In this fourth instalment of our Unsealed Guide to 4×4 tyres series, we go into detail about 4×4 tyre pressure!
“What tyre pressure should I run?” is a common question from 4×4 owners, and often, an absolute answer is given like 20 or 30psi. Humans like such simple answers to simple questions, but in reality, it is best to understand the principles behind tyre pressures. So, we’ve come up with this tyre pressure guide to explain it all so you can understand your 4×4 tyre pressures better.
4×4 tyre pressure guide
Air is forced into tyres to support the weight of the vehicle. The heavier the weight on a given tyre, the more air must be compressed into the tyre, something we know as tyre pressure.
Manufacturers of cars, not tyres, recommend pressures for their vehicles because the tyre maker has no idea how the tyre will be used, and the car maker does. Often two tyre pressure values are recommended – loaded, and unloaded, with slightly higher pressures for the loaded car to handle the extra weight.
These tyre pressures are found on a placard guide next to the driver’s door and are known as the placard pressures. The placard pressures are always your starting point, and for 4×4 driving, there can be a significant variation from the placard to the ideal for a given situation.
The first thing to know about pressures is that there are no absolutes. All that can be done is to explain the principles and give examples. Also, the precise pressure isn’t critical. You can drive around on say 34psi or 36psi and notice no real difference, unlike say a circuit racing car where a fraction of a psi can make a difference.
The relationship between weight, speed & tyre construction on tyre pressure
Weight
So, pressures. You need to go above the tyre pressure placard guide for three reasons: weight, speed and tyre construction. The weight we’ve covered, and it is fairly straightforward – more weight, more pressure.
Speed
The reason for the relationship between pressure and speed is heat. When a tyre rolls, it deforms – that’s the flat bit at the bottom. It’s known as the contact patch as it is the part of the tyre that touches the ground.
That deformation creates heat, so the greater and faster the deformation, the greater the heat. The greater the heat, the greater the chance of overheating the tyre, which is constructed from rubbers, silicone, glues and more. And when a tyre overheats, it can fail, sometimes catastrophically, with no prior warning.
Tyre construction
Tyre construction plays a part in pressure too. Heavy, thick tyres, such as light-truck construction tyres, do not dissipate heat as well as thinner tyres, such as passenger construction tyres, so as a guide, the LTs need slightly higher pressure than thinner tyres.
So, if your 4×4 is loaded to GVM, runs LT tyres, and you’re planning on hours of 110km/h cruising…then you would want to increase your pressures. How much by? Well, a good rule of thumb is the 4psi rule, where tyre pressures increase from cold by 4psi after about 20m of running on bitumen. By the way, towing heavy trailers – say 150kg of towball mass plus – will significantly increase the load on the rear axle, and therefore you’ll need more pressure on the rear tyres.
Why lower pressures are required for off-road driving
When it comes to offroad driving, the situation changes. We’re now at a lower speed, so heat management is less of a problem. By definition, ‘offroad’ is ‘not bitumen’, so the terrain is now either soft, rocky, loose or some combination. In all situations, lower tyre pressures improve performance for a few reasons.
Traction off-road
The first reason is traction. Offroad surfaces tend to be loose and, therefore, shear easily, unlike bitumen. So, if any given square centimetre of ground can only support a certain amount of shearing force, it makes sense to guide the force over as much ground as possible by increasing the contact patch, which means lower tyre pressures. Then, there is terrain softness. If a tyre sinks into the ground, it may well have good traction, but it will suffer from high rolling resistance, so it’s better to ‘float’ on top.
Bouncing
Another reason is bouncing. As a guide, soft tyres don’t bounce over things like rocks in the same way that tyres inflated to highway pressures do. The less bouncing, the greater the traction.
Puncture resistance & macro grip
Then we come to puncture resistance…less with lower pressures, even allowing for sidewall bulge. It wouldn’t be much of a tyre pressure guide if we didn’t mention macro grip. A soft tyre with lower pressure can wrap itself around rocks and the like easily, allowing big tread blocks to mould around obstacles.
All these reasons are why lower tyre pressures are favoured offroad, be that snow, sand, mud or rocks.
The downsides to lowering your tyre pressure
So are there any cons to lowering pressures? Yes. The speed we’ve covered, but ground clearance is reduced too, maybe by as much as 50mm, which can be significant. Aside from supporting the vehicle’s weight, air pressure keeps the tyre on the wheel. Lower pressures, therefore, risk the tyre separating from the wheel, known as debeading.
How far to go with tyre pressures?
The next question in this tyre pressure guide is how far to go. Again, we start with the placard because pressures can vary hugely from vehicle to vehicle. Some of you reading this will be driving Suzuki Jimnys, which run about 25pi on-road. Others may be in Fuso Canters, for which 60psi onroad may be appropriate.
So, the best way to think about tyre pressure reductions is in terms of the percentage of placards. As a rough guide, consider a 20% reduction for dirt roads, 40% for rocks and forest work, and 50% for soft surfaces such as sand. But those are just guidelines.
You can and should vary those tyre pressures – reduce if the vehicle struggles and a reduction will make a difference. No need to drop so low if the vehicle can perform without, for example on some sandy tracks in my Ranger I can run 25psi and still easily make progress. Other times, I need 10pi for really soft sand.
Do you need to run different tyre pressures if you have mud or all-terrains?
Another common question for this guide is whether you need to run different tyre pressures if you have mud or all-terrains. The answer is yes in theory, but not that you’d notice any difference. So, for practical purposes, just use the same guidelines I have here.
Do low-profile tyres need more of less pressure than high-profiles?
The interesting thing about low-profile tyres is that they deform just like high-profile tyres, so for a given pressure drop, you get pretty much the same contact patch increase as any other tyre. However, you do need to keep more air in them than high profiles, and while extreme, the photo below demonstrates why.
The answer is rim damage. As a guide, low-profile tyres simply do not have as much rubber between the rim and the road, so tyre pressures need to be kept up. This is less important in terrain like sand where you have all four wheel on the ground and it’s soft. But it is critical in rocky terrain like pictured below.
Driving technique with low tyre pressures
In this guide, we recommend driving slower with low tyre pressures because of the heat and also tyre debeading. Turning sharply or in ruts is a good way to debead a tyre. Other than that, just make use of your new-found traction to drive slower, which is safer and better for the car and the environment. Lower pressures do increase rolling resistance, so fuel consumption will increase too.
So now that you understand the how and why of tyre pressures…choose wisely!
