4wd tips and tricks – Tyre Pressures
When four wheel driving, changing the tyre pressure is one of the best 4wd tips and tricks you’ll learn. It is the most effective ways of changing the performance of your 4wd in almost all off-road situations. The only point of contact between your 4wd and the earth is its tyres. So it makes sense that modifying the tyres in some way has a huge impact on the performance of your 4wd when you’re off road. The cheapest and simplest modification you can perform on your tyre is to change the tyre pressure. And in almost all off-road situations, this means reducing your tyre pressure.
It’s important to mention, however, that not re-inflating your tyres when you get back onto the black-top is dangerous. You could risk your tyres, your 4wd, and even your and others lives. Driving with reduced tyre pressure at highway speeds for long periods of time will cause tyre failure. With reduced tyre pressure, the flexing in the rubber will cause heat build-up. This weakens the rubber causing tyre blow-outs. Also, high speed turns with reduced tyre pressure can cause your tyre to separate from the wheel rim, increasing the chance of vehicle rollover
Reducing tyre pressure is also better for the environment. With reduced tyre pressure, your four wheel drive will sit on top of the surface rather than digging in. When vehicles dig in, they cause damage to the tracks. Other vehicles then try to drive around this damage and the track becomes ever increasingly wider, encroaching into the surrounding environment.
Tyre pressure in soft sand
When driving on soft sand, the forward motion of your four wheel drive will create small sand mounds in front of each tyre. Although the ground may appear to be flat, as shown in the image on the left, your vehicle is constantly trying to climb a very steep slope. The only part of the terrain that is important to your vehicle is the part that is in contact with your tyres.
As you move forward, your vehicle is constantly forced to try to lift its entire weight up and over these tiny hills, however with harder tyres, rather than climbing these mountains, the sand is pushed in front of your tyres, and your 4wd effectively slides back down again. This constant effort and wheel spinning can cause your tyres to dig in further until you become bogged.
Reducing your tyre pressure increases your vehicles ‘foot print’. This increased surface area spreads the weight of your car over a larger area and helps to prevent your four wheel drive from sinking into the soft sand, thereby helping to prevent the build-up of these mountains and enabling your vehicle to maintain forward momentum with significantly less effort. And also greatly reduces the chances of you becoming bogged.
It’s important to note that this strategy can, and should be applied to any tyre in contact with soft sand, including non-powered wheels, or wheels on a camper trailer. Leaving the tyres on your camper trailer fully inflated will mean that the tiny sand mountains will be created, forcing your 4×4 to tow the trailer up a never ending sand mountain.
Reduced tyre pressure in other terrain
There are a number of advantages gained by reducing your tyre pressure in terrain other than soft sand. The three main advantages are:
- Larger footprint – this allows less sink-ability in soft terrain, and increases the surface area in contact with the ground, increasing traction. At very low pressures, the size of the contact patches can increase as much as 250%.
- More comfortable ride – Over rough and corrugated tracks, decreasing your tyre pressure softens the tyre. This then absorbs a significant amount vibration that would normally be transferred through to the suspension.
- Shape-ability – Increases the tyre’s ability to shape itself around obstacles, rather than have to climb over them. A softer tyre shapes itself around objects, where a harder tyre will need to drive over the top. This ability to conform to the terrain reduces the chance of puncturing the tyre, but also greatly improves traction. The more of the tyre in contact with the ground, the better grip the tyre will achieve.
However reducing the tyre pressure does decrease clearance under all parts of the vehicle, including the axles and diffs.
How low should you go for off-road driving?
Let me answer this question with another… How long, exactly, is a piece of string? The school of thought on how much you should reduce your tyre pressure is very varied. Asking 10 different 4wd specialists how low to reduce your tyre pressure will produce 10 different answers. But there are many factors contributing to how low you should go. These include:
- Sidewall stiffness
- Tire aspect ratio (vs.wheel diameter)
- Driving speed
- Trail surface/softness
- Vehicle Weight
If you’re tyres have stiff sidewalls, then the sidewalls will provide support and will prevent the tyre from flattening out. Reduce your tyre pressure more to allow the tyre footprint to increase.
Tyres with more flexible sidewalls will allow the tyre to balloon sooner, allowing a larger footprint to be created at higher pressures.
Tyre aspect ratio
The lower the profile of your tyre, the less you’ll be able to reduce the pressure without risking damage to the rim.
Driving quickly with reduced tyre pressure will cause damage to your tyres. The constant flexing of the tyre causes heat build-up and eventually will cause catastrophic failure of your tyre. There is also an increased possibility of the tyre separating from the rim in high speed turns.
Either of these events can cause you to lose control of your vehicle.
So if you intend to drive quickly then increase your tyre pressure without exceeding the recommended maximum.
On smooth surfaces, tyre pressure may not need to be reduced at all. And in fact perhaps should not be reduced. With smoother surfaces, the tendency may be to travel faster. Travelling at higher speeds with lower pressure tyres can be dangerous.
On more rugged surfaces or on steeper terrain a lower tyre pressure may be required. Remember, lowering your tyre pressure will increase the footprint of your tyre and therefore increase traction. On harder surfaces, however, the lower the tyre pressure, the greater chance of rim damage occurring. Harder tyres offer more protection to the wheel rims, but provide less traction.
On corrugated roads, reducing your tyre pressure will assist in the absorption of the corrugations, making your ride significantly more comfortable.
When it comes to driving on sand and mud, the softer it gets the lower you will probably need to go. It’s all about increasing your footprint to keep your vehicle sitting on top of the surface rather than digging in. Just keep in mind that lower tyre pressures increase the risk of popping your tyre off the rim.
Lighter vehicles, or less loaded vehicles, will need a lower tyre pressure to achieve the same ‘balooning’ effect.
Remember to reduce the tyre pressure of whatever you’re towing as well. If you’ve got a camper trailer hitched behind you when you’re driving on soft sand, and you don’t reduce it’s tyre pressure, then your car will be trying to drag the trailer up those constantly moving hills.
As a general rule, reduce the pressure in your trailer tyres to the same as what you’ve reduced your vehicle tyre pressure to.
Alternatively, before you leave on your trip, reduce your vehicle tyre pressure to what you expect to be using on the soft tracks. Once you have done that, measure the footprint length that you have achieved. Then reduce your trailer tyre pressure until you achieve that same footprint with your trailer tyres.
So how low then!
As a general rule, the looser the surface (snow, mud, sand) the lower the tyre pressure required.
For harder surfaces (rocks, hard clay, hard rutted dirt), a starting point would be to reduce your tyre pressure by one third of the standard road pressure.
On softer surfaces (Sand, snow, mud), you could start by reducing tyre pressure by one half of your usual road pressure. This will vary depending on the surface you’re driving on, and the types of tyre you’re using. If you notice your vehicle struggling then reduce the tyre pressure in small increments (2 to 4 psi at a time).
But remember, the lower you go, the slower you go.
TyrePressure – Profile – Harry Lewellyn – (Extract from book – SHIFTING into 4WD)
Sidewinder News – Week 4 2010
Warn Industries – Airing down your tyres for offroad use