Everything you need to know about towing heavy trailers
Towing big, heavy trailers is not a matter of hooking up and hoping for the best – here’s our guide to everything you need to know about towing heavy trailers.
Last time we looked at why a 3500kg tow rating is not necessarily a 3500kg tow rating. Now it’s time to look at towing in a bit more detail, and answer a few questions. Such as:
What do the manufacturers recommend for tow setup?
The answer there is pretty simple. I asked several, and here’s some example responses:
“Subaru specifies the maximum towing mass for both un-braked and braked trailers as well as maximum tow ball down mass. Subaru also advises against exceeding the maximum permissible weight (MPW) of the tow vehicle as well as the maximum permissible rear axle weight (MPAW) of the tow vehicle when towing.”
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“The operator needs to manage payload and towing to not exceed GVM or GCM” — Ford.
“A customer should never exceed the GVM of their vehicle or maximum axle weights stipulated in their owner’s manual/handbook” – Toyota
I could show you more, but they all say here’s the figures and the guidelines, now don’t go exceeding them. Many of the manufacturers pointed out that the information is available in the owner’s handbook.
That means the onus is back on you, the operator, to understand do the right thing with the data, in much the same way car manufacturers don’t teach you how to drive. This is fair enough, especially as the car makers have no idea what trailer you’re going to hook up.
The problem is that all these different weights and figures are confusing. Most people do not consider trailer setup to be a fun way to spend their time, and just want someone to tell them what to do so they are safe and legal. Many would even quite happily pay someone to set it all up and they drive away, never needing to worry about the difference between GCM and GVM.
Unfortunately, there are very few people who do that, as it involves understanding quite a lot of trailer regulations and dynamics. And then if you are paid to set something up you then take on some liability if it is wrong or if there is an accident, so you can then understand the reluctance of experts to certify car/trailer combinations.
So let’s assume you are an owner who wants to get their rig set up right. There is unfortunately no shortcut to setting up trailers correctly, you need to fully understand the forces at play and the various limits. That’s what we’ll address in this and subsequent articles, and maybe destroy a few myths along the way.
Bottom line: you either need to figure it all our yourself, or go to an independent towing expert to have your rig properly set up. Every car/trailer combination is different.
Types of trailer
There are pig trailers and dog trailers. These do not refer to transporting of animals but mean the axle configuration:
The red arrows show that the dog trailer supports its own weight on its own wheels, and the axles are at each end of the trailer. The dog is a steerable trailer, and only needs the towcar to provide motive force and some input to steering. This is why dog trailers are used for large, heavy loads, and why you see them behind heavy trucks.
Unfortunately, dog trailers are expensive, heavy and even more difficult to reverse than a pig trailer (although an advantage is you can generally turn tighter without a jacknife), and turntable models are unstable as you get closer to ninety degrees of turn. That’s why they haven’t caught on. I suspect they also look a bit commercial and ugly, but not to my eye, safe is beautiful. Dogs are easier to hook up as there’s much less towball mass (if any at all) and it is easier to move the drawbar as it’s just moving the front wheels not the whole trailer.
In contrast, the pig’s weight is supported mostly by the rear axles, but its stability is provided by the towcar and that means some weight on the towcar. Which is where our problems start.
Trailer forces – the key to understanding a safe tow setup
Here is a diagram of the forces:
Diagram not to scale
The trailer has a centre of gravity (CoG) shown by the small red circle with a white cross, which is ahead of the TA (trailer axle). This creates a force on the back of the towcar, the TBM or towball mass. That in turn creates a upwards force on the FA (front axle), pivoting around the RA (rear axle). Interesting fact – the force on the rear axle is greater than the TBM, typically by about 30%. More on that later.
When you tow a trailer you want it to be dynamically stable, which means that when it is disturbed by something – a gust of wind, a bump in the road – then the trailer returns to its position behind the towcar. The opposite of dynamically stable is unstable, where once disturbed the trailer naturally deviates even further from its original position.
The key to stability is the position of the centre of gravity (CoG) relative to the axles, which is similar to but not the same as the TBM. The diagrams below make the point:
Centre of gravity for illustrative purposes only. The exact CoG location is dependent on the trailer and the towcar.
