February 9, 2010 by markarndt
The US Federal Motor Vehicle Safety Standard (FMVSS) 120 covers tires and rims for motor vehicle other than passenger cars. The standard is applicable to multipurpose passenger vehicles, trucks, buses, trailers and motor cycles. The core of the standard attempts to assure that the tires and rims that are sold with a vehicle, and that the tires and rims that are specified on a tire label affixed to a vehicle, are capable of supporting the manufacturer’s specified gross axle load.
A manufacture can sell vehicles weighing more than 10,000 lbs with tires and rims different from its tire label specified tires, rims and inflation pressures. Such a vehicle must meet the requirements of FMVSS 120 with all sets of tires and rims listed on the label and mounted to the vehicle. The tires and rims on a vehicle are evaluated with information from the sidewall of the tire, including the maximum load at the sidewall inflation pressure. The tires and rims listed on a tire label are evaluated using a source like the Tire and Rim Association Yearbook to determine the load carrying capacity of each tire on the label at the recommended pressure. The evaluation process requires that the sum of load carrying capacity for all tires on an axle equal or exceed the manufacture’s front and rear Gross Axle Weight Rating (GAWR).
There are several problems that are presented by this standard including:
There is no assurance that the tire inflation pressure listed on the tire label will be compatible with different tires in-use on a vehicle. In other words, the inflation pressure listed on a tire label may be too low for the in-use tire given the load and service provided.
The requirement is that the combined tire carrying capacity of an axle must equal or exceed an axle’s weight rating, however in some vehicles large side to side weight differences exist rendering a tire or set of tires insufficient for the carrying load.
The standard does not assure compatibility for the variety of tire to rim and mixed tire combinations that might occur given differences between the tire label and the tires sold with a vehicle. For example, the tires listed on a tire label may not properly fit a wider rim appropriate for larger tires sold with a vehicle.