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What is tow ball weight, and why does it matter?

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 for safe towing.

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. For more information on weights in general, check out our page on 4WD Weights.

There’s a lot of illegal setups out there, and we’ve written a simple towing guide to make it easy for everyone to understand the 7 ways you can be over weight – Towing capacity; a simple guide to keep you legal.

Tow ball weight Hypercamper

Weighing the tow ball weight on our camper trailer

What is Tow Ball Weight?

What does tow ball weight mean? 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 or poly block hitch, 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 new 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 over weight.

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. The tow ball weight figure that you read from the manufacturers specifications is the maximum weight on the tow ball that you can legally apply.

Tow ball weight

Is your tow ball weight safe?

Tow ball weight explained; 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:

Trailer instability

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 trailer 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.

Towing with a Pajero

If you want a dangerous and unstable trailer, run the wrong tow ball weight

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 clarify, that means if you are applying 250kg to the tow ball, it can be adding around 375kg to the rear axle!

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. This is one of the important factors when deciding between a Wagon or Ute; they both have their compromises you need to make.

The best tow vehicle should be well and truly suited to the tow ball weight you are applying, and the conditions you apply it in. Off road you will put far more stress on the vehicle, and the ratings need to be seriously reduced.

Tow ball weight

How much overhang do you have?

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.

Dmax tow bar

Any weight you add to the vehicle adds more stress

Legalities

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.

The Mitsubishi Pajero Tow Ball Weight for example is capped at 180kg when you are towing over 2.5 tonnes on many of their models, and if you exceed this (which is extremely easy to do with many vans today) you are effectively driving an illegal vehicle.

Towing can easily make you illegal

Are you compliant?

Your tow ball weight changes

The most important thing to know is that your tow ball weight will rarely be the same. When your trailer is empty (its tare weight) the tow ball weight is supposed to match what is written on the nameplate of your trailer. It often doesn’t, and there are issues with suppliers providing trailers that are incorrectly stamped, or someone has messed with the weights.

Either way, once you load the trailer up, you’ll find the tow ball weight will change, and it often goes up considerably. If you start with 180kg on the ball, it can easily go to 250, or 300kg depending on how you load it up. The Aggregate Trailer Mass (ATM) is the maximum you can load the trailer to, and your tow ball weight is most important when your trailer is fully loaded.

It’s important to look at the Gross Trailer Mass (GTM) as well, which is the maximum weight that can be applied to the trailers wheels. Your ATM minus the GTM should leave you with a suitable tow ball weight, or you are in trouble.

Tow ball weight

Your tow ball weight will change hugely as you load the trailer up

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.

Measuring the tow ball weight

Measuring the tow ball weight on our Reconn R2

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 ideal tow ball weight 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!

4WD boat trailer wheels

The lighter the trailer, the lighter the tow ball weight should be

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 wanted to know how to reduce tow ball weight, installing a big toolbox on the back of your van would work, but it is a terrible idea as it moves weight away from the axles and creates a pendulum effect.

Many people do this, and the van becomes extremely unsafe and handles poorly, because a small twitch of the axles means a huge chunk of weight out far gets moved much more, and carries a lot of energy with it. Instead, move things around close to the axles, and avoid adding or taking away huge amounts of weight far away from the axles.

Tow ball mass

On our R2, just filling the fridge up increases the tow ball mass!

Trailer design

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:

Axle location

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 draw bar 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. Caravan Ball weight tends to have greater flexibility because of the extra room for water tanks to be mounted in different places.

The other issue on trailers 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.

Axle position on a camper

Where is the axle located?

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.

Dual axle vans

Dual axle caravans have different weight distribution

Draw bar length

The length of your draw bar also affects tow ball mass considerably. A longer draw bar 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 draw bar, 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 draw bar

I’ve seen all sorts of accessories mounted to draw bars, 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 draw bar by adding aftermarket accessories, expect your tow ball weight to skyrocket, and in some cases the draw bar to crack, or snap!

Some people have mounted motorbikes, or quads to an extended drawbar, and this can end very badly if its not extremely well engineered.

Drawbar weight

What accessories do you have on your drawbar?

Hitch orientation

In many circumstances, you can actually remove your tow hitch and flip it around the other way. This does a number of things including hugely changing your tow ball weight. Ideally, your trailer and vehicle should both be sitting level, and sometimes a simple hitch rotation does the trick.

