Heavy Truck Research 

BACKGROUND: Federal Motor Vehicle Safety Standards (FMVSS) 105 and 121 currently require medium and heavy duty vehicles (vehicles with a Gross Vehicle Weight Rating (GVWR) of 10,001 pounds to 26,000 pounds are medium vehicles, vehicless with a GVWR of more than 26,000 pounds are heavy vehicles) to stop, on a high coefficient of friction pavement and with properly working brakes, in the distances shown in Table 1. In comparison, FMVSS 135 basically requires light vehicles (vehicles with a GVWR of 10,000 pounds or less except for motorcycles) to stop, also on a high coefficient of friction pavement and with properly working brakes, in 215 feet (the actual requirement is 230 feet from 62 mph (100 kph)). These standards also set required failed system/emergency brake stopping distances (not shown in this document). Again, the required failed system/emergency brake stopping distances are substantially longer for medium and heavy duty vehicles than for light vehicles.

Current Stopping Distance Requirements for Medium and Heavy Duty Vehicles

Vehicle Type	Stopping Distance from 60 mph, Empty	Stopping Distance from 60 mph, Loaded
Buses (including School Buses)	280 ft	280 ft
Single Unit Trucks (except Buses) Weight > 10,000 lbs 8,000 - 9,999 lbs 7716 lbs - 7,999 lbs	335 ft 242 ft 216 ft	310 ft 216 ft 216 ft
Truck-Tractors	335 ft	n/a
Truck-Tractors with Unbraked Control Trailer	n/a	355 ft
Semi-Trailers, Trailers, and Converter Dollies	Dynamometer Requirement	Dynamometer Requirement

The original, 1970, proposal for FMVSS 121 would have required Buses, Single Unit Trucks, and empty Truck-Tractors to meet a 60 mph stopping distance requirement of 216 feet. (This is identical to the FMVSS 135 light vehicle requirement except for metric system round-off, i.e., the Europeans wanted to have a required stopping distance from 100 kph of 70 meters, not 70.4 meters.) Due to comments and objections from the trucking industry as to their inability, at that time, to meet the 216 feet requirement for all types of medium and heavy duty vehicles, the stopping distance requirements were relaxed to those in force today. However, we believe that due to recent technological advances in medium and heavy duty vehicle brakes, specifically in the area of disk brakes and electronic brake systems (EBS), required stopping distances for these vehicles can be made substantially shorter.

PROGRAM OBJECTIVES: The goal of this program is to perform all of the research necessary to allow Safety Performance Standards to upgrade FMVSS 105 and 121. The upgraded versions of FMVSS 105 and 121 would include:

• Substantially shorter required stopping distances for buses, single unit trucks, and empty truck-tractors on a high coefficient of friction pavement from 60 mph. Revised requirements would be developed both for vehicles with properly functioning brakes and for vehicles with failed systems/ emergency brakes.
  
  
• Either increased brake retardation force (this force is measured on a brake dynamometer) or stopping distance requirements for air-braked semi-trailers, trailers, and converter dollies towed by truck tractors. Again, requirements would be developed both for units with properly functioning brakes and for units with failed systems/emergency brakes.
  To accomplish the above, this program will focus on medium and heavy duty brake system performance improvements that can be accomplished with upgraded foundation brake systems using various axle configurations for disc brakes, and/or higher output s-cam brakes.