Why trailer weights matter
Overloading a trailer is dangerous, and there are rules about it that differ between states. Check with your state road authority. Understanding the weight ratings stamped on any trailer, and how they relate to your vehicle's limits, is worth doing 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 the figure on a particular trailer, check its compliance plate or ask the owner.
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 less than ATM because some weight is transferred to the tow ball.
Payload
The maximum load you can put in the trailer. Payload = ATM minus Tare weight. The trailer's compliance plate has the ratings you need to work it out.
Tow ball download
The downward force the trailer exerts on the tow ball. For the figures that apply to your setup, check your vehicle's owner's manual and the trailer's compliance plate. 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, and your owner's manual has the figures for both:
- Maximum towing capacity: the heaviest trailer (by ATM) your vehicle is rated to tow. Found in your owner's manual.
- Maximum tow ball download: the downward force your tow ball is rated for. This varies between vehicles.
Stay within both limits at the same time. If a trailer's ATM is within your towing capacity but the tow ball download goes past your vehicle's limit, it isn't a combination to tow at full load. Check your owner's manual for your vehicle's figures and your state road authority for the rules.