Car Advice

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.”

 “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:

 pigdogtrailer

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:

 diag2

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:

 t-top7

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:

 overhang2The 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:

defender-weight The 90, 110 and 130 all have different wheelbases and overhangs, hence 150kg of TBM has different effects on the rear axle.  The * refers to this:

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.

Holden said:

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.

 trl1

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.

trl2 

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:

trl3

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.

pitch-weights 

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:

dynamic-trolley

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:

overhang-pitch

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:

top-overhang

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:

navara-d40

clearly stating that the payload can be reduced by a smaller amount than the TBM.  Interesting, the Navara NP300 has this:

np300

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:

  1. 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.
  2. 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.
  3. Trailer load on the rear axle. Refer above for the formula. In our example we have a rear axle load of 398kg.
  4. 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.
  5. 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:

  1. Vehicle weight – weight of the vehicle without trailer, but with all occupants and load. Hopefully that’ll be less than the GVM.
  2. Trailer weight – weigh the trailer without the vehicle. This should be the ATM, or less.
  3. 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.
  4. 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.

IMG_1424
You generally get a shock when your car is on the weighbridge. Mine has a load in the back and a driver, but even after that’s been allowed for it is still well over the kerb weight. Good thing it is a ute with a large payload!

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.


  • Fantastic article – great advice!

  • Pingback: Big Toy Horse Trailer And Truck | Trailer()

  • Jason sabeeney

    Great article Robert, the best and most comprehensive I have seen on towing yet. You have very clearly articulated what can be a technically confusing subject for many. I like your challenge for trailer manufacturers to show evidence of scientific research around their trailer design and stability. I have no doubt many Australians are ignorant to the fact they are loading their vehicles and towing well beyond safe and legal limits. This is not assisted by new vehicles manufacturers all competing to promote their 3500kg towing limits when many vehicles are not safe when loaded to that capacity. It also seems crazy that manufacturers can’t follow the Australian standards for kerb mass and unladen mass as this further confuses the issue. Once again congratulations on a very informative article.

  • Pingback: Best Vehicle Towing Boat Trailer | Trailer()

  • Pingback: Alabama Car Trailer | Trailer()

  • Pingback: Entrepreneur finds niche in accessorizing work gloves (+ VIDEO)()

  • Pingback: 3 Axle Dog Trailer | Trailer()

  • Pingback: Al-ko Trailer Axles | Trailer()

  • Pingback: 2005 Toyota Solara Trailer Hitch | Trailer()

  • Pingback: How To Calculate Easton Arrow Weight | Fix How To()

  • Pingback: How To Calculate No Load Current Of Induction Motor | Fix How To()

  • Mandrake51

    Hi Great article – thanks. I may have missed it but I would like a more in depth article about the difference between 5th wheelers and caravans and how each can be towed behind a dual cab 4WD – is that possible?

    • It is not on the list but may be covered in future.

  • Pingback: What Is Atm Tare And Gvm | los-angeles atms index()

  • Pingback: Atm Australia Trailer Manufacturers P/l | guide - how to use atms()

  • Pingback: Difference Between Atm And Gvm | find atms abroad()

  • Pingback: Atm Australia Trailer Manufacturers | bank atm guide()

  • Pingback: Gvm Atm Tare | local atms guide()

  • Pingback: Gvm Atm Tare | locate an atm()

  • Pingback: Difference Between Atm And Gvm | local atms guide()

  • Pingback: Atm Gvm Tare | map - atms near you()

  • Pingback: Truck Towing A Trailer Destroys Atm | learn - how to use atms()

  • Brett Allen

    FYI in Australia the maximum towing speed is 100 kmh.

    Just thought i’d throw this in because every idiot on the Kwinana freeway this week obviously doesn’t know it.

    • In some states it is 100 eg WA, in others there is no specified limit so the normal limit applies.

      • Chris

        For NSW the speed limit for a braked trailer is 80 kph. Also if the combined GVM is over 4500kg, you need an heavy vehicle licence ( medium-rigid))

  • Pingback: Atm Australia Trailer Manufacturers P/l | finder - atmlocator()

  • Max Bancroft

    Excellent. Thank you.

  • Pingback: What Is Atm Tare And Gvm | atmbusiness()

  • Pingback: Amend Practical Driving Test Booking | lunchtime - booking driving test()

  • Pingback: Atm Australia Trailer Manufacturers P/l | eftposexplained()

  • Pingback: Atm Gvm Tare | worldwide atms()

  • Pingback: Gvm Atm Tare | rent -atmbusiness()

  • Pingback: Truck Towing A Trailer Destroys Atm | information - eftposexplained()

  • Pingback: Uk Driving Licence Towing Trailer | nice - driving schools()

  • Leif

    Sorry I am late but I just read the articles Why a 3500kg
    tow rating may not really be a 3500kg tow rating and Everything
    you need to know about towing heavy trailers.

