There are many things to think about when you’re looking to hit the road with your caravan or camper trailer, but one of the most important things to make sure you’re on top of is your weights, both in your tow vehicle and whatever trailer you’re towing. While knowing the weights of your caravan and may seem unimportant it’s critical for ensuring that you remain 100% legal and safe on the road.
While there are many different weights you should know about your towing setup, we’re focusing on the one that can be the trickiest to measure and easiest to overlook; the tow ball mass.
What is Tow Ball Mass (TBM)?
Put simply, tow ball mass is the amount of weight a caravan applies to the tow ball of a vehicle. This is essentially the tow bar capacity of your vehicle. When you’re towing, this is the amount of weight that is loaded on to your tow vehicle from your caravan or camper trailer. This weight can vary depending on how your caravan or camper trailer is loaded or packed.
Why is the TBM important?
The TBM is important for a couple of reasons. Firstly, because each vehicle has a specified TBM that should not be exceeded for the health and safety of your vehicle. This specific number is available on a sticker or plate on your towbar. Secondly, because when you’re adding a TBM on your vehicle this contributes to the overall payload of your vehicle.
The payload of your vehicle is the total weight you’re legally and safety allowed to add onto the vehicle. When you’re travelling or towing the payload can include occupants, luggage, aftermarket accessories and the TBM. Depending on your TBM, it can quickly eat into your payload and affect how much other gear and passenger weight you can carry.
Having a TBM that is too heavy for your vehicle also makes you prone to caravan sway while you’re driving, which can quickly be disastrous.
So, what should my TBM be?
Ideally, you want to keep your TBM within 7-10% of the aggregate trailer mass.
Aggregate trailer mass (ATM): the aggregate trailer mass is the maximum amount the trailer can legally weigh once everything is loaded into it. When your trailer is loaded it must not exceed the ATM. To calculate the ATM, add all maximum loads (luggage, passengers, water, fuel and accessories) plus the tow ball mass onto the weight of the trailer.
This will make your setup much less prone to caravan sway. It’s always a good idea to check what your vehicle manufacturer recommends and stick to that as well.
How do I calculate the TBM?
There are three main ways to calculate your TBM, with tow ball weight scales, household scales or at a public weighbridge.
Tow ball weight scales
If you’re planning to travel or measure your TBM frequently, getting a set of tow ball weight scales could be the easiest option. Having your own set of scales to store in the caravan or camper trailer is always handy to be able to check your TBM whenever you set off anywhere.
To measure your TBM with tow ball weight scales simply make sure your trailer is level and secured, raise the jockey wheel, place the scale directly underneath the tow ball coupling and lower the jockey wheel until the full weight of the caravan is on the scales.
If you’ve got a public weighbridge or industrial vehicle scales near you, this could be the simplest option. This is a slightly lengthier process as you will need to hitch and unhitch the trailer a few times to get all the necessary measurements. To get your TBM, you’ll need to get the following measurements.
Weight A – car + tow ball weight
To get Weight A, drive your tow vehicle onto the scales with the trailer hitched. Make sure just the four wheels of the car are on the scales.
Weight B – standalone car weight
Unhitch your caravan and drive your car back into the scales so you’re just measuring the vehicle weight.
TMB = Weight A – Weight B
Subtract your vehicle’s weight (weight B) from your vehicle and tow ball combined weight (weight A).
This is the most challenging and inaccurate way to measure your TBM as you’ll need a few extra bits and pieces, but if you’re in a pinch and don’t have access to a weighbridge this method will suffice.
To do this method you’ll need a 1m long piece of wood, another piece of wood solid enough to hold your caravan tow ball coupling, pavers, two pieces of pipe and household scales rated to have a limit of over 100kg.
- Measure the plank of wood and mark four lines, each 300mm apart, with a little overhang on each side. On one side of the wood, carve out a notch at the two end markings. This is where the pipe pieces will sit.
- Place your piece of wood, balanced on the pipe, underneath the towbar of your trailer. Make sure the tow ball coupling is over the one third marking on the plank.
- Place your scales under the piece of pipe that is the furthest away from the draw bar. Under the other pipe place a few pavers, lifting the plank up to be level. Double check it’s level with a spirit level.
- Secure your caravan and raise the jockey wheel. Place the other piece of wood at the one third marker. Make sure this piece of wood holds the trailer as close to the height that it will be when towing as possible.
- Lower the jockey wheel until the tow bar coupling is resting on the wood.
- Read the weight measurement and multiply it by three. This is your TBM.
This method can be a bit confusing but there are plenty of video resources and tutorials on YouTube if you’re looking for a bit more help.
There you have it, all the different ways you can calculate your TBM the next time you’ve got the caravan or camper trailer ready to head out. To learn more about some of the other calculations you need to know before towing on your next trip check out uncovering the truth about towing capacities or everything you need to know about towing.