Two pictures explain most of what people get wrong about towing weights. Here they are, with the arithmetic that goes with them.

Why 100 kg on the tow ball is not 100 kg on the axle

The tow ball does not sit above the rear axle. It sits behind it, typically about a metre back. That overhang turns the tow ball into a lever, and the rear wheels are the fulcrum.

The diagram above shows the result: roughly 39 kg comes off the front axle for every 139 kg the rear axle gains. Nothing may be legally over at that point, but the steering has gone light and the front tyres have less grip to brake with. This is the trade nobody mentions: nose weight buys you stability against snaking, and pays for it in front axle load — which is why the calculator reports front axle relief alongside rear axle load.

For the full derivation behind that 1.39 multiplier, see The limit nobody checks: your tow car’s rear axle.

What to do when the rear axle is your tightest limit

The most effective fix costs nothing. Move weight out of the car boot and into the caravan, over the caravan’s own axle.

The bag weighs the same in both positions. In the boot it sits behind the rear axle, so it is amplified by roughly 1.2 and costs you 24 kg of rear axle capacity. Over the caravan’s axle it costs the tow car nothing at all.

The three axle figures balance: the front gains 4 kg, the rear sheds 24, the caravan takes 20. Nothing appears or disappears; it simply moves. The small front axle gain is real, because boot weight behind the rear axle was levering a little load off the front, and taking it out puts that back.

The badge above the tow ball is a different kind of number. Those 24 kg of freed rear axle capacity convert into about 17 kg of extra permitted nose weight, because the conversion runs back through the same 1.39 lever. It is permission, not weight that appears by itself, and it is only available if the caravan’s plated nose limit and the tow ball rating both have room. On many outfits one of those binds first and the 17 kg stays theoretical.

One caveat if you do take up that extra allowance: adding nose weight lifts load off the front axle, roughly 0.39 kg for every kilogram added. Taking up the full 17 kg would cost about 7 kg at the front — more than the 4 kg the bag move gave back. You are buying towing stability and a legal rear axle, not front grip.

One warning

This only works if the weight goes over the caravan’s axle. Put the same bag in the front locker and you have moved it onto the nose, where it lands back on the car’s rear axle amplified by 1.39 — worse than leaving it in the boot.

For the fuller list of what else helps when the rear axle is your tightest limit, see The limit nobody checks: your tow car’s rear axle.