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Setting Your Trailer Towball Mass

Setting Your Trailer Towball Mass

by September 7, 2020

Towball mass is the weight that the loaded trailer places on the tow vehicle’s tow point when hooked up and ready to go. As a general guide, it should be ±9% of the fully laden trailer’s weight or ATM (Aggregate Trailer Mass – the weight of your unhitched trailer and its full load). In practise, it is calculated as ATM minus GTM (Gross Trailer Mass – the weight imposed on the same trailer’s axles when hitched to its tow vehicle).

You can check what your current towball mass is by placing just the jockey wheel of your loaded trailer on the scales at a weigh bridge, or on a set of dedicated towball weight scales. If you find your towball mass falls too far outside the general ‘9% of ATM’ rule, it might be worth visiting a trailer expert, like Oceanic Trailers or one of our dealers, to see if we can assist you in setting your trailer up correctly.


A common sign that your trailer’s towball mass may not be set up correctly is if the trailer and/or tow vehicle are not sitting reasonably flat when hitched up. If the connect point creates either a dip or rise in the angle of a line through the tow vehicle and the trailer’s drawbar, the towball mass is not set correctly.

The danger of having too much towball mass in relation to your trailer’s ATM is it un-weights the tow vehicle’s front end, leaving you with a reduction of front-end grip, and diminished front braking capabilities, while over-stressing rear suspension and braking components. It also tends to over-load mechanical trailer braking systems as the load’s weight is pitched more down/forward, which magnifies the force on the system’s actuator. The chance of striking the towball or tow point is also increased if the extra towball mass is causing it to sit lower than is usual.

If the towball mass is too low, the trailer will be more prone to weave and bounce, as the extra weight is carried over the back of the axles when in motion, and tends to make the rear of the tow vehicle feel light when going over undulations or bumps on the road surface. There’s also a tendency to overheat the trailer’s tyres if the towball mass is set too low, due to more weight being carried by the trailer’s axles. This becomes more critical if the trailer’s ATM is approaching the load limit of the tyres at their maximum inflation pressure.

If you are unsure about the towball mass of your boat, PWC/jet ski, or box trailer, contact Oceanic Trailers or one of our dealers who offer professional trailer set-up services. For more information, visit

Published in the August – November 2020 print edition.