Understand Axle Spacing Trade-offs

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.

And some regions of North America don't follow the federal Bridge Formula if you are not on the federal Interstate system.

Consequently, you end up with a patchwork of regulations that make it difficult to optimize a truck that operates across state lines. "Take the Southeast for example," cites Mike Cantwell, Volvo Trucks North America. "Vocational trucks in Georgia are strictly Bridge Formula 20,000-lb. front and 34,000-lb. rear with no lift axles. Alabama allows lift axles and Florida allows front ratings based on tire footprint, which could get you to 22,000 lbs. on the front."

If you're interested in hauling maximum payload and operate in a region that adheres to the Bridge Formula, the length of the wheelbase becomes a critical issue. "Bridge Formula states almost require maximum payload to maximum length or it is costing the owner revenue on every load," says Cantwell.

You really need to look at the regulations in the areas in which you operate and determine what trade-offs are acceptable. "A contractor that operates in several locales needs to take into account the types of roads he travels on, as well as his application and payload," says T.J. Reed, product manager, Western Star.

No matter what local regulations drive your truck configurations, you must adhere to a few basic guidelines. The Gross Axle Weight Rating (GAWR) determines the maximum load for each individual axle, and the GVWR must not exceed the capacity of the individual axle ratings.

Front axle selection
Axle-forward trucks provide a longer wheelbase when compared to an axle-back chassis with the same overall truck length; this allows maximum payload when operating under the Bridge Formula. The mix of axle-forward vs. axle-back trucks sold proves most contractors are bound by the formula.

"I thought we would reach 25% axle-back production due to its desirability in those states where the Bridge Formula is not being enforced," states Steve Ginter, vocational product manager, Mack Trucks, Inc. But apparently more contractors are using roads that require the Bridge Formula to be followed. "In 2006, we built five axle forwards for every axle back." He now states that the market will probably settle at 20% axle back and 80% axle forward.

There are other variables that can dictate whether you should choose an axle-forward or axle-back front axle configuration. "Many customers like the looks of the axle forward over the axle back," says Cantwell. "It is also better for the secondary or resale market to allow resale into different markets. The axle forward also allows the fuel tanks and battery box to be further forward."