Towing and Weight Terms Explained

Towing Capacity
The total amount of weight your vehicle can safely and legally tow.
Check your vehicle manual for both braked and unbraked towing limits.

Gross Vehicle Mass (GVM)
The maximum weight of the tow vehicle, including passengers, fuel, and cargo.
Exceeding this limit can impact braking, suspension, and safety.

Gross Combined Mass (GCM)
The maximum total weight of the tow vehicle and caravan combined.
You must stay within this limit when the vehicle and caravan are hitched together.

Axle Load Limits
Every vehicle and caravan has maximum weight ratings for each axle on the front and rear of the vehicle, and the axle group on the caravan.
These are set by the manufacturer and can usually be found on the compliance plate or in the owner’s manual.
Even if your total weight is under the GVM or ATM, exceeding an axle limit can still make your setup unsafe or illegal.

Rear Axle Weight (RAW)
The total weight supported by your vehicle’s rear axle, including the vehicle’s load and the downward force from the caravan’s tow ball.
It’s important to stay within the rear axle load limit to maintain safe handling and prevent suspension or tyre damage.
You can measure RAW at a weighbridge while hitched.

Axle Group Rating (Caravan)
The combined maximum load that all caravan axles can legally support.
Found on the caravan’s compliance plate - exceeding it can overload the suspension, tyres, and brakes.

Tow Ball Capacity
The maximum weight the vehicle’s tow ball can legally carry.
This rating is found on the vehicle’s tow bar or compliance plate - do not exceed it.

Ball Weight (also called Tow Ball Load or Tow Ball Mass)
The downward force of the caravan on the vehicle’s tow ball, usually 10–15% of the caravan’s ATM.
It can vary depending on how the caravan is loaded.
When measured unhitched, it’s referred to as Ball Weight; when measured hitched, it’s known as Tow Ball Load.
Always ensure it stays within your vehicle’s rated Tow Ball Capacity.

Weight Distribution Hitch (WDH)
A towing accessory that helps distribute the caravan’s ball weight evenly across all axles of both the tow vehicle and the caravan.
Improves steering, braking, and reduces rear-end sag in the tow vehicle.

GVM Upgrade (or GCM Upgrade)
A certified modification that increases your vehicle’s legal GVM (and sometimes GCM) limit.
Common among 4WD owners towing heavy caravans. Upgrades must be engineer-approved and listed on your compliance plate.

Gross Trailer Mass (GTM)
The total weight supported by the caravan’s wheels when it’s hitched to the tow vehicle.
It excludes the tow ball weight and is usually slightly less than the ATM, since part of the weight transfers to the tow vehicle.
The GTM is listed on the caravan’s compliance plate and must not be exceeded.

Aggregate Trailer Mass (ATM)
The total weight of the caravan when fully loaded and detached from the tow vehicle.
Includes everything in and on the van - gear, water, gas, and the ball weight.

Tare Mass
The empty weight of the caravan as specified by the manufacturer with no water, gas, or personal items included.

Payload
The maximum load you can add to the caravan, including water, food, clothes, camping gear, and accessories.
Payload = ATM – Tare Mass.

Kerb Weight
The weight of the vehicle when empty, with full fuel and all standard equipment, but no passengers or cargo.
Useful for calculating your available payload before reaching GVM.

Drawbar Weight (or Drawbar Load)
The portion of the caravan’s weight carried by the drawbar, including the coupling, gas bottles, and front storage box.
Excessive drawbar weight can increase ball load and affect towing stability.

Slip Load or Static Load Rating (Tow Bars)
Refers to the maximum vertical load the tow bar can handle - essentially the same as its ball load limit.
Check the rating stamped on your tow bar to ensure compatibility.

Overhang
The distance between the rear axle of the tow vehicle and the tow ball.
A longer overhang increases leverage on the rear axle, affecting stability and ball weight sensitivity.

Tyre Load Rating and Pressure
Every tyre has a maximum load rating and inflation pressure.
Overloading or under-inflating tyres is a major cause of towing instability and blowouts. Always match tyres to your GVM/GCM requirements.

Weighbridge Certificate
A printed report from an accredited weighbridge showing your vehicle’s and caravan’s individual axle weights and total masses.
Essential for confirming you’re within all legal limits before travelling.

Always check the vehicle and caravan compliance plates, manufacturer manuals, and your tow bar specifications before towing.
If unsure, visit a weighbridge to verify your setup is both safe and legal.