What is tow ball weight?

Tow ball weight or tow ball mass or towing ball weight is, in the simplest terms, the amount of downward weight your trailer or caravan applies to the tow-ball of the vehicle towing it.

Different makes and models of vehicle will have different limits to how much this weight can be, but in all cases, the limit can not be legally exceeded. Nor should it be on a practical basis, as too much weight on the tow ball is asking for a combination that is dangerous to drive and could easily cause an accident.

The tow ball weight limit will have been calculated by the car's manufacturer but, as a rule of thumb in Australia, it's generally 10 per cent of the Aggregate Trailer Mass (ATM weight; with all the gear you plan to carry already loaded on the trailer) being towed (not to be confused with the trailer's GTM which is the load on its own axle when it's hitched up, or tare weight which is the weight of the trailer empty, not in its as-towed, loaded state).

Where tow ball weight differs from other rules is that the limit is a double-edged sword: Even if your towing vehicle is rated to tow 3000kg, you might imagine you have a 300kg tow ball weight limit. Not necessarily. See, if the trailer being towed weighs 2000kg, then the tow-ball limit (using that 10 per cent rule again) will only be 200kg. The rule also states that you must use the lower of the two figures if the trailer and tow vehicle have different individual tow ball weight limits.

So, even if you have a popular brand of tow vehicle such as a Toyota Prado or Mitsubishi Pajero with a 3000kg towing limit (or even a Ford Ranger with its huge 3500kg towing limit) but one of the modern camper-trailer caravans with low tow ball weight (weighing, say, 1000kg) your effective tow ball weight limit is 100kg. Which is exactly the same as for vehicles with lower outright towing limits such as the Toyota Kluger (2000kg) or the Mitsubishi Outlander SUV (1600kg). In the case of towing caravans with 100kg tow ball weight, you sometimes don't need as much tow vehicle as you might have imagined.

Vehicles will have their theoretical tow ball mass limit listed in the owner's manual, while trailers generally have the tow ball limit on their build plate, usually somewhere near the hitch.

The importance of getting this right is in terms of stability at speed. Too much tow ball weight and the tow vehicle can easily have its nose pointed at the sky, compromising braking and steering. Too little tow ball weight also suggests a trailer that is tail-heavy, and that can lead to a jack-knife or trailer-sway situation.

Sometimes, a raised tow ball is enough to drop the rear of the trailer, so don't assume it's incorrectly loaded, because it might be incorrectly hitched instead.

So, how to measure tow ball weight? The best way is to use tow ball weight scales which can be purchased from most camping and four-wheel-drive stores. They're also available at towing equipment specialists such as Hayman Reese, but general hardware retailers like Bunnings are less likely to have anything as specialised as these scales.

By placing this under the trailer's hitch and allowing the scales to take the weight, you'll be given a quick, accurate idea of how much weight you're dealing with. The other common tow ball weight calculator is a set of home bathroom scales, but this method is more involved than you might think and isn't as accurate as scales designed for the job.

In some cases, you'll find that you simply need a bigger, heavier tow car to make this all work, but there's also the opportunity to distribute the load around the trailer to increase or decrease the tow ball weight. In fact, repacking a caravan is the first trick on how to reduce tow ball weight.