Why position matters more than weight
A truck rests on two supports. Anything you set on the deck is shared between them in inverse proportion to how far it sits from each, which is the same statics that governs a plank on two sawhorses. Put a load a distance d behind the front axle on a wheelbase L and the rear axle takes W x d / L. The front axle takes the rest.
The consequence that surprises people is what happens when d is greater than L — when the load is behind the rear axle, out over the tail. Then W x d / L is larger than W, the rear axle carries more than the whole item, and the front axle carries a negative amount. The tail is levering the nose up.
The default load, worked through
| Item | Weight | Behind front axle | Front | Rear |
|---|---|---|---|---|
| General load, spread 40 to 220 in | 3,200 lb | 130 in (its middle) | +863 lb | +2,337 lb |
| Heavy item 1 | 700 lb | 60 in | +464 lb | +236 lb |
| Heavy item 2 | 500 lb | 120 in | +163 lb | +337 lb |
| Heavy item 3 | 400 lb | 240 in | -139 lb | +539 lb |
| Cab | 350 lb | 0 in | +350 lb | 0 lb |
On a 178 inch wheelbase the third item sits 62 inches past the rear axle, so it puts 539 pounds on the rear and takes 139 pounds off the front. Four hundred pounds of load has moved 139 pounds away from the wheels that steer. Slide the same item forward to 120 inches behind the front axle and it contributes 130 pounds front and 270 rear instead: the front axle gains 269 pounds and the rear sheds the same 269, from one item and one decision about where to set it down.
What this model does and does not do
It treats the truck as a rigid beam on two axles and every load as acting at one point. The general load is handled as a uniform spread, which is exactly equivalent to a point load at its midpoint for axle purposes, and that equivalence is real rather than an approximation. What the model has no view on is side to side balance, the height of the load, or how the box itself is mounted. It also assumes the empty axle weights you entered are correct, and on a rental truck the only way to know them is a scale.
Tandem rear axles are treated as one support at the centre of the group, which is how the total on the rear is normally quoted anyway. For a trailer rather than a truck the geometry is different — the coupler is a third support — and the trailer tongue weight calculator and the trailer axle load calculator handle that case. For a pickup the towing and payload calculator works from the ratings side.
Where the load figures come from
Nobody weighs the boxes. The usual route is volume: the truck size calculator gives cubic feet from the rooms, and the moving load weight calculator turns cubic feet into pounds by category, which is the input this page wants. Densities vary enormously — books run around 35 pounds a cubic foot while a sofa is nearer 3 — so a load that is mostly a home library and a load that is mostly cushions can differ by a factor of ten in the same space. That is also why the heavy items are entered separately here: it is the few dense objects that decide the balance, not the general mass of boxes.
Questions people ask
Should heavy things go at the front of a moving truck?
Over and just ahead of the rear axle is where they load the axles most evenly, and pushed right to the front bulkhead is where they load the front axle hardest. Both of those are far better than the tail. The reason is the lever: an item behind the rear axle subtracts from the front axle rather than adding to it, and the front axle is the one that steers and does most of the braking. Enter your own distances above and you can see which arrangement produces which split.
How much weight does a load on the tail take off the front axle?
The item weight times the distance past the rear axle, divided by the wheelbase. Four hundred pounds sitting 62 inches behind the rear axle of a 178 inch wheelbase takes 139 pounds off the front. Doubling the overhang doubles that figure, which is why the last few feet of a long box are the part that changes the handling most per pound.
Do I need to weigh a rental truck?
A certified scale is the only thing that turns any of this into fact. Everything above is arithmetic on numbers you typed, including the empty axle weights, and those are the numbers most likely to be wrong. Whether weighing is required of you is a question for the rental company and for your state, not for a calculator. If the computed figures land anywhere near the ratings on your paperwork, the scale is the next step rather than a second opinion from a website.
What is the difference between GVWR and an axle rating?
GVWR is a figure for the whole vehicle and the axle ratings are figures for each axle separately. A load can sit under the GVWR while one axle is over its own rating, because the total says nothing about the split — that is the entire point of working out the split. Both figures come off your paperwork or the label on the vehicle, and this page only repeats what you entered beside what it computed.
Does the same arithmetic work for a cargo van?
Yes, with the caveat that a van has a short wheelbase and a long rear overhang, so the same item moved the same distance changes the split more. Measure hub centre to hub centre for the wheelbase and measure your load distances back from the front hub. The awkward part on a van is that the empty axle weights are harder to find than on a box truck, and without them the result is a shape rather than a number.