    Firstly, it is the first time anyone has mentioned the
    correct weight distribution hence an excellent article.

    In addition to the limits of the axels there are the limits
    of the tyres, my caravan is single axel with light truck tyres hence the limit
    is in the axel but my car has “sport performance” tyres hence the limits are
    the tyres not the axel’s.

    The next step is to address the weight distribution hitch
    when fitted.

    Firstly, it adds about 25kg to the ball weight hence the new
    weight is 130 + 25 = 155kg in the sample given

    This will increase the car rear axle load by 225kg and reduce
    the front axle load by 68kg.

    But then when using the load equaliser, the tow car will be
    brought back to original level but with an addition weight on each axle of 47kg
    for balance but to achieve this an additional weight is required on the caravan
    for leverage hence an increase of 62kg (my caravan axle to tow ball is 3.9m)

    I did write a program several years ago that also included
    the tyre pressure required

    As it is a estimate it shows only whole kg

    • Brilliant, thank you for your post!

      • Leif

        In a well written article like this there is no room for opinions hence we should only show facts.
        Added information of the estimated impact of a WDH

        But to do that we need some specific facts used in the sample
        Caravan Golf GTX Tourer
        Total weight fully loaded including tow-ball 2300kg (for this sample real 1335kg)
        Tow-ball weight 130kg
        Distance axle to tow-ball 3.9m
        Tyre 185/14
        Car Ford Falcon BF II 6speed auto sports suspension
        Total weight 1724kg equally distributed over front and rear axle 862kg
        Distance tow-ball to rear axle 1.4m
        Distance tow-ball to front axle 4.2m
        WDH EZA-lift

        Now with simple maths we can look at extreme values to give indicative picture of operation (in real life the vehicle may break before the values are achieved).
        1. Lift the front wheel of the ground with no jack
        a. Put a load on the tow-ball of 1725kg and the front axle will have a load of 0kg hence of the road
        2. Lift the rear wheel of the ground using the WDH
        a. Lift the levelling bars to give a compensation of 586kg and the load on the rear axle is 0kg hence of the road
        Rather than recalculate we can use the above figures
        A tow-ball load of 1725kg lifts all 100% of the front axle load 862kg
        Then:
        – Half or 50% of 1725kg lifts half or 50% of 862kg or 431kg
        – We can offcourse then calculate the % of the tow-ball load used 130kg and then apply that to the loss on the front car axle 7.53% of 862kg or 65kg

        We need to make sure the estimated 130kg tow-ball load is including all known loads and in my case there are:
        – The two bicycles mounted on the A-frame
        – The WDH mounted under the tow-ball

        All of these estimates is for the vehicles being parked so how about actually driving
        – When you go down a hill and apply the electric brakes on the caravan the brakes would not work if they could not stop the wheels from turning they do that by levering on the tow ball
        We know the weight is 2300kg and we know the tyre diameter and we know the distance from the axle to the tow-bar hence a simple task to estimate the maximum possible load on the tow-bar 178kg
        As we started with the static 130kg we need then to add the 178kg for the then total load on the tow-bar 308kg

        We can now go back up and use the estimates for the impact of the loss of weight on the front axle for that load
        And as a result we can see we will have a total front axle load at that time of 708kg from original 862kg a loss of 154kg

        But as we all know bad things come in 3
        – The reason we were breaking hard was for
        o We did see the pot hole late in the dark
        o It is a relative sharp bend in front
        The impact of hitting a pot hole and going through a sharp bend at the same time is only something an experience driver can explain and with the caravan pushing did the front wheels’ slip? but we all know what it is like when the oncoming car is overloaded and the lights points straight into our eyes.

        This then brings us the question what is the WDH doing to our weight on the tow-ball.
        Some says it transferee a small percentage in fact it transfers 100% of the weight on the tow-ball and distributes it over the axles hence the name Weight Distribution Hitch if we look at the sample it transferred 41% to the caravan axle and 29% to the car rear axle and 29% to the car front axle and left zilch or 0 on the tow-ball.
        This is for the stationary weights but a good quality WDH should also cope with the increases caused by driving but may not cover it 100%.
        This is when a car not suitable for caravan towing may not cope with the stress and the manufacturer recommends no usage of a WDH and encourages you to drive with the loss of weight on the front axle

        There are only two reasons for not using a WDH when towing with your family car
        1. You do not tow above walking speed
        2. You do not know what you are doing and is an accident waiting to happen

        If your car has self-levelling suspension it has no impact on the above calculations the only difference is your head lights do not point up but that is because the rear end is lifted but no weight is put back on the front axle and your car remains out of balance.

