Towing 3500kg with your 4WD ute: Worst idea ever

Think it’s a great idea hitching 3.5 tonnes to your 4X4 ute, loading up with gear, and your kids? You’re in extremely dangerous, potentially illegal territory. Here is why.

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Is your ute-trailer combination an illegally overloaded one? This report is inspired by a dude named Harvey Singh, a prospective Dingo Piss Creek visitor. 

Should you fail to get the details right here, and just stick a dirty big and insanely heavy trailer on the back of your ute, you are going to be a ticking time bomb out there on the road, among perfectly unsuspecting and innocent motorists and their families.

Not to mention your own. Let’s look at the fundamental problem and how to avoid it.

Here’s the question as Mr Singh put it to me:

Harvey, and all you other over-excited heavy haulers, that’s just not going to work. Here’s why.

The fact is utes and 3.5-tonne towing are a match made in engineering hell, because it’s entirely impractical and manifestly unsafe. Most ute manufacturers will tell you 3.5 tonnes is possible - they celebrate and advertise it, even. But in practise, you can’t, and the numbers don’t lie. If you attempt it, you are (unwittingly perhaps) a moron.

The solution to Harvey’s problem, and yours, potentially, is blindingly obvious.

D-Max is kinda okay; it’s pretty agricultural and brand/dealer support is mediocre, and they absolutely milk the claim about ‘truck-like’ toughness and reliability for all it’s worth. It’s just trying to marry up the Isuzu truck brand’s image with its passenger utes - which are definitely not in the same league. Not even the same sport.

In reality, Isuzu Ute Australia is a separate company to the trucks. And the engine in the much-hyped D-MAX is a disgracefully geriatric powerplant, far better consigned to the role of ‘museum exhibit’ in my view.

It’s essentially a brand new 1999 Holden Jackaroo diesel engine. (Remember them?) Isuzu Ute is just maintaining the hype, courtesy of the marketing department. Isuzu fan boys can argue the toss, but despite the quarter-century gap, these engines are, essentially, the same.

The architecture lives on. It’s like, on Isuzu-World, engine design peaked at the end of the 20th Century, and there was no point investing any more cash redundantly in R&D.

If you want to pay $70k for that, knock yourself out. But remember, Isuzu trucks are a very different breed of workhorse. 

Plus, mounting that $100k 3.5 tonne trailer onto the 50mm towball of a D-Max is a very different towing assignment than putting it on the back of an Isuzu NPR - a real truck.

But that 3.5 tonnes - you’ll be doing it illegally in any of the common utes. You can only hypothetically tow 3.5 tonnes - and if you do, you can’t carry the family, or even a cut lunch. And the manufacturers of these vehicles should hang their heads in shame for collectively suggesting not only you can do this, but that doing so is an excellent idea. Because that’s grossly irresponsible in my view.

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Towing the seeds of disaster: Why heavy trailers crash

There are two kinds of trailer, essentially. Dog trailers and Pig trailers.

A dog trailer has axle groups at the front and rear of the trailer and that location makes it very stable at speed and ideal for extremely heavy loads. Hence they’re used for trucks, primarily.

A pig trailer has its axle group right in the middle, which gives them certain dynamic compromise, and limits them to low static loads. Used for cars and utes, for light duty.

There are three planes these trailers operate under. Pitch and yaw are the primary ones, and roll is the secondary plane.

When you drive along with a dog trailer it’s quite restrained on the plane of pitch, which is what happens when a truck hits the brakes - the whole trailer pitches forward.

With a pig trailer, the central axle group means hitting the brakes forces the trailer to act in a see-saw motion and pitch forward, which alters how it travels on the road, while also pushing down on the tow vehicle’s towball, lifting the front axle, reducing steering accuracy and effectiveness of the brakes. That’s bad.

If you swerve while towing a pig trailer weighing 3.5 tonnes, you effectively have a pig heavy pendulum pushing the tow vehicle, like your D-Max, of course. And if you have to swerve to avoid a serious crash, again, that central axle group design means you’re going to introduce additional yaw, where the trailer wants to push sideways. That’s also bad.