Rig 1 is about right, the trailer is stable. If disturbed, the trailer will return to its place behind the towcar, yet be reasonably easy to manoeuvre.
Rig 2 has the CoG way too far forwards, so the trailer is very stable, but also hard to turn and creating too high a load on the back of the car. Rig 2 would have problems going around corners, the trailer would want to go straight on.
Rig 3 is very easy to turn with a CoG close to the axle, but it’s not a stable trailer, especially at speed. There would be little damping of disturbances, leading to instability of the towcar. But this is a good setup for low-speed offroad work where you want minimal towball downforce, and high-speed stability doesn’t matter.
Rig 4 is totally wrong because the CoG is behind the trailer axle, creating a dynamically unstable trailer where any deviation is magnified.
Many people talk of increasing towball mass to improve stability. That’s actually the result, not the action.
What’s really important is shifting the CoG further ahead of the axles, and that has a side effect of increasing the towball mass. It is also possible to hugely improve a trailer’s towability without changing its weight or towball mass, something explained later on. It is all to easy to just increase the TBM to solve stability problems, but there are big risks with that approach.
What happens with a heavy TBM and forward Centre of Gravity?
The more you move the CoG ahead of the axles, the better the straight-line stability. Unfortunately, the TBM also increases, and the greater the TBM, the greater the force on the car, in particular the rear axle. There is a corresponding reduction in weight on the front axle. This situation is bad news for vehicle handling and therefore safety:
- the high rear axle downforce creates stresses that the chassis may not be designed for, particularly on rough roads. Fun fact: all maximum weights are for bitumen roads, not corrugated dirt roads.
- the reduced weight on the front axle reduces the ability of the vehicle to steer and brake – at the same time trailer stability is increased, which means the trailer is more reluctant to go around corners and the towcar is less able to make the turn.
- the rear suspension is partially compressed so the suspension may not be able to handle bumps before bottoming out, and that can lead to loss of control.
Bottom line: if the trailer is not stable, increasing the TBM will help but beware of the disadvantages. There are other ways to achieve stability such as centralising the trailer’s payload, and weight distribution hitches.
Rear axle loading – it’s more than the TBM!
The front and rear axles on any vehicle had load limits which must not be exceeded. These limits are in addition to the vehicle’s GVM. It is possible to be under the GVM and still exceed an axle limit.
Unfortunately, when you connect a trailer the rear axle load is increased by more than the TBM. The reason is because the TBM force is some distance behind the rear axle. This is what the forces look like:
The TBM pushes the back of the car down, and the chassis pivots on the rear axle so there’s less weight on the front axle. That weight has to go somewhere, and it goes onto the rear axle. Intuitively, you know that if you pushed down hard enough on the towball, the car’s front wheels would leave the ground. At that point, you’d agree the entire weight of the car is on the rear wheels.
The formula to calculate rear axle load is:
Rear axle load as a result of TBM = (Overhang / Wheelbase) * TBM
For example, 1200mm overhang, 130kg TBM, 2750mm wheelbase = 57kg reduction in weight on the front axle. Add that to the TBM of 130kg and we have 187kg on the rear axle.
Now if we look at a 350kg TBM with the same dimensions we get 152kg weight reduction on the front axle, and 502kg (350+152kg) on the rear axle. That is a serious load for any 4WD you care to name, and no, stiffer springs won’t solve all your worries. Might crack your chassis though.
This brings home why a short overhang and long wheelbase is so important. The shorter the overhang, the less the rear axle load, and the better the trailer control as there’s less leverage for the trailer to disturb the towcar.
If we look at the modern ute then we find they have very long overhangs, and their relatively long wheelbases don’t entirely compensate. Utes that have the rear wheels close to the rear doors are great for turning circles, but hopeless for load carrying and towing. Most utes could be much better towers and load carriers if they had wheels about 200-300mm further back. This is also a reason to use towbar tongues as short as possible.
Here is part of Land Rover’s specification for the Defender which proves the point:
If it is necessary to increase nose weight up to a mean of 250kg, the vehicle load should be reduced accordingly.This ensures the GVM and rear axle load are not exceeded.
This means that the vehicle can tow with a 150kg TBM at GVM. Go beyond 150kg and you must reduce payload accordingly.
