Are you concerned about your tow ball mass but you don’t have tow ball scales to measure it with? We found this ‘MacGyver’ed’ method using a set of bathroom scales, a few pieces of wood and some pipe.
If your rig looks like the one in the above picture, then you very likely have an issue with excessive tow ball mass. Similarly, if your caravan isn’t as stable on the road as you would like it to be, then your tow ball mass could be too low. Either way, it’s a good idea to check it.
Understanding your caravan’s tow ball mass
Maximum Tow ball Mass (or TBM) is a rating that is not defined in the Australian Design Rules however it really is nothing more complex than the maximum weight imposed by the trailer onto the towing vehicle when it is connected and while the rig is at rest. The maximum TBM should be noted on the compliance plate of the trailer. You also need to know the maximum TBM rating of the tow vehicle itself. If you have both, your limit will be the lesser of the two.
As a general rule, you should aim to have a tow ball mass of approximately 8 – 10% of the total loaded weight of the trailer. As an example, a caravan with an ATM of 3,500kg will normally have a maximum TBM of 350kg.
Using a bathroom scale:
To use a regular bathroom scale for determining your caravan’s tow ball mass, you will need the following items:
- A set of bathroom scales (preferably not glass and with a weight capacity of at least 150kg)
- A brick or paver the same height as your bathroom scales
- Two small lengths of metal pipe (15-20cm each will suffice)
- A sturdy wooden plank (1-1.5m long)
- Some additional pieces of the same sturdy wood to form a T piece
- A spirit level
- A tape measure
- Wheel chocks
Park your disconnected caravan on level ground and ENSURE IT IS IMMOBILIZED using wheel chocks and the handbrake firmly engaged.
With the van resting on the jockey wheel, place the spirit level lengthwise on the drawbar and, using the jockey wheel, raise or lower the drawbar to the point where it is level.
Next, place the scales and the brick on the ground with their centres’ 90cm apart. Place a length of pipe on centreline of each and run the long wooden plank across the two pieces of pipe to form a bridge. Make sure you position this bridge so that it lines up under the tow hitch and about 60cm along the length from the centre of the bathroom scales.
Measure the distance from the bottom of the tow hitch to the top of the bridging plank of wood and, using the other pieces of wood plank, construct a T piece with a total length of this measurement. Once completed, raise the drawbar slightly.
Reset your bathroom scales to zero.
Now, place the wood T piece under the hitch and lower the jockey wheel so the T piece rests onto your bridge and is centred at the 60cm point along the bridge (from the centre of the bathroom scales). Recheck that the drawbar is level and that the pipes haven’t moved off their centre marks on the paver and the scales.
If this all sounds confusing, this picture should make it a bit clearer.
Record the reading on the scales, and multiply it by 3 to get your caravan’s approximate tow ball mass. As an example. If your scales show a reading of 100kg, your approximate tow ball mass is 300kg (100 x 3).
Keep in mind this is an approximate measurement only. To obtain an accurate measurement, you will need a quality set of tow ball scales. Alternatively, you can take your caravan to a weighbridge or engage the services of a mobile trailer weighing service.
We also strongly recommend you do not do this alone. Get a mate to give you a hand.
Thanks go to the folks at Caravan RV Camping for allowing us to share this with you.