Bridge formula for weights

Special transport, semi-trailer truck with many axles and wheels to transport very heavy goods.
Special transport, semi-trailer truck with many axles and wheels to transport very heavy goods.

For reasons unknown, each state and province have determined their own weight and wheelbase dimensions. In a perfect world, these requirements would be universal across North America, but unfortunately, they aren’t, so we need to be vigilant regarding axle and gross weights, and axle spacing. As a rule of thumb, having the centre of the rear axles between 37 ft and 41 ft from the trailer king pin means your axle spacing is legal, except in California.  Axle weight allowances also vary between jurisdictions, especially in the western US states, so before leaving on a trip, be sure to review the requirements for all jurisdictions, and set your equipment up to be following the most restrictive conditions.  One area where drivers can find themselves in violation is with the US Federal Bridge Formula. Trying to use this to determine if you’re legal or not almost requires a university degree and for that reason, hardly anybody uses it, just following the 37-41 ft rule on a 53 ft trailer, and 99.999% of the time, you’re in compliance with the bridge formula.

Finding all the requirements can be a daunting and time-consuming task. State DOT websites can really help, but some of them are hard to navigate and they tend to use technical terms rather than plain English. One good resource for all provincial and state regulations is the Rand McNally Motor Carriers Atlas. In addition to all the pertinent regulatory information, it also lists low cleanses, restricted and designated truck routes. It’s also a good resource to have handy, because unlike Google maps, Apple maps, or the best GPS system available, it never goes offline for any reason.

Below is the federal bridge formula, with an explanation of the variables:

  • W = the maximum weight in pounds that can be carried on a group of two or more axles to the nearest 500 pounds (230 kg).
  • L = spacing in feet between the outer axles of any two or more consecutive axles.
  • N = number of axles being considered
Don Taylor has been a professional driver since March 1985.  In 1994 he made the jump to driving tractor trailers, and has accumulated over 3.5 million miles, including over 4 years of driving turnpike doubles in Manitoba, Saskatchewan and Alberta.  He is currently hauling flat decks across North America.