Car and Trailer Weights – The Words (and Letters) We Use
3. Tow Ball Mass
A Recap of ATM
As I discussed in my previous article on Gross Vehicle Mass (GVM) and Aggregate Trailer Mass (ATM), we learnt that the Actual ATM of your trailer can be considered as the weight of the whole trailer suspended from a set of fish scales.
When not hanging in mid-air, the total weight of the loaded trailer (ATM) either in your back yard or on site in camp is shared between the trailer’s axle or axles and the jockey wheel.
Now don’t worry too much about the jockey wheel because that’s just a support – a bit like the wind-down legs. This is important to bear in mind because the jockey wheel is say a metre away from the actual tow hitch and bears a little more weight than what we’re chasing.
Getting Hitched
So, now let’s move on and think about the trailer hitch.
This is quite simply the ‘thing’ that connects the trailer to the tow vehicle. They come in many forms but all do the same job – help the car to drag the trailer, keep the trailer connected and pull it up when you want to stop.
In doing all this, it takes part of the trailer’s ATM load and puts it onto the towing vehicle. This is a bit like throwing say 10-15 bags of cement on the car’s rear bumper bar.
More guidance on Tow Ball Mass
Generally (very generally) this load can be around 10% of the trailer’s total weight. So, for a trailer that actually weighs 3 tonnes (its actual ATM) that’d be about 300kg on the ball.
I say generally because for some modern camper trailers (like mine) the trailer axle is a long way back from the hitch and I run about 13-15% of the Actual ATM on my ball. This design allows for better exit clearances when on off-road tracks. Additionally, I find that a more rearward axle gives me better tracking behind the car.
Tow Ball Mass
Now that we’re hitched up we now have the trailers Actual ATM spread between the trailer’s axle/s and the tow ball on the car.
This load on the tow ball of the car is called the Tow Ball Mass. Again, it is important to know what your trailer’s actual Tow Ball Mass is – DON’T rely on the sales brochures, the compliance plate, a 10% calculation, the scales under the jockey wheel or what a mate told you.
Back to ATM
Let’s not forget about the trailer’s Actual ATM!!!!
Just because the load at the front of the trailer is now supported by the tow vehicle it does not mean that the trailer just got 200-300kg lighter (say). The additional weight that your car is now trying to control in its acceleration, braking and cornering is still the full Actual ATM of the trailer.
Don’t count twice
Similarly, the load on the tow ball is not counted twice in the trailer and on the car.
The Trailer’s Actual ATM remains the same whether it is hitched or unhitched.
This is where we need to explore Gross Combination Mass (GCM) in my next article.
Accuracy When Weighing Dual and Triple Axle Trailers
As I’ve said earlier, it is important that you know your trailer’s Actual ATM.
If you aren’t using the services of a professional weighbridge, I have provided a guide on how weigh your car and trailer on this page – https://campandtravel.com.au/weighing-your-car-and-trailer/.
Riding Level You Say
But here’s something else to be aware of… Your trailer must be riding level.
Otherwise you’re putting unequal loads on the trailer’s axles.
