Truck Axle Weight Distribution Calculator

How it works

Kingpin load = Σ(weight × distance ahead of trailer tandem) ÷ (kingpin-to-tandem); trailer axles = trailer + cargo − kingpin; steer = bobtail steer + kingpin × (fifth-wheel offset ÷ wheelbase); drives = the rest

A tractor-trailer is two levers. On the trailer, the load and the trailer’s own weight balance between the trailer tandem and the kingpin: the closer the cargo centre of gravity sits to the kingpin, the more goes onto the tractor. On the tractor, the kingpin load lands mostly on the drive axles, with a share reaching the steer axle in proportion to how far the fifth wheel sits ahead of the drive tandem. US federal law caps a standard tandem at 34,000 lb and the whole rig at 80,000 lb, with the steer axle governed by its tyre rating (typically 12,000–12,500 lb); the bridge formula and state rules can be tighter. Sliding the trailer tandems forward moves weight off the drives onto the trailer axles (roughly 250–400 lb per hole), sliding the fifth wheel forward moves weight to the steer.

Worked example

An 18,500 lb tractor, 14,500 lb trailer and 44,000 lb of cargo centred 22 ft behind the kingpin on a 40 ft kingpin-to-tandem: kingpin 27,050 lb; steer ≈ 13,250 lb, drives ≈ 32,300 lb, trailer ≈ 31,450 lb — 77,000 GVW, but the steer is over 12,000 lb, so slide the fifth wheel back or move the load rearward a foot or two.

Frequently asked questions