Calculator 08 / Axle Weights
Axle Weight Distribution Calculator
Weight does not split evenly between axles — cargo loads each one in proportion to how far its centre of gravity sits from the front axle. Find how much weight lands on the front (steer) and rear (drive) axle of a straight truck based on cargo weight and where it sits.
How this calculator works
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This treats the truck as a simple beam supported at two points (the axles) — standard statics, the same principle taught in CDL load-securement training. Moving cargo toward the rear axle shifts more weight onto it and less onto the front axle, and vice versa. This model covers a single straight truck with one front and one rear axle; tractor-trailers with multiple axle groups need a more detailed multi-axle analysis.
Worked example: 8,000 lb on a 16-foot wheelbase
A straight truck with a 16 ft wheelbase, empty axle weights of 5,200 lb front and 4,800 lb rear, carrying 8,000 lb whose centre of gravity sits 10 ft behind the front axle:
- Share carried by the rear = 10 ÷ 16 = 0.625
- Cargo on the rear = 8,000 × 0.625 = 5,000 lb
- Cargo on the front = 8,000 − 5,000 = 3,000 lb
- Front axle total = 5,200 + 3,000 = 8,200 lb
- Rear axle total = 4,800 + 5,000 = 9,800 lb
- Gross = 18,000 lb
The load is a simple lever about the two axles. Slide the same 8,000 lb forward to 4 ft behind the front axle and the split inverts to 6,000 lb front and 2,000 lb rear — the gross weight never changes, and both axle weights change by 3,000 lb.
What moving the load actually does
Same truck, same 8,000 lb, only the centre of gravity moves. This is the table to look at when an axle is over and the gross is fine — the fix is almost always position, not weight.
| CG behind front axle | Cargo on front | Cargo on rear | Front axle | Rear axle |
|---|---|---|---|---|
| 2 ft | 7,000 | 1,000 | 12,200 | 5,800 |
| 4 ft | 6,000 | 2,000 | 11,200 | 6,800 |
| 6 ft | 5,000 | 3,000 | 10,200 | 7,800 |
| 8 ft | 4,000 | 4,000 | 9,200 | 8,800 |
| 10 ft | 3,000 | 5,000 | 8,200 | 9,800 |
| 12 ft | 2,000 | 6,000 | 7,200 | 10,800 |
| 14 ft | 1,000 | 7,000 | 6,200 | 11,800 |
| 16 ft (over the rear axle) | 0 | 8,000 | 5,200 | 12,800 |
Note the two ends. Load directly over an axle puts all of itself on that axle and nothing on the other. Load placed behind the rear axle — a rear overhang, which this calculator will happily model if you enter a distance greater than the wheelbase — goes further still and lifts weight off the steer axle. That is real physics, not a bug, and it is a genuinely dangerous condition: an unloaded steer axle means light, vague steering and reduced braking exactly where you need it most.
Federal weight limits
These apply to the Interstate System in the United States. <strong>The calculator does not check them</strong> — it compares your axle weights only against the ratings you type in. State limits differ, are frequently lower, and often govern on non-Interstate routes.
| Limit | Maximum | Notes |
|---|---|---|
| Single axle | 20,000 lb | Includes the steer axle; tire ratings often bind first |
| Tandem axle group | 34,000 lb | Two axles spaced more than 40 in and not more than 96 in apart |
| Gross vehicle weight | 80,000 lb | The whole combination |
| Bridge Formula | varies | Caps any group of axles by how far apart they are spread |
The Bridge Formula
W = 500 × ( LN ÷ (N − 1) + 12N + 36 ), where L is the distance in feet between the outer axles of a group and N is the number of axles in it. It exists to stop concentrated loads damaging bridge spans, and it is the limit people forget — a combination can satisfy every individual axle limit and still be illegal because the axles are too close together.
| Axles (N) | Spread (L) | Formula allows | Which is |
|---|---|---|---|
| 2 | 4 ft | 34,000 lb | A standard tandem — where the 34,000 lb limit comes from |
| 2 | 8 ft | 38,000 lb | Capped at 34,000 lb anyway by the tandem rule |
| 3 | 10 ft | 43,500 lb | A tight tridem |
| 3 | 20 ft | 51,000 lb | Spreading the same three axles buys 7,500 lb |
| 4 | 30 ft | 62,000 lb | Tractor drive tandem plus trailer tandem |
| 5 | 51 ft | 79,875 lb | A typical 5-axle tractor-semitrailer, just inside 80,000 |
| 6 | 60 ft | 80,000 lb | Formula gives 90,000 — the gross limit caps it |
The fifth row is why a standard tractor-semitrailer is built the way it is: at a 51 ft outer spread the formula allows 79,875 lb, which is as close to the 80,000 lb gross limit as five axles can get. Sliding a trailer tandem forward shortens L and lowers what the formula permits — which is why moving the tandems is a legal adjustment and not just a ride-quality one.
Where this goes wrong
- Measuring the CG from the wrong place. The distance is from the front axle, not from the front bumper or the front of the box. Getting the datum wrong shifts thousands of pounds between axles.
- Treating a mixed load as one point. A single centre of gravity is a fair model for a uniform load. For several distinct heavy items, work out each one's position and add them, or the answer is fiction.
- Assuming legal gross means legal axles. Being under 80,000 lb says nothing about whether the tandem is over 34,000 lb. Axle limits are what weigh stations enforce.
- Forgetting the Bridge Formula entirely. Every individual axle can pass and the group still fail.
- Applying this to a tractor-trailer. This is a two-axle static model for a straight truck. An articulated combination transfers load through the fifth wheel and needs a different calculation.
- Ignoring fuel, driver and equipment. They are part of the curb weights you enter, and a full fuel tank on a heavy truck is several hundred pounds that moves as it burns off.
- Stopping at the arithmetic. A certified scale is the only weight an enforcement officer accepts, and it is the only one that catches what this model leaves out.
Where these numbers come from
Axle weight is where a legal gross weight still fails a scale. The limits below are per axle and per axle group, and they are what a weigh station applies.
- 23 CFR Part 658 — Truck Size and Weight — Single-axle, tandem-axle and bridge-formula limits, in the regulation itself.
- FHWA Freight Size and Weight Program — The bridge formula explained, plus where state limits diverge from the federal ones.
Why does cargo position matter so much?
Weight distributes between axles in proportion to how close the load's center of gravity is to each axle — cargo directly over the rear axle puts nearly all its weight there, while cargo near the front axle puts most of its weight on the front instead.
What if my front or rear axle comes back over its rating?
Reposition the cargo's center of gravity toward the axle with spare capacity, or reduce total load — running over an axle's rated capacity is both a safety and a legal weight violation.
Estimates only. This is a simplified two-axle model — always verify actual axle weights on a certified scale before a DOT weigh station, especially for loads near a rating limit.