        The law of physics and the maths is simple but to not have to do the calculations I did write a simple estimate program in VB hence only working in Windows.
        I fill in the known facts and the estimates arrive in the picture

        A sharp eye will see that my van axle weight is 1205kg but the one in the calculation used was 2170kg to maximize the impact of the breaking sample in fact I should not have used my figures but the maximum tow-ball weight allowable for the car 230kg and the total tow-ball weight when breaking would have been 230kg plus 178kg a total of 408kg and the front axle then would have been 658kg a loss of 204kg

        This raises the question is the Ford limit the total limit or the static limit
        With exclusion of extreme incidents, we should expect dynamic additions of say 130kg hence is the Ford then only recommended for caravans with 100kg tow-ball weight?

    • G H

      A small correction here A WDH does NOT affect ball weight because it is attached to the tug and not the van. It adds to the payload of the tug only

      The ball weight of a trailer is weighed disconnected from the tug and is whatever weight that it is ALWAYS. The only variance is that when you pull up a WDH you distribute a small % of the weight forward onto the front axle and a small amount rearward onto the van axle. It does NOT alter the original weight.
      By the action of pulling up the bars you apply more FORCE to the joint but do not increase or decrease the weight. Only a Jack or a skyhook can do that.

      Vehicle weight limits are set by a manufacturer to cover a wide range of situations and as a lot dont use a WDH there is no tolerance for the use of one.

      The only time is when one is specified for having a tow weight over a certain limit. EG Ford heavy towing packs.

  • G H

    Here are some questions for you Mr Pepper. Firstly the law says you may not load your vehicle beyond its GVM so how can you say to load it to GVM and then add ballweight as long as it doesnt exceed rear axle limits. That makes it illegal whether its a Ford or a Landrover

    Another question. How do you make out that the GCVM is the sum of the ATM and GVM Its not. Its the Sum of the GVM and the GTM otherwise you are counting the TBW twice.

    If you dont believe me try this example.

    Go to a weighbridge with your rig and I will use these figures as an example GCVM 6000kg GVM 3200kg ATM 3100kg

    Ballweight 300kg

    Drive the whole rig on the bridge 6000kg

    Back the van off weigh the tug 3200kg. INC BW

    Pull forward weigh the van 2800kg

    Disconnect the van and reweigh it 3100kg

    Reweigh the tug on its own 2900kg.

    So if you add the GVM and the ATM you will get 6300kg because you have weighed the ballweight twice

    So you are factually incorrect.
    The towbar photos you have with lessening weights with higher ball weights means that you must reduce load accordingly to stay within GVM. Eg with 150kg within GVM standard limit

    To go up to a 250kg BW you must unload 100kg of gear and for 350kg BW 200kg of gear.

    WHY Because its illegal to exceed any vehicles GVM

    From your previous article

    “Be aware of your weights as there are legal, insurance and warranty implications if they are exceeded;”

    • Hello G H, thanks for your post.

      The article discusses what some manufacturers specify and say in their documentation. That doesn’t constitute a recommendation. In fact, the article ends with:

      “Vehicle weight with trailer attached less than GVM (TBM is added to payload)”

      GCM is GVM plus ATM. GCM does not make a differentiation as to where that weight is carried, which is where you’re going wrong.

      The towbar is very clear, it says for a given TBM the weight below GVM should be reduced, but not by as much. That’s unusual, hence the photo. It doesn’t carry a recommendation, the recommendation is as above.

      “Remember there is usually some rear axle load left once the vehicle is at GVM. That can be used for towball mass'”

      Yes I’ll need to correct that. It should have read:

      “Remember there is usually some rear axle load left once the vehicle is at GVM. That can be used for towball mass provided the load is reduced to keep the vehicle below GVM.'”

      • G H

        Did you read the example of what I posted. Its pretty plain. If the load is reduced back to GVM with a higher tbm you wouldnt have to worry about axle mass as you would be back where you started
        This has been debated extensively on a van forum recently and with input from a dealer an engineer and owners and all have come up with GVM and GTM as what makes up GCVM.

        Put it this way You load up the tug to its GVM of 3200 kg and then your van has an ATM of 3100kg Where is your ballweight going to disappear to as it cant go on the tug as its already at Maximum legal weight.

        • Think of it this way – take both trailer and towcar and put both on a weighbridge. The maximum weight there is the GCM.

          The split between towcar and trailer is not relevant to GCM. The split is highly relevant to ATM, GTM and GVM, and TBM, and rear axle loads.

          There are vehicles with GCMs less than their GVM + ATM, and vehicles where that’s equal which is mostly, but not always the case.

          In this example:

          “Put it this way You load up the tug to its GVM of 3200 kg and then
          your van has an ATM of 3100kg Where is your ballweight going to
          disappear to as it cant go on the tug as its already at Maximum legal
          weight.”

          the error is loading the car to its GVM first, then adding the TBM. The correct way would be to load the car to GVM – TBM, then add TBM to get the car to GVM.

