six have truck speed limits of 60 or less, NHTSA considered stopping distance requirements from 75 mph. That would have required a complete restructuring of FMVSS 121 and its established test procedures. If higher speeds had been adopted, air disc brakes would have been needed all around, especially if the standard had required multiple stops within a short period of time.
Multiple stops would have more closely simulated actual driving demands and the need for fade resistance. With nine states currently allowing trucks to operate at 75 mph, consumer safety groups favored higher speed and multi-stop requirements.
Brake size and type are not the only solutions to shortening stopping distances. Brakes are systems that use friction to generate heat. Linings or pads are forced against rotating drums or discs. Technology is improving the linings’ and pads’ ability to generate heat and dissipate it to the metallic mass of the brake assembly.
Transfer film technology deposits lining material on drums, and the cohesive effect of similar surfaces rubbing together increases the brakes’ coefficients of friction. Metal fibers in modern friction materials help transfer heat.
Because linings are an important part of the braking process, you should always replace brake shoes with OEM or equivalent products. Avoid bargains, especially unbranded products. They may be counterfeits. Also, check torque ratings for brake linings published in TMC Recommended Practice RP 628A. FF linings, for example, have been measured to have as much as 14 percent variation between manufacturers. Always replace brake components on both ends of each axle to prevent side-to-side variations that lead to lateral instability.
Brakes work by converting the truck’s energy of motion to heat. High-friction materials rub brake drums or discs, generating heat. The drum or disc conducts the heat away from the lining or pad, to be dissipated to the air flowing over the brake. If heat builds up more rapidly than it can be drawn away, the friction material in the shoe or pad cooks. That releases gases, which lubricate the contact area. That reduces friction and creates what we feel as brake fade, or “the brakes going away.” When it happens, friction is reduced and the truck does not slow as it should.
Disc brakes are better able to dissipate heat and to release any gases – although according to WABCO, they weigh about the same as high-performance drum brakes. They are inherently more resistant to fade than drum brakes. Air disc brakes have been available in the U.S. since the 1980s, but have been used in limited applications. Today, three companies supply them: Bendix, MeritorWABCO and Haldex. They are available as original equipment through all truck manufacturers.
The augmented brakes will add to the cost of new trucks. When NHTSA issued the final rule, it estimated that enhanced drum brakes would add $85 for each steer axle and $65 per drive axle for a three-axle truck. The agency’s estimated increase in prices for disc brakes is $500 per axle regardless of position.
Freightliner indicated that the incremental cost for enhanced drum brakes would be $222, while disc brakes would add $963 to a two-axle rig and $1,627 for three axles, according to comments made to NHTSA during deliberations.
The added costs for these brakes will be offset by lower maintenance costs. Most application pressures will remain normal, with the exception of panic stops, and braking forces will be distributed through larger brakes. That lessens unit stresses, decreasing brake wear and lengthening brake life.
One of the greatest benefits expected from these shorter stopping distances will be the reduction of impact velocity in rear-end, truck into car crashes. In some cases, according to the final rule, “reduced stopping distances will stop a crash from occurring.” In most cases, the ability to slow a truck more rapidly will result in fewer or less severe injuries and less property damage. LL