The Meritor ES22525 heavy-duty air disc brake.
The Meritor ES22525 heavy-duty air disc brake.

Ready-mix producers are on notice: Starting in 2011, stopping distances for ready-mix trucks must be greatly reduced.

The National Highway Traffic Safety Administration (NHTSA) issued its long-awaited revisions to Federal Motor Vehicle Safety Standard FMVSS 121 in the July 27 issue of the Federal Register. The final rule calls for a 30% reduction in stopping distances.

While written around heavy-duty tractors, the stopping distances apply to all air-braked vehicles and hydraulic-braked vehicles greater than 10,000 pounds gross vehicle weight rating. Manufacturers of chassis for ready-mix trucks, dump trucks, and pumps will use the same braking systems as tractor-trailers.

Current regulations call for a tractor pulling an unbraked test trailer to stop from 60 mph in 355 feet. Virtually all of today's tractors exceed the requirement by 15% or more, stopping within 302 feet. The new stopping distance will be 250 feet.

Today's brakes, 15x4-inch drums on steer axles, 16-1/2x7-inch drums on drive axles, cannot achieve the new target. The best stopping distances today are between 255 and 292 feet, varying with truck configuration.

Larger front brakes

Tests conducted by brake system suppliers and fleets and reported at TMC meetings, indicate that at least front brakes will need to be larger to achieve the 30% reduction. Air disc brakes (ADBs) on the steer axle coupled with standard drive axle drum brakes can approach the 250-foot target.

Front drum brakes 16-1/2x5-1/2-inch coupled with normal 16-1/2x7-inch-wide drums on drive axles can meet the requirement, but not after a moderate number of stops. Even more effective in consistently stopping test trucks from 60 mph are wider 16-1/2x8-5/8-inch drums on drive wheels with the larger 16-1/2x5-1/2-inch-wide front brakes.

Today's brake configurations stop typical test trucks in 330 feet. Just replacing steer-axle brakes with ADBs lowers that to 301 feet. ADBs all around will stop those trucks in just 234 feet. Cars typically stop from 60 mph in less than 130 feet.

Brakes work by converting kinetic energy to heat. Linings or pads made of high-friction materials rub brake drums or discs. 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, releasing gasses that lubricate the contact area. That reduces friction and creates what we feel as brake fade, or “the brakes going away.”