The applied physics of heavy towing
If you’re thinking about towing an extremely heavy load, or even a moderate-size trailer, this is the science behind why and how it can go wrong - and how to stay safe…
This report is aimed at you if you are towing a heavy trailer like a boat, a camper or caravan, or even if you're contemplating doing so for the first time.
Towing, by nature, is a potentially dangerous task, where a vehicle with a great deal of acquired kinetic energy is in control of another. There are so many ways this situation can go wrong, and the most common way it does, is by loss of stability.
What is stability?
Stability sounds like a pretty boring term, but it’s the most important aspect of towing. Towing should be an event-free task - you don’t want anything exciting happening while you’re towing. The way you achieve this is through mitigation; reducing risk and avoiding disaster. But you have to do this constantly.
Some of the first intentions of a ute owner is to modify it to ‘cope’ with heavier loads, so let’s dispose of the non-issues first, starting with height, suspension and upgrades.
Obviously these things do play into stability, but they’re not primarily related.
If the suspension is not deficient, don’t go ‘fixing’ a ‘problem’ which doesn’t exist. Don’t go modifying your suspension, because you probably don’t need to.
Height of the vehicle is not really relevant to stability, unless you change it (with some kind of lift kit or modification) to be higher than normal, which will make its stability worse. Again, don’t change it; it’s probably unsafe and pointless, and makes the vehicle worse.
Okay, so let’s get to the five main issues here, relating to stability when towing.
1. Grip
Essentially the vehicle is not a free body, because it’s anchored by four contact points at the tyres.
Tyres are a big deal - especially pressures, their condition, and their maintenance. Also, the kind of tyre you use matters.
Many people put knobbly off-road tyres on their caravans and campers, which is only going to make it less stable on freeways and back roads because the chunkier the tread pattern, the less rubber is connecting with the road in the majority of driving. Prioritise the tyre performance for the 90 per cent of the driving you’re doing, where the trailer needs utmost stability - on public roads. All-Terrain tyres are the best option here for all-round grip with sealed roads and dirt roads. You cannot improve stability and performance on the highway (90 per cent of your driving at 80-110km/h) and the bush (at low speeds).
Also, don’t tow 3.5 tonnes off-road. And on dirt roads - slow down. It’s very simple; reduce the amount of energy you impose on the vehicle and trailer, by just going slower.
2. Wheelbase
Track often gets included when talking about wheelbase, but they’re both kinda irrelevant. Track has an influence, but virtually all vehicles are pretty identical; they’re all kinda the same when it comes to track.
Wheelbase also has an impact, but all the utes and 4WD wagons are quite similar, in relation to their effect on towing stability.
Nobody will argue a 300 Series LandCruiser is a lesser towing vehicle because it has a shorter wheelbase than a Ranger, and nor is Ranger a poorer tow vehicle because it has the longest wheelbase of D-Max/BT, Triton, Hilux or LC.
Length between these vehicles are all very close, so it’s also pretty irrelevant.
The biggest determinant of stability is their overall kerb weight.
3. Relative size & mass
It’s important to have a much heavier prime mover relative to a trailer, when it comes to utes and towing. You have greater stability when that relativity is greatest.
Of course, trucks don’t work like that, because they’re designed completely differently, with groups of axles and wheels right at the ends and corners, there’s maximum possible weight over the driving wheels of the prime mover and the whole combination is designed specifically to carry extremely heavy loads.
Utes and caravans are not designed like trucks, B-doubles, dog trailers et al. Towing is a complete afterthought when designing a ute. The ute’s primary purpose is to haul light loads, very well. That is its mission, like the T-800 in Terminator 2: Judgement Day.
Overwhelmingly, the LC300 is the best tow package with the most grunt and the most kerb weight, so it is therefore the most stable tow vehicle. But it is also overwhelmingly the most expensive.
4. Load distribution
What really matters when towing with a ute is load distribution.
You have lots of different masses, when loading your ute and trailer, and it’s highly important where you locate them.
It’s more important to put heavy stuff between the rear wheels in the ute’s back wheels, which is often hard to do in utes. This is due to the design of utes where often the rear wheels are underneath the rear of the cabin, meaning they’re at the front of the loadspace, not in the centre; they abut the cabin.
Where you put the load in the back matters, and often it creates a laziness with ute owners who can’t be bothered putting the heaviest items at the front of the tray.
How you load your vehicle is pivotal to stability.
5. Rotation ( ‘I’ - in scientific terms)
Linear movement is relatively simple: An object has force applied upon it, so it accelerates forward, which is called displacement. Linear movement is up/down, left/right, forwards/backwards.