Then there’s the secondary roll moment, where the whole entourage wants to tip over because the centre of gravity is high enough on an object heavy enough to do that. The whole family ends up on the roof. That’s very bad.

Why do we use trailers like this? I suspect it’s because pig trailers are easy to make, and for light duty jobs they’re fine. But over time, they’ve grown bigger, and heavier and the regulations have not really changed to kerb that evolution.

Failure mechanisms

There are failure mechanisms all around us, designed to stop rooves collapsing on top of us, preventing bridges from collapsing and keeping the world around you from crumbling into the sea. Isn’t it nice that a bunch of brainiacs have put all these systems in place to make sure you’re kept safe?

Let’s carry that philosophy into utes towing trailers. These same protections are systematically baked into your trailer, and vehicle you use to tow it. 

There are powertrain failure mechanisms like transmissions overheating or gearboxes not strong enough to restraint the kinds of inertial loads when configured with 3.5 tonnes on board. There are brake failure mechanisms, where the brakes can’t reject heat away quick enough under heavy braking applications. But overall, these failures aren’t as big a problem as they used to be, brakes and transmissions are pretty good these days.

The biggest issue to be wary of here, is dynamic instability in pitch and yaw - this remains the biggest potential for disaster. Sometimes known as the tail wagging the dog.

You do not want the trailer wagging the car - avoid it at all costs - just like engineers try to avoid at all costs the failure mechanisms in steel beams when building a skyscraper or a bridge.

You need to think about this predisposition for a heavy trailer to be unstable in pitch and yaw.

Instability mitigation

You want to be safe. You need to keep your family safe, and everybody else’s family, out there on the roads.

The best way to mitigate this disaster waiting to happen, is to minimise the mass disparity on the trailer.

This doesn’t really apply to short trips like to a local boat ramp, because you’re probably not going to reach high speeds.

But at highway speeds of 80km/h, 100km/h or 110km/h, you have acquired a huge amount of energy. When that energy is dispersed or transformed in a short period of time, that’s bad. 

When the tail nudges the dog, the only thing keeping you stable is the traction available through the tyres.

There are geometric deficiencies when driving off-road or unsealed rural roads with 3.5 tonnes. Hitting a hole or trench or a rock or standing water - whatever - you’re going to bleed that energy away very quickly, which is going to damage something.

You cannot realistically take 3.5t for severe off-roading. Pig trailers are not designed to pitch, roll and yaw in extreme ways. Sure, off-road trailers with a treg hitch help in these situations, but they remain limited in how much punishment they can take.

Just because the specs sheet says you can tow 3.5 tonnes, that does not make it safe to do so. You need to keep the weight slow as possible to reduce the amount of energy you carry in your entourage - the more energy, the harder it is to keep under control.

Compliance Pt. 1

Compliance is an important issues and there are two aspects you need to be aware of.

Use a public weighbridge at every opportunity to keep track of what weight you’re carrying and how much you have to play with - and that doesn’t mean load it up to the limit.

There’s GVM (Gross Vehicle Mass) and GCM (Gross Combined Mass).

GVM relates to the vehicle doing the towing: LandCruiser, Patrol, D-Max, Triton - whatever. The total weight of the fully loaded vehicle must not exceed the GVM, by law. If they police weigh you with their portable weighbridge, and you’re over, you’re getting a fine and you won’t be allowed to proceed.

To found out what your vehicle is prospectively going to weigh, start by looking at the specs of your vehicle. You start at the kerb weight, add your passengers, your luggage, the towbar, any accessories, and - crucially - you must include the towball download weight, which is generally 10 per cent of the trailer, in this case 350kg.

Just because you add all these weights together and it adds up to something like 2840kg, with a GVM of 3100kg (like on D-Max), that doesn’t mean you’re good to go.

Compliance Pt. 2

GCM is Gross Combination Mass, and it’s one of those limits not to be exceeded.

It’s the grand total weight of the loaded trailer and the grand total weight of the vehicle, also fully loaded.

