Most people towing a caravan know two numbers off by heart: the caravan’s fully laden weight, and the 85% guideline. Plenty know their car’s maximum braked towing capacity as well.

Far fewer know what their rear axle is rated to carry, or how much of that is already used up before the caravan is anywhere near the tow ball.

That’s the gap this page is about, and it’s the reason the HitchGoblin^™ calculator has a second tab.

Why the rear axle is the one that catches people

Your outfit has to satisfy five separate limits at the same time, and you are only ever as legal as the tightest one:

• The caravan’s plated maximum nose weight
• The tow bar and tow ball rating
• The car handbook’s maximum nose weight
• Whatever payload the car has left
• The rear axle’s plated maximum

The first three are printed on plates and in handbooks, and people generally find them. The fourth is arithmetic most people manage. The fifth is invisible, because working it out needs a number that no document in the glovebox actually contains.

So it gets skipped. And it’s frequently the tightest of the five.

The lever nobody accounts for

Here is the part that surprises people.

The tow ball does not sit above the rear axle. It sits behind it, typically a metre or so back. That overhang turns the tow ball into a lever, pivoting on the rear axle.

The consequence is that a kilogram on the tow ball does not put a kilogram on the rear axle. It puts considerably more.

The arithmetic is straightforward. If your wheelbase is L and the distance from the rear wheel centre back to the tow ball is c, then:

Rear axle gain = nose weight × (1 + c ÷ L)

For a typical car with a 2,800 mm wheelbase and a 1,100 mm overhang, that multiplier is about 1.39. So 100 kg of nose weight puts roughly 139 kg on the rear axle.

If you have been assuming your 100 kg nose weight costs you 100 kg of rear axle capacity, you are out by nearly 40 kg. On a car where the gap between the empty rear axle load and the plated maximum is only 300 kg or so, that is a large fraction of your entire margin.

The same lever works at the other end. Roughly 39 kg comes off the front axle. Lighter steering and reduced front braking grip arrive well before anything is legally over, which is why the calculator reports front axle relief as well.

Luggage in the boot sits behind the rear axle too, though closer to it than the tow ball. It gets amplified by around 1.2. Rear seat passengers sit close to the axle and count at roughly 0.85 each.

Where the numbers actually come from

The calculator only asks for things you can genuinely lay hands on.

The statutory plate, on most other cars in the driver’s door jamb, carries four weight figures in a fixed order: maximum laden mass, maximum train mass, maximum front axle, maximum rear axle. The second and fourth are both used. Photograph the plate while you’re there.

The V5C gives O.1 for maximum braked trailer mass, F.1 for maximum permissible laden mass, and G for mass in service, which is the kerb weight including a 75 kg driver and a full tank. The 85% guideline is measured against G, not against F.1 — a distinction that trips up a surprising number of online calculators.

The caravan’s own plate gives its fully laden weight and its maximum nose weight.

A tape measure gives the last piece: horizontally from the centre of the rear wheel to the centre of the tow ball. Thirty seconds, no jacking, no scales. Wheelbase comes off any online specification sheet.

That leaves exactly one number unaccounted for, and it is the awkward one.

The number that isn’t written down anywhere

To know what the rear axle is carrying before you hitch up, you need to know how the car’s empty weight divides between its two axles.

Nothing on the V5C carries it. Nothing on the VIN plate carries it. Most handbooks don’t either.

Real cars vary more than people expect. A front-drive hatchback might carry 43% of its weight on the rear axle. A large premium 4×4 sits close to even. Within a single body style the spread is wide enough to matter: on a 1,900 kg car, five percentage points is 95 kg.

There are three reliable ways to deal with this, and the calculator supports all three.

Estimate it from the type of car

Pick the description that fits and the app uses a typical figure, deliberately chosen at the heavy end of the plausible range. That bias is the whole design. Because the assumption errs heavy, the tool will never tell you that you are fine when you are not. It may occasionally tell you the rear axle is your limiting factor when a more precise figure would have given you a little more room.

If you can’t place your car, take the “not sure” option. It assumes the least favourable figures in the table, so it fails safe.

Worth knowing: a Certificate of Conformity is easy to get hold of (manufacturers will usually email one free for a registered vehicle), and it does give the total mass in running order. But on every production-vehicle CoC we’ve checked, it does not break that figure down between the axles — only the maximum permitted load on each axle, which is a legal ceiling, not the actual empty-car split. It’s not a shortcut to the number you need here.

Check the handbook or manufacturer data sheet

Some handbooks and manufacturer technical data sheets list unladen axle loads directly, though this is inconsistent across makes — some publish it, many don’t. Worth five minutes of searching before you assume you need a weighbridge.

Weigh it

The gold standard. Take the car to a weighbridge unhitched, with just the driver aboard, a full tank of fuel, and nothing else loaded, and read the rear axle line off the ticket. Both caravan clubs also run weigh-ins at rallies, which is a much lower barrier than tracking down a public weighbridge, and there are mobile services.

Enter that figure and the app stops estimating entirely.

Whichever route you take, the input is a weight in kilograms — because that is what a weighbridge ticket actually gives you. You are never asked for a percentage, because nobody has one.

How this reaches the gauge

The calculator works out all five nose weight limits, takes the lowest, and tells you which one is binding. That single figure becomes the ceiling your HitchGoblin^™ Gauge colours against.

This matters more than it sounds. A gauge that only knows the caravan’s plated limit and the tow ball rating can show you a comfortable green reading while the rear axle limit has already been passed. Feeding the true ceiling through means the colour on the display reflects the constraint that actually applies to your outfit.

When the rear axle turns out to be the binding limit, the app says so, and tells you what taking a bag out of the boot buys you back in nose weight.

What to do if it says you’re over

Nothing about being over is a disaster, provided you find out in the driveway rather than at the roadside.

The levers available, in rough order of effectiveness: move weight out of the car boot and into the caravan over its own axle, take a passenger out of the back seat, reduce nose weight if there is room above the 5% minimum, and reconsider what is genuinely needed for the trip.

And if the app says you are over while running on an estimated figure, that is exactly the moment to get the real one. A weighbridge visit or a club weigh-in usually costs little and takes minutes, and it removes the guesswork entirely.

One thing that is not a solution

Air suspension and self-levelling systems do not raise what an axle may legally carry. They hold the car level while it carries exactly the same plated maximum.

A level car and a legal car are not the same thing.