Some manufacturers contacted for this article were clear that the rear axle load was not increased by any more than the TBM. I can’t explain why they said that and requests for clarification so far have not been forthcoming.
Generally the impact of towball mass on the rear axle is greater than the actual towball mass, due to the towball being placed well behind the rear axle. We recommend that customers measure a vehicle at a weigh bridge if concerned they will be exceeded their rear axle mass. We do not publish an approved ratio [ of TBM to rear axle load ] due to the many different towball and tongue configurations available.
However, as you can see above it is not hard to calculate for yourself.
Bottom line: the TBM puts a lot of stress on the rear axle, more than the actual weight of the TBM itself.
Kerb & tare weights
As a result of the last article several people wrote in to say there was a definite definition of tare and kerb. Amusingly, each one had a slightly different definite definition, which was the point made in the original article. Here are some example definitions:
- EC Directive 95/48/EC which defines kerb weight as; ‘a car, in ready to drive condition, with the fuel tank 90% full, a driver on board, weighing 68kg and luggage of 7kg [ Euro standard commonly used by European manufacturers ]
- General Motors (Holden) defines Kerb mass as vehicle with full tank a fuel and all fluids, Tare mass is same configuration as Kerb but with only 10 L of fuel, Payload is (GVM-Kerb) [ GM Holden ]
- Kerb weight is the total weight of a vehicle with standard equipment, all necessary operating consumables (e.g. motor oil and coolant), and a full tank of fuel, while not loaded with either passengers or cargo. Also, it is worth noting that any accessories affixed to the vehicle are also considered part of the kerb mass and will thus increase the kerb mass. [ Mitsubishi ]
- Kerb Mass or Weight – The weight of the vehicle including all options, fluids and full fuel tank, but not including accessories.
Tare Mass or Weight – Is the same as Kerb Weight but with only 10 litres of fuel in the fuel tank instead of a full tank. [ Toyota ]
- Minimum kerb weight is the mass of vehicle with additional features, no driver, 10 litres fuel, spare wheel & tools. [ Land Rover, from the Defender specifications ]
- UNLADEN MASS – the mass of the vehicle in running order unoccupied and unladen with all fluid reservoirs filled to nominal capacity including fuel, and with all standard equipment. [ Vehicle Standard (Australian Design Rule – Definitions and Vehicle Categories) 2005 Compilation 4 ]
And there are others. Again, you get the idea…there are many standards, and while there are Australian standards these are not always adhered to by manufacturers, thus confusing what is already a difficult situation even further. It is even known for manufacturers to use differing standards for different models, and certainly over time. Don’t blame me, I just report what’s happening.
Technically, tare weight is the vehicle barely ready to drive with no occupants and little fuel, and kerb weight is the vehicle ready to operate. But the terms are often mixed up, and neither has a definition everybody uses.
Bottom line: when you hear “kerb weight”, “tare weight”, or “unladen mass” find out exactly what it means in terms of fuel and driver/passenger load.
The big mistake with trailer loading
So you have yourself a 3500kg trailer. Let’s say we have the major weights represented by black rectangles.
The TBM is a bit low, so you decide to bring it back in limits by adding or moving weight. Logically, you’d do this by adding weight to front of the trailer.
Lots of weight at the front and back of the trailer. But at least the TBM is now within limits.
Unfortunately, you have totally ruined the trailer dynamics.
There’s a simple test you can do to prove the point. Grab two heavyish weights, maybe two bottles of wine. Stand in the middle of room, and stretch your arms out wide, holding the bottles. Now spin around 180 degrees as fast as you can, stop.
Next, do the same again but clutch the wine bottles to your chest, then spin.
Notice the difference? The total weight is the same, but because it’s all concentrated in one spot it is easier to start spinning and to stop. Here’s the equivalent load on the trailer:
Exactly the same principle applies to trailer loading. A trailer with its weight over the axles is a much easier and safer tow than one with the weight at either end of the trailer. A weight-at-each-end trailer is a nightmare to tow for two reasons, even if the TBM is the same or lower:
- it’ll be hard to turn, and when it does turn it’ll be hard to stop turning.
- it will pitch up and down, disturbing the towcar and reducing the ability of the towcar to maintain traction
Here’s a video by one of the few people that have done research on the topic:
And here’s a diagram to show how weights at each end make the trailer pitch up and down. Not good at all, as that pitching up and down reduces traction on the rear of the car which can lead to trailer sway.