However, there’s a bit more to it than this, and you can read more here; Can you flip your tow hitch?

How to increase tow ball weight capacity

If you need a greater tow ball weight capacity on your vehicle, you need to start at the tow tongue and work your way back. In some cases, you can just upgrade the tongue, or the tow bar. If you need more than that though, you are going to have to consult the manufacturer, or an external engineer, and even then I don’t know if you’ll have any luck.

In general, unless you’ve had extensive modifications done to accommodate a higher weight, applying more tow ball weight than the manufacturer set as the limit is not a good idea.

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.

Do you know what your tow ball weight is?

What’s your tow ball weight?

What’s your experience with tow ball weights? Have you had issues and changed things around so it handles better?

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10 comments… add one
  • Wayne November 29, 2020, 7:33 PM

    We often see tandem axle caravans with 450+kg more on one axle than the other. This makes for a very unstable towing experience. Adjusting the tow ball ride height by as little as 50mm up or down can make all the difference. We ideally like to have 80kg to 110kg more on the front axle. Not only does it better balance the weight distribution between the axles it also helps address tow ball downforce irregularities as well.

  • Aaron Schubert November 29, 2020, 8:17 PM

    Hey Wayne,

    Interesting information; 450kg is shocking! It’s amazing how many people who tow with a setup that is not level. Cheers for the useful comment!

    All the best
    Aaron

  • Sandra January 30, 2021, 12:28 PM

    I have a nissen xtrail 4×4 2011 model, Hayman Reese tow bar, I purchased a coromal convair 2004. Apparently my tow ball weight is 190 kg, but was told my car can only have 100 kg, what do I have to do, can I have something fitted that will allow me to have a higher towball weight? All other weights are within the cars capability

  • Aaron Schubert January 30, 2021, 12:46 PM

    Hi Sandra,

    You should do two things – get the van weighed and verify that the weights are correct, and then speak to Hayman Reece, and Nissan. Hayman Reece may offer something that suits the tow ball weight, and Nissan may also.

    If neither do, you cannot legally tow the van with that vehicle.

    All the best
    Aaron

  • Muhleisen John February 19, 2021, 2:45 PM

    I have a caravan with a tare of 2080kg and an ATM of 2790. I cannot see anywhere on my van that tells me what the legal ball weight should be. I have a Prado and the towbar is rated at 2500kg tow and 250kg ball weight, however there is nothing on the caravan compliance plate that tells me what my maximum ball weight should be. Would appreciate your comments.
    Many thanks John.

  • Aaron Schubert February 19, 2021, 8:33 PM

    Hi John,

    There is no maximum ball weight supplied by most caravans. If there’s a ball weight on the nameplate, it will be the weight off the factory, not when its loaded. Ideally you aim for around 10% of the weight as tow ball weight, as long as your vehicle is suitable for it.

    There is one other way of looking at it, if you have the GTM and ATM figures. Generally GTM take away ATM is your tow ball weight. You cannot exceed either, but it can be useful to understand.

    All the best
    Aaron

  • Dennis Hopper May 22, 2021, 1:45 PM

    I find there is an inaccuracy in subtracting the trailers GTM from the ATM to obtain the actual towball weight. By reason of the leverage, from where the jockey wheel is positioned to the actual hitch location.
    The towball weight will be less than the previous calculation.

  • Aaron Schubert May 22, 2021, 1:50 PM

    Hey Dennis,

    Yep, that is possible to get a bad reading depending on how you do it. You should always get the tow ball weight from the hitch, and not the jockey wheel, and also ensure that when weighing the tow ball weight its sitting at a similar height to how it would be on your vehicle. If you get that right, it should be accurate.

    All the best
    Aaron

  • Wayne Sproull June 30, 2021, 9:48 AM

    We recently purchased a used vancruiser caravan that has a tare of 2580 kgs and ball weight of 140kgs. Towing it with my 2020 triton was not great as over 80ks it got sway. I towed it with our 2016 prado and it towed beautiful. I’m confused as what to do to remedy this for the triton. Any suggestions?

  • Aaron Schubert June 30, 2021, 12:41 PM

    Hi Wayne,

    In general, 140kg tow ball weight for a 2.6 tonne van is extremely light. The difference is probably the distance between your rear axle centre line and the tow bar. I expect the Triton would be much longer. The best remedy is probably to increase the tow ball weight. 10% is the ideal amount, depending on how its all set up

    All the best
    Aaron

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