Tow ball weight, or Tow Ball Mass (TBM) is a term that’s being thrown around more and more today, and for good reason. Understanding how it works, how to measure it and what is acceptable is critical to towing safely and successfully.
You don’t need to know it to the exact kilogram, but having an idea makes for safe travel, mechanical sympathy, staying legal and reduced chance of things going wrong. This is especially the case if you are running near the edge of your 4WD’s towing or weight capacities or if you load the trailer and vehicle up differently each time you travel.
What is Tow Ball Weight?
Tow ball weight, or Tow Ball Mass is quite simply the weight applied from your trailer to the tow ball on your 4WD. When you are towing, the bulk of the trailer weight is taken by its wheels, and a small portion of it is passed onto the tow vehicle via the tow ball.
If you run a different hitch (like a Treg, DO35, or McHitch) the same principle applies; its still called tow ball weight. A trailer that is heavy behind the axles will have a low tow ball weight, and if its heavy at the front (like a lot of imported camper trailers) then it will have a heavy tow ball weight.
In general, a lot of new 4WD’s are capped at 350kg tow ball weight, but this is very misleading, depending on what vehicle you own, how it is loaded and what tow bar you have installed. If you have a tow ball weight that is significant, you need to be extremely careful that you are maintaining a legal setup as a high tow ball weight is one of the most common ways to become overloaded.
Some vehicles have a much lower tow ball weight allowed than 350kg, and some even have additional penalties to be added when you go above a certain weight. You can find all this in your owners manual, or the stamped tow bar, or you can ring your vehicle manufacturer with the VIN number ready.
Why does it matter?
There are lots of reasons why Tow ball weight matters, and getting it wrong can, and has resulted in some very nasty accidents over the years. Lets start with the basics of why tow ball weight is so critical:
If there’s one thing that affects trailer stability to the extreme, its your tow ball weight. If its too light, you will end up with terrible sway issues. In general, the heavier the tow ball weight the more stable it will be (to a point!), but a heavy tow ball weight isn’t good for other reasons, so you need to find the right balance.
Of course, just because you have the right tow ball weight doesn’t guarantee trailer stability; its only the first step. You need to make sure the majority of the weight is down low, close to the axles and that you don’t have anything heavy way out the back, or front of the van. There are also a number of manufacturing designs that will determine stability levels, which we go into further down.
Significant extra stress on the vehicle
Tow ball weight applies a huge amount of stress to a 4WD. Next time you walk past your vehicle, have a look at how much distance there is from the centre of your rear differential to the tow ball. For wagons, this tends to be less (around the 1000 – 1400mm mark). For dual cab Utes, it can be around 1200 – 1600mm, and this creates significant leverage.
The leverage can easily be worked out, and its not uncommon to see the actual weight applied to the rear axle 50% higher than the tow ball weight. To work it out, just divide your overhang by the wheelbase length of your vehicle, and then multiply the result by the tow ball weight. The figure you get is weight shifted from the front axle of the vehicle to the rear.
Overhang is the distance from the centre of your rear axle to the tow ball. Wheel base is the distance between the front and rear wheels.
For example, our Isuzu Dmax has an overhang of 1375mm and wheelbase of 3095mm. If the tow ball weight is 120kg, you divide 1375 into 3095 and get 0.444. Multiply that by 120 and the additional weight applied is 53kg, meaning on our setup a 120kg tow ball weight actually applies 173kg to the rear axle. It does this by removing 53kg from the front axle and transferring it to the rear.
This is a major problem, especially if you are going off road. If you hit a dip and it jerks the trailer and vehicle, you can end up with much, much higher forces going onto the vehicle, and the risk of chassis damage is greatly increased.
There have been plenty of cases of 4WD’s loaded within their legal capacities that ended up with bent chassis simply from hitting a hole or drop and passing too much force through. Of course, if you are not legal (like thousands of people) you are much more likely to do this.
There are a huge range of legalities that you have to meet when you jump in your 4WD and take off down the road. In fact, there are 32 ways to make your 4WD illegal, and overloading it in one of about 10 ways will render your vehicle illegal. Not only can this put you in a nasty place with the law, but it can also void your insurance, as you’ve broken your part of the deal with the insurance company by not driving a roadworthy vehicle.
I mentioned above that there are numerous weight restrictions to adhere to. I’ve written a post that makes this all super clear here – How to keep your 4WD and towing setup legal.
How do you measure Tow ball weight?
Measuring tow ball weight can actually be done pretty easily. You can buy dedicated tow ball weight scales from any decent auto shop, or you can use two (or even one sometimes) bathroom scales with a block of wood on top of them.
