AxleLoad 53 guides

California’s 40-foot kingpin-to-axle rule

Updated October 18, 2026

In California, a 53-foot trailer can be illegal even when it is under weight, if its axles are slid too far back. This is the 40-foot rule, also called KPRA (kingpin to rear axle).

What the rule says

For a semi-trailer longer than 48 feet, the distance from the center of the kingpin to the center of the rear axle group must not exceed 40 feet. For a tandem, measure to the midpoint between the two axles.

The rule is about maneuverability: the farther back the axles, the more the trailer cuts corners and swings into other lanes in town. Other states have similar rules, often around 41 feet, sometimes measured differently. Check every state on your route.

How to check it

  1. Find the kingpin center under the trailer nose, usually about 36 inches from the front.
  2. Measure to the midpoint between the two tandem axles.
  3. Many trailers have a decal or mark on the slider rail showing the 40-foot legal position.

How it changes your axle weights

Sliding the axles forward to meet 40 feet shortens the trailer’s lever: the trailer axles carry more, and the drives carry less. With a heavy load, the trailer tandem can then go over 34,000 lb even though the combination is under 80,000 lb.

You then need to move pallets forward or lighten the load. The guide Balancing a 53-foot trailer explains how.

In AxleLoad 53, choose United States (federal) and turn on the 40 ft kingpin-to-axle rule. The dimension line under the trailer turns red when the axles are too far back, and Best slider only suggests legal positions.
Open the AxleLoad 53 calculator →Free, in English, French and Spanish. Works on a phone in the cab.

More guides

Axle weight limits in Canada and the United StatesSteer, tandem, tridem, gross weight and the Bridge Formula: the limits that apply to a tractor with a 53-foot semi-trailer.Balancing a 53-foot trailer: slider, fifth wheel and palletsHow pallet weight splits between the kingpin and the trailer axles, and how many pounds each slider hole or fifth-wheel notch moves.Quebec spring thaw: zones, dates and load reductionsWhy allowed loads drop every spring in Quebec, how the thaw zones work and how to plan your loads.

This guide is for information only. Legal limits change and depend on the road, the season and your permits: always check with the competent authority. Certified scale weights govern.