If you need to increase TBM, it is better to shift weight from behind the rear axles forwards, than to shift weight ahead of the axles even further forwards. Weight centralisation should be your goal.
The less centralised the weight, the more TBM you’ll need for stability, and the greater the TBM, the more stress on the towcar and less able the towcar is to turn the rig around corners.
The scientist in the video above did the research and testing to produce a paper on the subject (publically available, but you have to pay for it). Having read it, the conclusion is clear:
It was found that the dominant factors affecting stability were the trailer yaw inertia, nose mass (mass distribution), and trailer axle position. It is interesting to see that the trailer mass alone does not dramatically affect the high-speed stability, as this runs contrary to current guidelines relating to limits on the relative mass of the car and trailer.
In other words, the distribution of weight in a trailer is more important than the total weight. If you want to buy the paper for yourself you can do so here.
If you want a practical demonstration of the effect of trailer weights find a four-wheel-steer trolley and load it like so:
Two 20kg weights should do it. Then push it around corners, and at a brisk pace in a straight line, waggle the trolley. When you’ve finished re-stacking the shelves you knocked over the point will be made.
Bottom line: centralisation of weight is critical, and even more important than total weight. A lighter trailer which is badly balanced may well be worse than a heavy one that is well balanced.
Another reason why rear overhang is evil
A long rear overhang not only increases rear axle loading well beyond the TBM, it also decreases trailer stability both vertically and horizontally.
The diagram below just changes the overhang:
The lower vehicle has a greater overhang, so a little force from the trailer is translated into a big one on the car thanks to leverage.
Exactly the same thing happens if we look at it from the top way:
Ever see someone look at a towcar and remark on how good the small rear overhang is? If you take just one point away from this article how about – there’s more to towing than an impressive torque figure and springs so stiff they may as well be concrete.
Is the TBM part of the payload?
The GVM, or gross vehicle mass, is the maximum a vehicle can weigh. That weight includes the driver, fuel, accessories…everything added to the car from stock standard. The amount a vehicle can carry is its payload, which is the difference between its stock weight (kerb) and the GVM.
The TBM creates a force on the vehicle which is in theory part of the payload. Many manufacturers are clear on this point, for example Toyota. So if you have a payload of 700kg, and a TBM of 300kg, then you’ve got 400kg left over for everything else. Remember that 300kg of TBM is more than 300kg on the rear axle for reasons explained above, but it’s 300kg of payload as the front axle weight reduces in proportion to the rear axle increase.
However, this TBM/payload rule is not always applied. Land Rover, like all other manufacturers, are very clear that the GVM, TBM and rear axle loads cannot be exceeded. But they have this in their manual for the Discovery 4:
” when towing, the maximum permissible GVM can be increased by a maximum of 100kgs provided that the vehicle speed is limited to 100kph.”
And Nissan have this specification for the Navara D40:
clearly stating that the payload can be reduced by a smaller amount than the TBM. Interesting, the Navara NP300 has this:
which goes the opposite way and reduces payload by more than the TBM.
Bottom line: it is best to assume the TBM is part of the payload, even though there are the odd exceptions. Remember that the rear axle load is more than the TBM.
How can I tell if my trailer is safe, legal and within limits?
First be clear – safe, legal and within limits are three different concepts.
What is safe is not always legal or within limits, and what is legal is not always safe. The example above of having heavy weights at either end of the trailer is an example of legal, but not safe.
The instructions below can be used as an approximate method for calculating whether the trailer and towcar are within limits, which is not the same as safe. You will need to amend the process for your specific vehicle and trailer’s limits.
Before you start you need:
- Towcar GVM and kerb weight – on the vehicle weight placard, usually around the driver’ s door. We’ll use 3000kg and 2200kg.
- Towcar payload – this is the kerb weight (refer above for a definition) subtracted from the GVM. Let’s say that’s 3000 – 2200 = 800kg.
- Towcar rear axle load limit – refer to handbook or manufacturer website. We’ll use 1800kg.
- Towcar GCM – this is the Gross Combined Mass, or the maximum that the towcar and trailer is permitted to weigh. This will be in the owner’s manual. We’ll use 6500kg.
