Tow-ball weight: how to actually measure it (bathroom scales won't cut it)

I had a bloke ring me from the side of the Newell last year, hazards on, ute squatting at the back end like it had been sat on. Tow-ball weight, he reckoned, was "about right, maybe a bit heavy." Turned out he was 60 kg over what his tow vehicle's tow-ball rating allowed, and the rear coils were doing a job they were never built for. That's the trouble with tow-ball weight. Almost nobody measures it properly, and almost everybody thinks they've got a rough idea.

Twenty years fixing rigs before I started writing about the tracks they broke down on taught me one thing above everything else: tow-ball weight is the number that quietly wrecks tow vehicles. Not GVM, not ATM — tow-ball weight, because it's the figure people guess at instead of measure, and it's the one that loads the rear axle, compresses the springs, lightens the steering and makes a van want to sway at 100 km/h.

What tow-ball weight actually is

Tow-ball weight (or tow-ball mass, TBM) is the download the coupling puts on your tow vehicle's tow bar when the van is hitched and loaded for travel. It's not a fixed number. It changes every time you load the van differently — water tanks full versus empty, jerry cans on the A-frame, the annex kit shoved forward or back in the boot.

The rule of thumb the industry has used for decades is 10 percent of the trailer's actual loaded mass, though most off-road van manufacturers will quote a workable range of 8 to 12 percent. Too light and the van can start to sway behind the tow vehicle at speed, particularly in crosswinds or when trucks pass. Too heavy and you're overloading the rear axle of the tow vehicle, which affects steering weight, headlight aim and braking balance — and can push you over the tow vehicle's tow-ball rating even when you're nowhere near your GCM.

That last point catches people out constantly. Your tow vehicle has its own tow-ball rating, separate from the tow bar's coupling rating and separate from the van's ATM. On a lot of dual-cab utes it's sitting around 350 kg, some higher, some lower depending on model and GVM upgrade status. Ford, Toyota and Isuzu all publish it in the vehicle handbook. If you don't know your number, ring the dealer or check the compliance plate — don't assume.

Why bathroom scales don't tell you the truth

I've seen every home-brand method going. Bathroom scales under the jockey wheel. A mate standing on scales holding the A-frame. None of it accounts for the geometry properly, and most of it reads low because the coupling height isn't matched to actual towing height, which changes the leverage on the number you're trying to measure.

The only way to get a genuine figure is one of three methods. A tow-ball weight scale — a proper dedicated unit, not improvised — set up at the correct hitch height with the van level and loaded exactly as it will be for travel, water in the tanks, gear in the lockers, gas bottles on. Alternatively, a set of weighbridge readings: weigh the tow vehicle and van together hitched, then weigh the van alone on its jockey wheel unhitched, and the difference in the tow vehicle's rear axle reading between the two gives you TBM — though this needs a public weighbridge and a bit of arithmetic. Or the simplest for most tourers: a certified caravan weighing service, several of which operate at caravan and camping shows and via mobile units, that'll give you a full weight report including TBM, GVM and axle loads for around $60 to $100.

I'd argue every off-road van owner should do a full weighbridge check at least once, fully loaded as they actually travel — not as the dealer set it up on delivery. The number on the compliance plate is meaningless once you've added a second battery, an awning, water in both tanks and three weeks of tinned food.

What happens when tow-ball weight is wrong

Too light, under about 6-7 percent, and you're in classic sway territory. Vans start to hunt behind the tow vehicle, particularly when a road train goes past or a gusty crosswind hits on the open Hume or across the Nullarbor. A weight distribution hitch can help manage this in some cases, but it doesn't fix a fundamentally wrong load — we've covered when you actually need one and when you're wasting your money in our weight distribution hitches explained piece.

Too heavy, and the symptoms show up in the tow vehicle first. Rear springs sag, front end gets light, steering feels vague, and headlights point at the tree tops instead of the road. I've pulled apart enough rear ends to know a consistently overloaded tow-ball mass accelerates wear on rear shocks and bushes, and on some utes it'll chew through leaf springs years before they should be due. It also messes with your GVM. Tow-ball weight is a real, physical load transferred onto your tow vehicle's rear axle — it counts against your GVM whether you remember that or not, and plenty of rigs are over GVM the moment the van's hitched, even if the tow vehicle looks fine unloaded.

Getting the load right before you even hitch up

The fix starts inside the van, not at the tow bar. Heavy gear — batteries, tools, water containers, the generator — belongs low and as close to the axle line as practical. Stuff crammed into the front boot or nose cone adds tow-ball weight fast, which is fine until you're 40 kg over what your tow vehicle can handle. We've gone through the layout mistakes that cost people both storage and stability in off-road van storage: the layout mistakes that cost you space and sleep, and tow-ball weight is one of the reasons those mistakes matter beyond just convenience.

Water is the sneaky one. A single 95-litre tank up front, filled before you leave, adds close to 95 kg right where it does the most to shift your tow-ball figure. Some tourers fill front tanks only when they need the water and run rear tanks for daily use — worth thinking about if your van has the option.

Checking it on the road, not just at home

Tow-ball weight isn't a set-and-forget number. It shifts every time you restock at a supermarket in Roma or fill jerry cans before the Oodnadatta. A tow-ball weight scale is cheap insurance — the decent dedicated units run somewhere in the $50-$150 bracket — and worth keeping in a side locker if you're doing serious kilometres. Fifteen minutes at a rest stop before a long leg beats finding out the hard way on a descent with a loaded rig and a van that's started to wander.

Before you even get to the point of checking tow-ball weight on the road, the hitch-up itself needs to be right every single time — no shortcuts, same routine, same checks. Our coupling up checklist covers the driveway disasters that happen when that routine slips, and tow-ball weight sits right alongside coupling height as part of getting the geometry correct before you roll.

A number worth respecting

Every serious weight figure on your rig — ATM, GVM, GCM — gets talked about at caravan shows and in forums until people's eyes glaze over. We've broken those down properly in towing weights explained: ATM, GVM, GCM, tow ball and payload. But tow-ball weight is the one you can actually control day to day, hitch to hitch, load to load. It doesn't need a compliance certificate or an engineer's report to fix. It needs a scale, five minutes, and the discipline to check it again after you've repacked.

The bloke on the Newell got himself sorted with a call to a mobile weighing service in Dubbo two days later. Turned out he'd had close to 90 kg of tools and recovery gear sitting in the front boot the whole trip. Moved half of it to the rear tunnel boot, checked the figure again, and the ute sat level for the first time in months. Cost him nothing but an afternoon.