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Technical Top Tips

4X4 Tyre Pressures … decoded

 

After looking into tyre pressures for 4×4’s especially during off road use there is a distinct lack of clarity when it comes to good information, plus there is a lot of conflicting advice out there.

The rules for tyre pressures vary according to tyre specifications maximum load, maximum pressure, tyre construction, sizes, type of vehicle, vehicle loading, suspension characteristics and tyre applications.

There is no single rule that can be universally applied throughout the spectrum.

The information given here is a rule of thumb that can be applied under any circumstances. It’s not perfect as we are not experts on tyres but we have spoken to tyre experts on researching this articles subject.

It must be understood there is no perfect pressure for any one situation. Changing pressure to improve one thing invariably makes something else worse. So a personal choice or judgment has to be made dependent on the situation.

So with all the information accumulated and the advice from the experts reviewed, here is our interpretation of that information as simply put as possible.

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The only point of contact between your 4×4 and the ground is the tyres. So it makes sense that modifying the tyres in some way has a huge impact on the performance of your 4×4 when 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.

Remember, by not re-inflating your tyres when you get back onto the highway is dangerous, you are risking your tyres, your vehicle, and even lives. Driving with reduced tyre pressures at highway speeds for a period of time will cause tyre failure, the flexing in the rubber will cause heat build-up weakening the rubber causing a tyre blowout. Also, high speed cornering with low 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 vehicle 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 increasingly wider and encroaches into the surrounding environment.

 

A Golden Rule to Remember – Going Lower – Drive Slower

 

Reducing Tyre pressure will give;

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  • Larger footprint;

this allows less sinking 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.

  • Tyre shape;

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.

A point to remember, reducing tyre pressure does decrease clearance under all parts of the vehicle, including axles and diffs.

Tyres with more flexible sidewalls will allow the tyre to balloon sooner, allowing a larger footprint to be created at higher pressures. The lower the profile of your tyre, the less you’ll be able to reduce the pressure without risking damage to the rim. If you’re tyres have stiff sidewalls, then the sidewalls will provide support and will prevent the tyre from ballooning sooner. You must therefore reduce your tyre pressure more to allow the tyre footprint to increase. Lighter vehicles, or less loaded vehicles, will need a lower tyre pressure to achieve the same ballooning effect and footprint.

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As a general guide, the looser the surface material e.g. snow, mud and 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 around 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 are using. If you notice your vehicle struggling then reduce the tyre pressure in small increments of 2 – 4 psi at a time.

 

MUD Pressures;

Pressure when attacking mud is a compromise between flotation according to the same principle as driving on sand, and biting into firmer soil afforded by harder tyres. Reasonable pressure needs to be maintained to ensure tyre safety when bouncing around over obstacles and ruts. A starting point may be around half of the highway pressure. Watery mud with a hard base may be ok at full highway pressure. Thick, deep mud with no base at all may require pressure closer to that of sand driving.

 

SAND Pressures;

Sand requires the lowest pressures. The softer the sand, the lower the pressure, by reducing pressure it makes a huge difference in soft sand keep reducing until your tyres “float” across the surface. A typical sand pressure may be around half highway to one third highway pressure for very soft sand.

Keep in mind the increased risk of sidewall damage and busting the bead as pressure is reduced, so keep the speed down.

In an emergency pressure can be reduced further, but do not reduce pressure to the point where the rims are contacting the tread, and never below five psi to ensure the tyre bead stays seated on the rim. These extreme low pressures are purely for recovering the vehicle and should not be used generally.

 

ROCK Pressures;

For rock traversing low pressure improves traction by assisting flexibility and helping to maintain tyre contact with the ground. It allows better mechanical “keying” between tread and rock edges which further improves traction.

Hard tyres will bounce over rocky terrain and causing wheel spin and dangerous shock loads through the vehicle’s drivetrain. Low pressure also reduces jarring impacts that are transferred to the suspension and through to the cabin.

Rock is however risky in terms of sidewall damage and low pressure could run the risk of pinching the tyre tread between the rim, causing tyre damage or puncture, as too by grinding the sidewall against the side of a rock could cause pinch damage.

So rock may call for a pressure of about two thirds of highway pressure, but again it is personal situation assessment and choice. By reducing pressures further will improve traction, but expose the tyres to greater risk of sidewall and pinch damage.

 

CORRUGATIONS Pressures;

Corrugations are probably the worst type of obstacle to encounter. Heavy abuse to the driver, cargo and vehicle namely the suspension. Pressure is recommended between two thirds to one half of standard highway pressure, dependent on the severity of the ripples.

People have experienced dangerous loss of traction when encountering very bad corrugations with the feeling of the rear end of the vehicle drifting sideways across the track. Reduced pressure prevents this situation. Lower pressure also reduces wear on suspension components reduces the risk of mechanical failure, improves comfort for occupants and reduces the risk of nuts and bolts working loose.

Reduced tyre pressure means the tyre deforms more and the suspension articulates less, so the suspension’s effect on creating corrugations is reduced. So lower pressure also reduces the formation of corrugations, which is a good move too.

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Other Points to Consider;

 

 

 

Suspension Wear;

Pneumatic tyres are able to respond quickly to uneven surfaces because they have very little inertia to resist movement. A pneumatic tyre easily absorbs sudden impact loads, this is the reason why pneumatic tyres are so universal. A hard tyre will transfer more of the load to the suspension, resulting in a severe spike in stress transfer to suspension components as impact loads are encountered. Low tyre pressure increases the work done by tyres in response to surface irregularities, therefore reducing the force transferred to the suspension. So lower tyre pressure reduces suspension wear.

Comfort;

Lower pressure improves comfort. Low pressure allows a tyre to deform quickly to any bumps in the road, insulating the vehicle from the impact. Higher pressure reduces tyre flex and makes for a firmer, bumpier ride.

Steering;

Lower tyre pressure increases the effort required to steer the vehicle.

Traction;

Lower pressure means the tyre stays in better contact with the road as it conforms to bumps. Higher pressure yields less traction over bumps as a tyre bounces and load is temporarily reduced or completely removed if the tyre becomes airborne.

Speeds;

Lower pressure increases tyre temperature at speed, so when running reduced pressure, speed must also be reduced to offset the additional heat load and maintain a safe tyre operating temperature. Failiure could result in a catastrophic failure (blowout).

Fuel Economy;

Hard Surface; harder tyres have less rolling resistance and so yield improved fuel efficiency. Lower pressure increases rolling resistance, overcome by a harder working engine burning more fuel.

So higher pressure improves fuel economy on hard surfaces.

 

Soft Surface; In dirt or sand, hard tyres dig in and create extra drag, causing the engine to work harder so more fuel is used. Soft tyres float on top and do not dig into the surface, hence reduced pressures offer improved fuel efficiency when crossing softer surfaces.

 

Tyre Wear;

Wear to you tyres will happen at lower and higher pressures just as at higher and lower speeds. The more grip, the more speed, all will have detrimental effect to your tyre wear. The tyre will deteriorate even if parked up, over time, doing nothing it is unavoidable.

 

 

For our recent review on All terrain Tyres, click here

 

 

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