- Wheelbase of vehicle – refer to handbook or manufacturer website. We’ll use 2800mm.
- Overhang of vehicle – you’ll need to measure this, refer above for a definition but in brief, the distance from rear axle to the towball. We’ll use 1200mm.
- Trailer ATM – on placard on trailer. We’ll use 2500kg.
- Find a weighbridge. Your local tip might have one, and some servos like larger BPs now run them.
Remember that the above is very specific to vehicles and trim levels. My favourite example is the GU Patrol which for three different cars of the same model year had a braked tow limit of either 2500kg, 3200kg or 3500kg depending on engine or transmission. Also, very often higher-spec cars have lower payloads than more basic trim levels. This can be seen with the Patrol Y62 and LC200 which seat 7 in higher spec models and 8 in lower spec…because if 8 adults got in the top models the car would most likely be overweight!
What we want to know is:
- The TBM we need – which we’ll assume will be 10% of the ATM. A 3000kg ATM trailer = 300kg. This is not a fixed rule, but will work as an example.
- Available payload after trailer is hooked up – subtract TBM from total payload. 800 – 300kg = 500kg. That’s how much payload you have left. If we add the kerb weight of 2200 to 500 we get 2700kg, which is the maximum the vehicle can weigh before you hook up the example trailer. The important thing about towball mass relative to GCM is that it shouldn’t be double-counted; the TBM of 300kg transfers 300kg from trailer to towcar, so the TBM is (generally) part of the payload, but makes no difference to the GCM.
- Trailer load on the rear axle. Refer above for the formula. In our example we have a rear axle load of 398kg.
- Available rear axle load. Subtract the rear axle load from the load on the rear axle – 1800 – 398 = 1402kg left to load on the rear axle.
- Are we within GCM limits? – add the trailer ATM to the vehicle’s GVM. In this example, 2500 ATM + 3000kg GVM = 5500kg. The GCM is 6000kg, so all good. If the total of GVM and ATM exceeds GCM you can still tow, but you couldn’t then load both the towcar and trailer to the maximum. Chances are you can’t even load the towcar to the maximum anyway.
What we’ve learned
We now know the vehicle can weigh no more than 2700kg when towing this trailer, leaving 500kg for the driver, accessories, gear and everything else. And we know the trailer puts 398kg of weight on the rear axle, leaving 1402kg available. If we divide 2700kg by two (front and rear axles) we get 1350kg…very close to what’s left, assuming a 50/50 front/rear weight distribution. Typically, the front axle isn’t rated as highly the rear. And the GCM is within limits, as our car and rig will be under 6000kg.
Remember that most manufacturers say the TBM is part of the payload. However, the weight addition to the rear axle is more than the TBM but that doesn’t matter for payload as there is a weight reduction off the front axle.
At this point you may realise you’re already over the limits, as 500kg is not a lot of payload. Two adults of 80kg plus two kids of 50kg makes 260kg, leaving you 240kg.
If you have a touring 4WD the story gets worse. A bullbar, winch, second battery, roofrack, spotlights, aftermarket suspension and tyres, cargo system and all the rest typically add up to 250-400kg. Which leaves 100kg for the occupants, food, camping gear and everything else. Probably not enough.
However, the above is just a desktop exercise, and there is no substitute for real life measurements.
The exercise is useful to get an initial idea of where you’re at, or if you are considering buying a trailer and towcar. But to check your real-life situation the next step is to set up your towcar as you’d normally tow with it. At a minimum, that’s with the driver inside. Might be with all your 4WD touring gear, horse gear, fishing gear…whatever you’d normally take. Then hook up your trailer, as you’d normally tow it, and go to the weighbridge. What we want is:
- Vehicle weight – weight of the vehicle without trailer, but with all occupants and load. Hopefully that’ll be less than the GVM.
- Trailer weight – weigh the trailer without the vehicle. This should be the ATM, or less.
- TBM – weigh just the jockey wheel. This is not precisely the TBM as the jockey wheel isn’t directly under the towball, but it’ll be close enough. Add 5% to get a bit closer. You can actually find the TBM at home easily enough, and Harding Swift Caravans have described how on this page.