As soon as you start learning about angular movement, things start to get complicated, especially when it comes to towing.
It’s all about Newton’s second law, which is arguably the hardest to learn and understand. It basically says that the amount of force applied to an object will have a proportional response on a mass, depending on its weight, and depending on the amount of force applied.
The rate of acceleration your vehicle will have will be proportional to how heavy your vehicle is and how hard it gets hit by the vehicle behind (how much force it applies).
When it comes to rotation, or angular movement, shape and size matters.
Your ute has a geometric centre of mass roughly in the centre of the four wheels, and the rotational inertia applies here. If you add weight to the object (the ute) you dramatically change the stability and how it will react out on the road. When your heavily laden ute rotates, or turns, or swerves, it’s much, much harder to slow down or control that movement compared to when it’s empty.
And your trailer, depending on its size and shape, has the ability to apply force on your ute and influence its stability, which we’ll refer to as ‘disruption’.
Disruption 1: Pitch
Your trailer has the potential to disrupt your vehicle. The fundamental truth is that the biggest influence in stability with trailers and caravans is weight, size and shape.
A 1500kg pop-top camper is not trivial, but has a very different capability of influencing your vehicle than a long, tall, 3.5-tonne caravan. It’s much easier for your tow vehicle to control the disruption of a small 1500kg pop-top than a 3.5-tonne caravan combination.
Pitch is basically a see-saw and that’s the one of two primary directions of movement of any trailer/caravan.
Yellow: Pitch + mass centre (see-saw or fulcrum point).
The centre of mass is about 10 percent forward of the axles, and pushes down on the vehicle, which is good because you need the tow vehicle’s rear axles loaded so the caravan follows around corners.
Every time you hit a bump, the trailer/caravan pitches, the axles unload somewhat, and the mass-centre pushes up very briefly before pushing back down again in the see-saw motion.
When this happens, as the trailer pushes down on the tow vehicle’s towbar and rear axles, it lifts the front end, making the steering lighter because there’s less weight pushing down on the wheels to grip the tyres and the road.
Again, the heavier the trailer, the more profound all of these these effects will be on the road in terms of stability. The bigger and heavier the object, the less stable the caravan and tow vehicle are.
Disruption 2: Yaw
Yaw is the second primary influence of a caravan and the tow vehicle, like your ute.
Yaw (too much yaw) is kinda what happened here >>
Every time you drive around a corner, towing your caravan, the trailer has to yaw. At the end of a corner, the vehicle has to tell the caravan/trailer to track straight, which means stopping it from rotating in yaw. Otherwise it will want to keep rotating.
Unfortunately, both pitch and yaw disruptions can happen at the same time. It’s as simple as hitting a bump mid-corner on an off-camber corner.
The trailer rotates in yaw because of the bend, it pitches because of the bump, and if the weight is great enough, it will overwhelm the tow vehicle’s ability to resist the influence of the disruptions caused by the trailer. There’s only so much compensation you, as the driver, can execute in this moment.
Overhang
Too many people fixate on rear overhang, behind the rear axle, when it comes to towing assignments.
What really matters is where the vehicle is yawing around. Where is the mass centre?
It’s not about how much rear overhang there is, it’s about the distance between the mass centre and the towball (where the caravan/trailer is connected) to disrupt the tow vehicle. Essentially, the longer the distance, the longer the cantilever is and the easier it is for the trailer to influence the vehicle’s movements - disrupt.
Overhang is more applicable to the failure of the vehicle’s chassis when it comes to overloading.
Overloading a chassis
When you think about the loads imposed on a ute, you’re looking at the towball, the tray payload, people in the cabin, engine and transmission up the front, and whatever accessories you mount (like a bullbar).
In terms of responding to all these loads, you have the springs, where two attachments to the chassis front and rear (on each side) for leafs, or one front and rear on each side for coils. These springs restrain the loads.
The problem with this system is you have a rigidly-mounted cabin to the chassis, but the tray/tub is mounted also to the chassis, but it’s not bottled to the cabin - there’s a gap.
So adding load into the tray is the worst possible design because the chassis failure mechanism - the weakest point - is the gap between tray and cabin.
Adding 350kg of ‘static’ towball download, acting upon the chassis, turns into a recipe for a hinge on the chassis when you hit a serious bump towing 3.5 tonnes. That ‘static’ load isn’t actually static, because every bump in the road accelerates that load downward, onto the towbar/chassis.
All those loads imposed on the vehicle will accelerate (proportionally; Newton’s 2nd law) when you hit a bump, because force is acting upon that trailer/vehicle. If that acceleration is great enough, the G-force could very easily be enough to bend your chassis if it’s a 3.5 tonne caravan and the bump is sufficiently deep/high enough.