When calculating GCM, do not add the 350kg towball download twice, because that figure is simply taken off the trailer’s weight. Simply add trailer, plus vehicle. But you do need to include the towbar.

If your trailer weighs 3.5 tonnes, plus the vehicle’s kerb weight at 2130kg, plus the 50kg towbar, you’re only left with 270kg of legal payload to use, without even putting people in the vehicle or accessories or luggage, or even any equipment. Your family is going to weigh up to or more than 300kg and that’s a big, fat problem.

When you look at GCM compliance, it’s almost impossible to comply with the limits while towing a 3.5 tonne trailer and doing anything practical with it.

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Some of us still live in the real world where the facts really matter - even the inconvenient ones. Especially the inconvenient ones. And these facts are staring you in the face.

Carmakers should hang their heads in shame advertising the virtues of towing 3.5 tonnes and being able to take payload in the tray and seats.

Pro Tip: Ignore that misleading claim about taking everything + 3.5 tonnes of trailer.

Pull the other one, Isuzu Ute. You cannot put an average human adult in four of the vehicle’s five seats, with a 3.5-tonne trailer behind. In fact, you need to leave just about everything out of the ute itself if you tow 3.5 tonnes. 

With 80 kilos per person on board - which is roughly average in North America and Europe - that’s 320 kilos of flesh, plus 50 for the towbar plus 2130 for the D-MAX X-Terrain, plus 3500 kilos for the caravan out the back, that equals 6000 kilos even. That’s 50 kilos over the D-Max’s gross combination mass. Before carrying a single item in the cab or the tray (such as Jerry cans, water containers, recovery gear), or fitting a single accessory, such as roof rack, driving lights, second spare wheel and tyre, compressor, a pedestrian-shredding, crash safety-compromising bullbar.

Therefore, that quote above, from Isuzu, is a disgraceful misrepresentation of the reality of towing 3.5 tonnes with that vehicle, in my estimation. It’s from isuzuute.com.au/d-max/towing, today. I’ve archived it, in case they change it.

‘You don’t need to leave anything behind’? You need to leave virtually everything and everyone behind.

To be fair, D-MAX is probably a better choice for heavy towing than Navara, which is a proper heavy towing shitbox, owing to the grubby compromises Nissan did with Mercedes-Benz, to platform-share the ill-fated X-Class with Navara. Well done there, another great call, Nissan.

Special mention to Mitsubishi, which declines to participate in the 3.5-tonne arms race, and limits the tow capacity of Triton to 3.1 tonnes - far more reasonable and practical. Not to mention safe.

I’m just a mechanical engineer - with no axe to grind, except that I don’t want to see well-meaning people like Harvey and his family set off on the big adventure only to find themselves parked on the roof, hours from a trauma centre with traumatic brain injuries thanks to a high-mechanism crash.

Such a horrific crash can be traced back, in my view, to some marketing executive’s ego telling the board they can sell more utes if Product Planning would just agree to let their ute tow 3.5 tonnes.

My strong advice about towing 3.5 tonnes is: Don’t. Or if you do, buy a 4WD truck, upgrade your licence and tow that big, heavy van, with the kind of big, heavy tow platform which will remain stable with such a load slung behind.

An Isuzu NPS will move 3.5 tonnes every day of the week, including public holidays and RDOs - because it’s designed to - safely.

Seriously, buy an actual Isuzu truck. They come in all shapes and sizes, with 4X4, long wheelbase, quad-cabs, with tray packs, automatic transmissions, and you can even have that more appropriately de-tuned 3.0-litre 4JJ engine - or you can have a 5.4-litre version with GVM and payload figures that leave a D-Max for dead. 

An Isuzu NPS murders a LandCruiser 300, a Ford Ranger, VW Amarok, Pajero Sport and even flips the bird at a RAM 2500, every day of the week.

If you’re not prepared to buy a truck and do this 3.5 tonne towing assignment properly, buy your ute. Just don’t tow anything heavier than 2.5 tonnes. Because physics doesn’t give a shit whether you live or die.

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