          The use of “GVM” does not mean the vehicle is loaded with luggage to GVM, it could and would be also loaded with TBM.

          • G H

            Finally we are getting somewhere. As you say we should allow for TBW in GVM I agree. So why add the ATM to get the GCVM as you already have the TBW in the GVM so why are you adding it again in the ATM That is double dipping. The example of my weighing gives the figures with the TBW in the GVM

          • I’ll say it again – the GCM is how much the total weight of car+trailer can be. There is nothing in the definition about where that weight has to be distributed – towcar or trailer. The TBM for GCM is a furphy as it’s included in the vehicle’s mass.

  • Kim West

    Hi Robert, I know I’m late to the party, however I was wondering if you have or could do an article on towing horse floats. Like the variety of tow vehicles available, there are just as many different floats now as well. The float I tow has a tare of only 860kg. Some friends have 4 horse angle floats, some have double straight load floats, and some have goose neck trailers.

    Thanks heaps,

    Kim

    • Hi Kim. I don’t know anything about horses, just trailers in general. If there are any horse experts I’d be happy to work with them on a trailer. I imagine that horse trailers are a bit different – they look to have the weight quite well centred, unlike the average caravan, but on the other hand are heavy and the horses can move.The basic principles described here apply.

  • Jacqui Sharkey

    Hi I am so confused and hopeless at Maths. Who could you recommend to get advice on the right vehicle to two a 4 horse float (roughly 1400kg) plus 4 horses (roughly 2000kg) plus four people plus gear. I am thinking a Landcruiser but would it be too much. Would a land cruiser 70 series twin cab be better with a gooseneck (roughly GTM 4800kg Tare 2900kg) plus 4 horses (roughly 2000kg).

    • Hi – 1400 + 2000 = 3400, so in theory the Cruiser would be ok. However, then you want four people plus gear, which is likely to be 80 x 4 =320 plus say 80kg of gear = 400kg of payload. The 3400kg trailer will need a towball mass in excess of 300kg, so 300 + 400 = 700kg of payload, more than a 200 Series can offer, but just the 70 twincab should work, just. All this is a dependent on other factors such as the towbar rating, rear axle load etc – bottom line is might be ok but only way to know for sure is to have a towing specialist assess your specific combination after a weighbridge check.

    • Hi – 1400 + 2000 = 3400, so in theory the Cruiser would be ok. However, then you want four people plus gear, which is likely to be 80 x 4 =320 plus say 80kg of gear = 400kg of payload. The 3400kg trailer will need a towball mass in excess of 300kg, so 300 + 400 = 700kg of payload, more than a 200 Series can offer, but just the 70 twincab should work, just. All this is a dependent on other factors such as the towbar rating, rear axle load etc – bottom line is might be ok but only way to know for sure is to have a towing specialist assess your specific combination after a weighbridge check.

      • Jacqui Sharkey

        Thanks Robert for your help. I am guessing from the figures in your article and some basic calculations that I did, a Ford Ranger would not do the job. To calculate the “pulling power” for a gooseneck horse float is it the same calculations? Thanks

  • Dean

    Hi, you touched on it with the discussion of utes, their overhangs and relative wheelbases, but is there any chance you’d be able to source and discuss some figures for the ratio between overhang and wheelbase, perhaps some vehicle comparisons (manufacturers don’t state overhang specs so it’s hard to work out) and discuss it’s relative importance for choosing a tow vehicle, or indeed choosing a tow hitch? Thank you!!

  • Graham Walker

    I think the “industry” needs to take a good look at itself. I have heard of too many accidents involving very big Caravans. A “block of flats” with a tow frame is a great description of some of them. Also the extra endorsement on your license is a brilliant idea. You see a lot of people who tow a 6×4 trailer hook up a 21′ van and think it`s the same……nope, not even close!
    Great article.

  • G H

    One thing that has not been mentioned in either article is this rule
    The max towing capacity of a vehicle is the max limit imposed by the manufacturer or the towbar

    WHICHEVER IS THE LESSER

    To explain A tug with manufacturer limit of 1800 kg fits a 2200kg towbar. Limit is 1800kg

    A tug with manufacturers capacity of 2500kg fits a 2000 kg capacity towbar Limit is 2000kg
    EG a 1995 80 series Cruiser had a capacity of only 2500kg but there was a 3500kg towbar that fitted. Toyota refused to uprate the vehicle.

Robert Pepper

Robert Pepper

Robert Pepper is the editor of PM4x4, an offroad driver trainer and photographer interested in anything with wings, sails or wheels. He is the author of four books on offroading, and owns a modified Ford Ranger PX which he uses for offroad touring. His other car is a Toyota 86 which exists purely to drive in circles on racetracks. Visit his website: www.l2sfbc.com or follow him on Facebook https://www.facebook.com/RobertPepperJourno/