This is why I constantly advise people like you not to tow more than 2.5 tonnes, ever, with any ute (or 4WD wagon). I also encourage you to think carefully about where and how you load your vehicle and trailer.
Don’t overload the ute’s loadspace. Think about where you locate heavy payloads.
Utes are ‘light duty’ vehicles only. Three tonnes of ute and three tonnes of trailer is right at the extreme limits of what’s operationally safe.
Bullbars and ‘safety’
It’s important to remember when loading up vehicles like utes and 4WD wagons, especially when you’re planning to tow, that every accessory you add, increases its overall weight.
It also reduces you available payload for people and their gear. Every lightbar, every CB radio, ever snorkel you don’t need, every roof rack you can’t strap stuff to, every winch you may not need, limits how much water, fuel and food you can carry.
Every can of baked beans adds to your GVM limit and payload. And the heavier your trailer, the less stuff you can carry and the fewer things you can bolt to your ute. This includes the most popular of all, the beloved bullbar:
I know bullbars are popular, but there’s no way they can have an effect on airbag deployment. And the only logical reason for wanting to fit one, is to protect the vehicle from damage caused by animal strikes.
The countermeasures to use instead include doing an advanced driving course which includes learning how to brake properly and how to swerve, you can reduce the weight of the vehicle to make it more responsive, you can avoid driving at dawn and dusk or overnight (when animals are more common), you can simply reduce your speed, too.
However, if you’re travelling at about 100km/h into an immovable gumtree, that’s probably going to be an unsurvivable crash. Any crash at over about 65km/h is going to seriously reduce your likelihood of survival.
Thousands of engineers for that last 40 years have diligently tried to design vehicles for energy management, in the millisecond domain, to extend the duration of a crash - to keep you alive.
Fitting a bullbar will change the dynamics of that interaction between you, the vehicle and the gumtree.
No bullbar manufacturers crash test vehicles with their bullbars attached, and there’s no public testing videos. But there have been leaks from mining companies doing their own independent crash testing on fleet vehicles, but found they performed worse at protecting occupants.
During a crash, the vehicle is not important; your life is. The vehicle is designed to save your life, by being sacrificed.
Can a few hundred kilos up the front help stability? Maybe, but it’ll be very minor. In fact it’ll probably have more adverse effects like reduced ground clearance and ride height, tyre wear, nose-heavy steering.
Weights and balances
I get the point of these kinds of discussions, but what matters is the vehicle’s weight on the road versus the trailer.
It’s always better for the tow vehicle (ute, 4WD wagon etc) to be heavier than the trailer/caravan/camper.
And this doesn’t apply to trucks because they’re completely different propositions. Trucks are carefully designed to stay on the road, despite their huge relative weight of the prime mover versus the trailer. The whole business of trucking is designed based around the business of maximising efficiency for carrying load as safely as possible. Maximum load: minimum weight of truck. You get paid for the weight of cargo you move, not the truck’s weight.
Many utes are rated to tow 3.5 tonnes, but it is completely unrealistic. Ranger has a gross combination mass of six tonnes (6.0 metric tonnes). If you’re pulling a 3 tonne trailer, the Ranger cannot weight more than 3 tonnes, but it has a kerb weight of 2.2 dead empty, which leaves less than 400kg for you and your stuff, including family, gear, and accessories.
With a dual-cab ute, towing three tonnes is on the raggedy edge.
The vehicle doing the towing needs to be heavier than the trailer, for safety and stability. And the less your entire convoy weighs, the less fuel you will consume and the less wear-and-tear you’ll impose on the ute, and the longer your tyres will love.
When you say a Ranger ‘approximately’ loaded to roughly three tonnes, towing a caravan weighing ‘about’ three tonnes, that’s a red flag.
You need to get the whole thing weighed on a public weighbridge as soon as possible. Otherwise, without knowing what your combination weighs, you are not only at risk of being caught and fined (and being forced to leave something behind), but you’re at risk of a stability and safety issue out there on the road.
Once that six tonne combination becomes unglued, a lot of damage can happen very quickly when pitch and yaw disrupt your vehicle.
I don’t want to see anybody making these mistakes because you and your family have been through enough in the last couple of years.
So take the time to get this right; reduce the weight as much as possible, be lighter, pack smarter, take less and think carefully about how you load your vehicle, make sure everything is safe and correct, slow down, get there safely and come home in one piece.
The soft-roading Subaru Forester packs a punch, and is one of the safest, most practical and capable SUVs on sale today. It’s also a great value, nicely driving and popular five-seater.