Why trailer weights matter
Overloading a trailer is illegal and dangerous. Understanding the weight ratings stamped on any trailer — and how they relate to your vehicle's limits — is essential before you hitch up and drive.
Key weight terms defined
Tare weight
The weight of the trailer itself, empty, with no load. Think of it as the "empty" weight. For a standard cage trailer, tare weight is typically 300–500kg.
ATM — Aggregate Trailer Mass
The maximum total weight of the trailer AND its load combined. This is the maximum the trailer is rated to weigh when loaded. ATM is stamped on the trailer's compliance plate.
GTM — Gross Trailer Mass
The maximum weight that can be applied to the trailer's axle(s) when it's hitched to a vehicle. GTM is always less than ATM because some weight is transferred to the tow ball — typically 8–12% of the total trailer weight.
Payload
The maximum load you can put in the trailer. Payload = ATM minus Tare weight. For example, a trailer with an ATM of 750kg and a tare of 300kg has a payload of 450kg.
Tow ball download
The downward force the trailer exerts on the tow ball. This should be 8–12% of the total loaded trailer weight. Too little and the trailer will be unstable; too much and your vehicle's rear will squat and steer poorly.
What limits apply to your vehicle?
Your vehicle has two key limits:
- Maximum towing capacity — the heaviest trailer (by ATM) you can legally tow. Found in your owner's manual.
- Maximum tow ball download — the maximum downward force allowed on your tow ball. Typically 150–350kg depending on the vehicle.
Always stay within both limits simultaneously. If a trailer's ATM is within your towing capacity but the tow ball download exceeds your vehicle's limit, you cannot legally tow it at full load.
Practical example
You're hiring a cage trailer with an ATM of 750kg and a tare of 320kg. Your SUV has a towing capacity of 2,000kg and a tow ball download limit of 200kg.
- Maximum payload: 750 − 320 = 430kg
- Expected tow ball download at full load: 750 × 10% = 75kg (well within your 200kg limit)
- Result: You can load up to 430kg. Your vehicle handles it easily.