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The Bridge Formula often drives the front axle configuration (axle forward or axle back) and the rear axle spacings of your vocational truck fleet. Congress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. It establishes the maximum weight any set number of axles may carry on the Interstate highway system.

Under the Bridge Formula, the gross weight limits are determined by the length of the vehicle, the number of axles it has and how they are spaced. The formula is: W= 500 [(LN/N-1) + 12N + 36]

In this equation, W is the maximum weight (lbs.) that can be carried on a group of two or more axles to the nearest 500 lbs. L is the spacing in feet between the outer axles of any two or more consecutive axles. N is the number of axles being considered. All combinations of axles on a vehicle must be within the bridge formula limits.

In addition to the Bridge Formula, federal law limits the weight on any single axle to 20,000 lbs., and axles closer than 96 in. apart are limited to 34,000 lbs.

In some states, gross weight and axle weights may be higher due to "grandfather" rights. This is where things can get complicated. When the federal Bridge Formula limits were set, states were allowed to keep, or grandfather, weight limits that were higher for trucks operating on state roadways.