Tow ball mass: what it is, and why "10%" is a guideline

Tow ball mass is the downward load your trailer puts on the tow ball. Here is what the 10% guideline really means, and which ratings actually limit it.

What tow ball mass is

Tow ball mass (TBM) is the mass the trailer presses down through its coupling onto the tow vehicle. You will also see it called tow ball weight, ball load, tow ball download or tow hitch download. They all mean the same thing.

It is an actual figure. It changes every time you load the trailer differently. When the trailer is hitched, its total mass splits two ways: part goes onto the tow ball, and the rest goes onto the trailer's own axles. That is why the trailer has two ratings, ATM for the total and GTM for the axles (see ATM vs GTM).

Why it matters

Tow ball mass does two jobs at once, and both need it in the right range.

Too little, or a drawbar that wants to lift, makes the combination unstable. The WA Department of Transport says tow ball download above or below the right range reduces the stability of the combination. A trailer with too little weight at the front is more prone to sway.

Too much overloads the back of the tow vehicle. Tow ball mass:

counts toward the vehicle's GVM. WA's guide says so explicitly.
sits behind the rear axle, so it loads the rear axle by more than its own weight and lifts some weight off the front axle. See front and rear axle limits.
must stay within the ratings of the vehicle, the towbar and the coupling

The 10% guideline, and what the sources say

You will often read that tow ball mass should be about 10% of the trailer's mass. The authorities put it in slightly different ways:

The WA Department of Transport says tow hitch download, as a guide, should be 8–10% of the ATM, up to the towbar's load limit. It also notes that some trailers, European designs for example, may be built for a lower figure.
RACQ says that as a general guide, unless otherwise indicated, a ball load of 10% of the trailer's ATM should not be exceeded. That makes 10% a ceiling, not a target.
RACQ also notes that vehicle tow-ball ratings are often around 10% of the vehicle's maximum towing mass, but not always. Some are lower and some are higher.

These do not quite agree, so treat 10% as a starting guide, not a rule. It gives you a sensible range to aim for. It is not a legal limit. The limits you must stay within are the ratings below, and the trailer and vehicle manufacturers' own advice takes priority over any rule of thumb.

The ratings that actually limit it

Tow ball mass must stay within all of these, and the lowest one is the one that applies to you:

The vehicle's tow ball rating, from the owner's manual or towing specifications for your variant. It may be given in kilograms or as a percentage.
The towbar's rating. Towbars must be marked with their load capacity. The WA guide says this applies to towbars made after 1 July 1988; Queensland says passenger-vehicle towbars made after 1 January 1992. If yours is older or unmarked, ask a towbar specialist. Transport for NSW points to the coupling manufacturer's handbook.
The coupling and tow ball markings, which state the mass they are rated for
Anything the trailer manufacturer specifies for your trailer

Remember that tow ball mass also uses part of the vehicle's GVM and rear axle limit. A tow ball mass inside its own rating can still push the vehicle over GVM or its rear axle limit if the vehicle is heavily loaded.

What changes your tow ball mass

Where the load sits in the trailer. Mass ahead of the trailer's axles adds to tow ball mass. Mass behind them takes it away. A drawbar toolbox, jerry cans or gas bottles up front add a lot.
Water tanks. A full front tank and an empty rear one give a very different reading from the reverse.
Weight distribution hitches. These spread some of the ball load to the vehicle's front axle and back to the trailer. They do not reduce the mass. Queensland and WA both warn never to use one to fix a badly loaded trailer, excessive download or an overloaded rear axle.

Measuring it

Transport for NSW suggests using a tow ball scale. You can also work it out from weighbridge readings. See how to weigh your rig, including why a reading under the jockey wheel is not the same as the load at the coupling.

Model your own rig

LoadSavvy's Load check models your tow ball mass as you place each load in the trailer, and shows it against the vehicle's rating, GVM and the rear axle limit. It is a planning tool, not a certified measurement. A scale or weighbridge measures your rig.

Model your rig in the Load check →