F/CMVSS 121: Air Brake Systems

Brakes are arguably the most important safety system in a vehicle. Federal/Canada Motor Vehicle Safety Standard (F/CMVSS) 121 establishes performance requirements for all air-braked vehicles, such as trucks, buses, tractors, trailers, etc. (some exceptions apply). This includes service brakes, parking and emergency brake functions, and performance requirements for subsystems and components. What you need to know Following are key requirements of this standard to note when upfitting air-braked vehicles and/or performing chassis modifications affecting the brake system. This isn’t a complete list of the standard’s requirements, and many additional specifics accompany these items. Stopping distance — The standard specifies loading, speed and other conditions in which vehicles must stop within specific maximum distances. Parking brakes, grade holding — With all parking brakes applied, the vehicle must remain stationary while facing uphill, and then again facing downhill, on a 20% grade when loaded to gross vehicle weight rating, as well as other loading requirements and specifications. Stability and control during braking — Trucks, tractors and buses must remain within a 12-foot-wide lane through a curved roadway under various specified conditions. Reservoir volumes — The combined volume of all service and supply reservoirs must be at least 12 times the combined volume of all service brake chambers for trucks and buses. For trailers, the total volume of each service reservoir shall be at least eight times the combined volume of all service brake chambers serviced by that reservoir. Air compressors — Air compressors must be of sufficient capacity to increase air pressure in supply and service reservoirs from 85 psi to 100 psi when the engine is operating at the vehicle manufacturer’s maximum recommended rpm within a time, in seconds, determined by the quotient (Actual reservoir capacity × 25) / Required reservoir capacity. Brake actuation time — This test requires the air pressure in each brake chamber to reach 60 psi in not more than 0.45 seconds for trucks and buses. Vehicles designed to tow other air-braked vehicles and trailers have different actuation times and conditions to achieve under these tests. Brake release time — This test requires air pressure in each brake chamber to fall to 5 psi in not more than 0.55 seconds for trucks and buses. Vehicles designed to tow other air-braked vehicles and trailers have different actuation times and conditions to achieve under these tests. Vehicles designed to tow another air-braked vehicle have additional requirements that include provisions to receive an antilock malfunction signal from the towed vehicle, as well as a towing vehicle protection system. Given these tests and their performance intentions, modifications such as adding axles, changing wheelbase or adding tow packages to trucks not originally equipped can directly or indirectly affect any number of these system and subsystem tests. For example, adding axles (and consequently the additional brake chambers that are included) will reduce the ratio of combined reservoir-to-chamber volumes, so additional reservoir capacity may be required. Other potential downstream effects include the ability of the original air compressor to keep up with extra demand, as well as increases to brake actuation and release timing. Further, there may be spill-over effect by the same modifications to other related F/CMVSS, requiring analysis beyond the brake system, such as F/CMVSS 106: Brake Hoses F/CMVSS 119: New Pneumatic Tires for Motor Vehicles with a GVWR of More than 4,536 Kilograms (10,000 Pounds) and Motorcycles F/CMVSS 120: Tire Selection and Rims and Motor Home/Recreation Vehicle Trailer Load Carrying Capacity Information for Motor Vehicles with a GVWR of More than 4,536 Kilograms (10,000 Pounds) F/CMVSS 136: Electronic Stability Control Systems for Heavy Vehicles As always, consult the OEM incomplete vehicle document and other technical materials for guidance to comply with this and all other applicable F/CMVSS.

F/CMVSS 121: Air Brake Systems

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