Car and Trailer Weights – The Words (and Letters) We Use
Gross Trailer Mass (GTM)
4. Recap
If you’re new to the world of travelling loads and trailers, you could be forgiven for thinking that everyone is against you when it comes to the nomenclature of loads and limits.
So far we’ve looked at Tare Mass, Gross Vehicle Mass, Aggregate Trailer Mass and Tow Ball Mass. This article addresses Gross Trailer Mass.
Most importantly for now, we also found that the trailer’s Aggregate Trailer Mass (ATM) is made up of both the Tow Ball Mass and the load that rides on the trailer’s axles when hitched to the tow vehicle.
This load on the trailer’s axles is referred to as the Gross Trailer Mass (GTM).
Gross Trailer Mass
The best way to determine the Actual GTM of your trailer is to drive on to a set of scales (refer https://campandtravel.com.au/weighing-your-car-and-trailer/)
Knowing this actual load is important because your trailer’s axles or axle group (where you have more than one axle) have maximum limits that are not to be exceeded.
Don’t be surprised if the load carrying capacity of the axle or axle group (as stamped on your compliance plate) is close to, or perhaps even more than, your trailer’s Maximum ATM. This is a legacy of the engineering of your suspension and axle/s versus the design of the trailer chassis and its members – and the weights the manufacturer wants to promote.
At the same time though, don’t be tempted to load your trailer up to the Maximum Axle Loads because you still need to keep within the trailer’s Actual ATM within the Maximum ATM limit. Plus, the general rule-of-thumb is that 10% of the Actual ATM ought be on the Ball – meaning that 90% (approx) is on the axle’s (GTM).
If ever you are wanting to increase the Maximum ATM of your trailer through a licensed Engineer, room to move on the Maximum axle loads is most likely the first place they will look for what gains they can possibly offer.
Where You Carry Stuff Impacts GTM
As you will find when exploring my other articles on Trailer Loads and when using my App LoadGood (refer https://campandtravel.com.au/loadgood-towing-and-travel-loads-app/), where you carry stuff in or on your trailer can have a big impact.
First, carrying heavier loads over the axle/s is perhaps the best place to stow these items in transport. This is because their load impact is directly over the axle/s with lower leverage impact or increased loading on the Tow Ball (well not much).
Load distributions
Loads carried in front of the trailer axle/s are shared between the axle/s and the Tow Ball – the amount depending on their distance in front of the axle/s.
However, items (like bikes, generators and jerry cans) carried behind the trailer are perhaps the worst offenders here. Because of the overhang behind the trailers axle/s their load impact on the axle/s can be considerable – most likely 1.5 times. What that means is that two jerry cans (say 40kg) on the rear bumper could mean an extra 60kg on the axle/s. You’ll need to do the sums yourself (https://campandtravel.com.au/loadgood-towing-and-travel-loads-app/).
The impact of load distributions
Just as you need to be aware of the multiplied impact of loads placed behind the axle, you need to be equally aware of their impact on the Tow Ball. That 20kg that ‘magically’ appeared on the trailer’s axle/s in the example above was effectively taken away from the Tow Ball Load – meaning that where your Tow Ball Load may have been 250kg now becomes 210kg. Such a a drop could affect your trailer’s handling on the road and perhaps could impact the dreaded wobbles.
Always take care when moving, removing or adding loads.
Big Tip – Look after your trailer’s axle/s – they’re so much more than just the tyres.
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