How to read weight figures and examples
- Ratings are maximum limits, not measurements of your loaded setup. Confirm the exact variant and approved ratings.
- Measure loaded coupling weight. A percentage or the difference between ATM and GTM ratings does not establish today's coupling weight.
- An added load changes both axle reactions according to its position. A change is not a total axle load; use measured starting axle weights and add each load once.
- Combination mass is unhitched loaded vehicle plus total loaded trailer, or the sum of all coupled axle readings. Count coupling mass once.
- Examples and AI suggestions are planning aids, not verified measurements or compliance clearance. Unknown figures require measurement; active weight-distribution hitches and gooseneck arrangements need their own assessment.
The lever effect
A towball load is applied behind the tow vehicle's rear axle. That position matters.
The downward force at the towball creates a turning effect around the rear axle. The front axle can lose load while the rear axle must support both the towball download and the load transferred away from the front axle. As a result, rear-axle load may rise by more than the towball download.
Front axle, rear axle and towball relationship
For a simplified static model, three measurements matter:
- Towball download
- Wheelbase, measured between the axle centres
- Horizontal distance from the rear axle centre to the towball
A longer rear overhang increases the leverage effect. A longer wheelbase reduces it. The relationship is a planning model rather than a substitute for scale readings.
Worked example
Using a 300 kg towball download, a 3,220 mm wheelbase and a 1,200 mm rear-axle-to-towball distance:
- Estimated front-axle change: -112 kg
- Estimated rear-axle change: +412 kg
- Net change across both axles: +300 kg
The figures balance because the vehicle gains the 300 kg vertical coupling load overall, but its distribution changes between the axles.
Why this matters in a touring vehicle
Rear-mounted accessories may already consume rear-axle capacity before a caravan is connected. Common examples include a canopy, drawers, spare wheels, tools, batteries, auxiliary fuel and cargo stored behind the rear axle.
Towball leverage is then added to that existing rear loading. A vehicle may therefore remain below its overall GVM while exceeding its rear-axle rating.
What a weight-distribution system changes
A correctly selected and configured weight-distribution system can redistribute some load through the coupled combination. It does not create extra legal capacity or increase manufacturer ratings.
Follow the vehicle, towbar, coupling and weight-distribution-system manufacturers' instructions. Some vehicles or hitch arrangements may restrict or prohibit their use.
Calculation versus measurement
Use the free calculator to test how geometry changes the static estimate. Then obtain measured axle weights for the loaded travel configuration.
Measurements are particularly important when margins are small, the vehicle carries substantial rear accessories, the towball download is high, or a modification changes vehicle mass or geometry.
Decision checklist
- Confirm the exact towball download
- Measure wheelbase and rear overhang consistently
- Check the vehicle's front and rear axle ratings
- Include accessories, occupants, fuel and cargo
- Confirm vehicle, towbar and coupling limits
- Weigh the final setup in travel condition