Front and rear axle limits: why where you put the load matters
Your vehicle can be under GVM and still overload an axle. Here is how axle limits work, and why load behind the rear axle counts for more than its weight.
Each axle has its own limit
As well as a GVM for the whole vehicle, manufacturers set a maximum load for each axle. The Australian Design Rules call it the gross axle load rating: the most the manufacturer allows on that axle. The WA Department of Transport lists front and rear axle capacity among the ratings to check before towing. You will find them in the owner's manual or specifications for your variant, and sometimes on the vehicle's placard.
The two axle limits usually add up to more than the GVM. That gives some room to distribute the load. It also means you can be under GVM and still overload one axle. Rear axles are the ones that usually go over when towing.
Tyres have load ratings too. Transport for NSW includes tyre carrying capacity in the limits a caravan combination must stay within.
The lever effect
Picture the vehicle as a plank resting on two supports, the front axle and the rear axle. A load placed between the axles is shared between them. The nearer it is to one axle, the more of it that axle carries. Put 100 kg exactly halfway and each axle takes 50 kg. Put it right over the rear axle and the rear axle takes all of it.
A load behind the rear axle behaves differently. The rear axle becomes a pivot. The load pushes the back of the vehicle down and lifts the front. The rear axle ends up carrying more than the load itself, and the front axle carries less than it did before.
Invented figures, for illustration only: a vehicle with a 3.0 m wheelbase and its tow ball 1.2 m behind the rear axle, carrying 250 kg of tow ball mass.
| Change | |
|---|---|
| Rear axle | +350 kg (250 × 4.2 ÷ 3.0) |
| Front axle | −100 kg (250 × 1.2 ÷ 3.0) |
| Total (GVM) | +250 kg |
The vehicle's total mass rose by exactly the tow ball mass. The rear axle rose by 40% more than that. This is why a rig can look fine against GVM and still be over on the rear axle.
The same applies to anything hung off the back: a rear wheel carrier, a bike rack, a heavy drawer system at the rear of a long tub.
Why a lighter front axle is a problem
Taking weight off the front axle is not a free bonus. The WA Department of Transport describes what happens when the rear sags and the front rises: steering and front braking performance can suffer, and headlights aim too high. Queensland's towing guidance makes the same point from the other direction: loading toward the rear reduces the load on the front axle.
Position matters in the trailer too
A trailer is the same kind of plank, resting on the tow ball at one end and its axle group at the other. Where you put a load decides how it splits between the two:
- Load ahead of the axles adds to tow ball mass
- Load over the axles goes almost entirely onto the trailer's axles
- Load behind the axles takes mass off the tow ball, and can lift the drawbar
Invented figures: a trailer whose axles are 4.0 m behind the tow ball. Move a 100 kg load from 1 m ahead of the axles to 1 m behind them, and the tow ball mass drops by about 50 kg. For the vehicle above, a 50 kg tow ball change moves the rear axle by about 70 kg and the front by about 20 kg. One box moved inside the caravan changes both axles of the car.
This is why Transport for NSW advises packing the heaviest items low and centred over the caravan's axles, and the WA guide says to keep heavy items close to the trailer's axle group and as low as possible.
What not to do
- Don't rely on a weight distribution hitch to fix it. WA and Queensland both say not to use one to correct an overloaded rear axle, excessive tow ball mass or a badly loaded trailer.
- Don't guess from total mass alone. Axle loads need axle-by-axle readings. See how to weigh your rig.
Your state road authority sets the rules on overloading and enforcement where you drive. Check with them for what applies to you.
Model your own rig
LoadSavvy's Load check places each load at its real position on the vehicle and trailer, then shows the front axle, rear axle, tow ball, GVM and GCM as you move it. It is a model for planning, not a measurement. A weighbridge gives you the real axle loads.
Model your rig in the Load check →