- Vehicle rear axle load – weigh just the rear axle of the towcar, no trailer attached. One way to do this is to connect the trailer, drive only the front axles on, note the weight, drive the rear axles on until the trailer is just about to enter the weighbridge, note the weight and subtract that from the front axle weight.
Note – the front axle is typically less of a concern as it has weight removed from it. Weighbridges are most accurate in the centre so ensure loads are as central as possible. Include all occupants, or make an allowance later for their weight.
What you’re looking for out of this exercise is:
- Vehicle weight without trailer less than GVM.
- Vehicle weight with trailer attached less than GVM (TBM is added to payload)
- Trailer weight less than ATM.
- TBM around 10% of ATM.
- Rear axle load less than rear axle load limit.
- Combined weight of towcar and trailer less than GCM.
If that’s all within limits you may be legal, and may be safe. If it is not within limits you’re definitely not legal, and unlikely to be safe.
Crikey, I’m over the limit! What do I do?
Simple to say but not do – reduce weight, or increase capacity. That means reducing the weight of the towcar and/or trailer, or using a higher-rated towcar.
A weight-distribution hitch (WDH) can help, but remember all that does is transfer weight from the rear axle to the front axle and to the trailer’s axle. The key word is distribution, not reduction. Not all vehicles can use WDHs. And often people will add WDHs, sway devices and the like to mask a poor or overloaded setup. Also, the new trailer stability control (both on the car and the trailer versions from Dexter and AL-KO) is there as an emergency aid, not as a way to tow even bigger trailers.
What else do I need to worry about?
Lots of factors affect the safety of a trailer/rig combination. Aside from the weights discussed here there is also:
- Tyres – good condition, and with trailers there is a real danger of aged tyres because many trailers are not often used. We have covered this topic in detail with stories about aged tyres from our readers
- Tyre pressures – appropriate to the trailer, which is rarely the same as being set to the same pressure as the towcar. More on that in future articles.
- Suspension – both the towcar and trailer need quality suspension that is not worn.
- Driving techniques – huge impact on safety, and trailers take a skill all of their own.
What are other countries doing?
Sensibly, the UK has from 19th January 2013 brought in a towing license. Here are the details:
From 19 January 2013, drivers passing a category B (car and small vehicle) test can tow:
- small trailers weighing no more than 750 kg
- tow a trailer over 750 kg as long as the combined weight of the trailer and towing vehicle is no more than 3,500kg Maximum Authorised Mass (MAM)
If you want to tow a trailer weighing more than 750 kg, when the combined weight of the towing vehicle and trailer is more than 3,500 kg, you’ll have to p ass a further test and get B+E entitlement on your licence. You’ll then be able to tow trailers up to 3,500 kg
Sound like a good idea for Australia? I think so. We’re not born knowing how to do much more than breathe and scream, certainly not tow. We do have the biggest and heaviest trailers in the world, with the possible exception of the USA – but they have F250s, 350s and the like to tow them.
How good are the big trailers?
As far as I can see, some manufacturers of large trailers basically build a block of flats with a drawbar, sling wheels underneath and slap a pricetag on it. Take a look at the heavy components of a trailer such as fluid tanks, bathroom, kitchen, gas bottles, spare wheels. Are they centrally located?
There’s good reasons for them not to be central, but that doesn’t make it right. I have seen little evidence of design for weight distribution, cornering dynamics or braking. Or any formal, scientific testing in real-world conditions like car manufacturers do.
I’m issuing a challenge to be proved wrong on this point.
If any trailer manufacturers read this and would like to provide evidence of trailer dynamics design and test let me know, and we’ll work on an article highlighting what you do and how you do it. Over 100,000 people have seen posts on this topic, so you’ll get your message across.
Also, it is not enough to say you use independent suspension or some such impressive feature. Unless properly designed, calibrated and tested design features by themselves mean nothing. It is better to have a well designed, live-axle leaf spring system than a poorly designed independent coil spring system, even if the shocks are from a big-name brand.
Thanks to all the automotive PR people who responded to yet more of my arcane technical questions, Harding Swift Caravans and John Eggenhuizen of Tow-Ed for their assistance with this feature. Both Harding-Swift and Tow-Ed have said they are happy to take calls from owners about trailer setup, or you can comment below.
You may also like to calculate your caravan’s towball mass using the interactive Ball Weight Calculator.