Simply load your trailer up as you would normally for a trip away including full water tanks, and park it on level ground. Put the scales under the tow ball (or as close as possible and lower the jockey wheel down so all of the weight is sitting on the tow ball scales. It’s important to think about the height that your trailer sits at on your vehicle too, as if its significantly different the reading will not be the same.
Alternatively, you can do this at a weigh bridge fairly easily, by weighing the 4WD only when the trailer is attached (so just have the car on the weighbridge), then detach the trailer and weigh just the vehicle. The difference is the weight the trailer is applying to the tow ball.
For example, if we take our Dmax to the local weighbridge, and drive the vehicle on to the bridge with the trailer attached, it might come back at 2700kg. If we then detach the trailer and weigh the vehicle again, it comes in again at 2580kg, which means we have a tow ball weight of 120kg.
What’s the recommended Tow Ball Weight?
People will tell you all sorts of random tow ball weight recommendations. In Australia, if its between 6% and 15% of the total trailer mass, you are in the right territory. In Europe the recommendation is much lower, but things are done very differently over there too.
For a 2500kg trailer, that means your tow ball weight should be in between 150 and 375kg. Of course, the ideal towing weight also needs to fit into the requirements and limitations of the vehicle and tow bar manufacturer, so you need to double check everything is legit before taking off!
Can you adjust your tow ball weight?
Yes, you can very easily adjust your tow ball weight. However, do so with caution as if you deviate too far from a good initial trailer design you may end up with other issues. It’s important that you don’t spread weight too far from the centre of the axles, as this affects handling in a very bad way. For example, if you found out that your tow ball weight was too heavy, installing a big toolbox on the back of your van to reduce the tow ball weight would be a terrible idea.
Many people do this, and the van becomes extremely unsafe and handles poorly, because of how far the weight is away from the axles. Instead, move things around close to the axles, and avoid adding or taking away huge amounts of weight far away from the axles.
If you look at a variety of trailers on the market, you will see that there are some very different designs out there. The design ultimately determines what sort of range of tow ball mass you are going to sit in. Of course, moving things around will change this, but there are a few things that determine essentially create the base line for how heavy your tow ball weight is going to be:
The position that the axle is fitted on your trailer will make a huge difference to the tow ball mass. Even moving it a few hundred millimetres can change the weight dramatically, and manufacturers often spend a lot of time getting this right. In essence its like a see-saw, and the wrong axle position will put you at the back of the pack before you even start.
You’ll see a lot of trailers have their axles located very far back, with a long drawbar and front section. This is an issue, as most of the space you have available to load is in front of the axle, meaning its only going to get heavier as you load it up.
The other issue is that they often rely on having a full water tank at the back to counter the heavy tow ball weight, and at the end of a trip your water tank isn’t going to be full! When buying a trailer, think very carefully about where the axle is located, and how that is going to affect your tow ball weight in relation to where you add weight.
Number of wheels
Most trailers have two wheels, which means the weight is spread over 3 places (each wheel and the tow ball). Once you start moving into the larger weight trailers you get dual axles, so the weight is spread over 5 places, and occasionally you will see trailers with tri axle and even quad from time to time
The more wheels, the better the weight distribution, and its not uncommon to see a reduction in tow ball weight for vans that have more wheels.
The length of your drawbar also affects tow ball mass considerably. A longer drawbar reduces the weight applied to the tow ball, and a lot of manufacturers offer this as an option (usually to fit more accessories on the drawbar, or to help with twitching).
Water tank positions
It’s worth paying careful attention to where your water tanks are mounted on your trailer. If they are forward of the axle, as you empty them your tow ball weight is going to decrease. If they are behind, the opposite happens. Ideally, water tanks should be mounted as close as possible to the axle as they are one of the heaviest items on a trailer.
Accessories on the drawbar
I’ve seen all sorts of accessories mounted to drawbars, from firewood boxes, to bike storage, outboard motor mounts, fuel storage and motorbikes. If the trailer has been drawn with these in mind you’ll have no issues with it, but if you add a lot of weight to the drawbar by adding aftermarket accessories, expect your tow ball weight to skyrocket, and in some cases the drawbar to crack, or snap!
Do you know what your tow ball weight is?
You should know what your tow ball weight is, in various scenario’s. It will change depending on how you are packed, how full your water tanks are and how long you are going away for. Particularly those with heavier trailers (1000kg +) this is critical, and if you get it wrong you will likely have a trailer that yaws, and handles terribly.