Tyre pressures and 4WD operation

Tyre pressure is critical and needs to be adjusted based on circumstances. On the open road, you want high pressures to minimise sidewall flex and stop overheating, otherwise, the low tyre pressure can cause tyre failure.

Tyre pressure and contact patchOff-road you want low tyre pressures to increase the tread surface area, maximising grip

The more extreme the conditions, the lower the tyre pressure, even down to 12 – 15 psi if warranted. You just have to remember that you cannot drive at speed with pressure this low, the sidewall, rim and tyre tread will feel very disconnected, ( this is why we took you for a drive on a bitumen road with low and high tyre pressure so the comparison was immediate.

There are ways to lower and adjust tyre pressure quickly and accurately, this section is to show a few approaches and tools to help you.

 

Tyres are the most important part of the car and the least understood. One of our key learning is that everything and I mean everything about driving is an equation between grip vs force. The force increases to the square of the speed applied, double the speed, four times the mass, or 20 kph – 40kph = 2500kg – 10,000 kg and you still only have the same four contact patch areas to stop, steer and control the car.

Tread depth is critical to tyre performance in the wetTread is important to, its primary role is a water pump, at 100kph, it can displace 10 lts water/ sec. that is faster than you can throw it out of a bucket. But it can only do this when the tread is new.

The difference in stopping distance between a worn tyre and a new tyre, can be equivalent to 30 metres further into the crash at 80kph.

To check tyre tread depth, look for moulded rubber blocks between the tread grooves, as the tyre wears, they will form a solid line across the tyre face.

Ideally, we want to replace the tyre at or before a minimum tread depth of 3mm. This tyre will already take up to 12 metres longer to stop in the wet at 80kph

Always have a pre drive inspection, take the long way around to the driver’s seat. Get to know what your vehicle looks like and should look like, including the side wall of the tyre.

 

Adjusting the air pressure in your tyres to suit different driving conditions will not only improve your vehicle’s performance, but also aid tyre life, help prevent tyre and track damage, and result in a much more comfortable ride, especially as we get of the black tarmac onto some of Australia’s notorious roads. The bigger the ruts, the corragations, the lower tyre pressure will help to absorb and provide not only a smoother ride, but also protect our car mechanically.

The suggested pressures listed here are for vehicles fitted with Light Truck (LT) construction tyres, which have a stronger carcass than Passenger Car (P) rated tyres. Also bear in mind that heavily laden vehicles will need to run higher air pressures in their tyres than lightly laden vehicles.

On-Road Pressures

As a general rule you should follow the recommended tyre pressures as stated on your vehicle’s tyre placard for on-road driving which, for most 4×4 wagons and utes, will be between 30psi and 38psi. Some vehicles and tyre design run pressures up to 60psi. The tyre placard will usually state different pressure recommendations dependent on the tyre size and the load the vehicle is carrying; the higher the load the higher the tyre pressure.

For drivers of 4×4 utes, for example, this might mean running 32psi in all four tyres when the vehicle is unladen, but when laden running 32psi up front and 36psi in the rear.

Finding the ideal tyre pressures for your vehicle may take some time, and factors to help you arrive at the ideal settings will include preferred ride quality, steering response, vehicle handling and tyre wear. A tyre that’s been overinflated will wear prematurely in the centre of the tread area, whereas a tyre that’s been underinflated will exhibit more wear on the outside of the tread area. If you have the pressure right, the tyre should wear evenly across the tread.

Rough Gravel Roads

When you reach the end of the blacktop and you’ve got gravel under your wheels, it’s time to start thinking about your tyre pressures.

If the gravel road is smooth and well graded, and you’re maintaining a similar speed to when you were driving on sealed roads, you can leave your tyre pressures as they were. But if the road surface deteriorates and you have to lower your speed to suit the conditions, you should probably think about lowering your tyre pressures. By how much will depend on the load your vehicle is carrying and the speed you’re travelling at; the higher the load and the higher the speed, the more heat will be generated, so you don’t want to go too low. I would recommend a minimum of 25psi – 30psi with a speed no greater than 80kph.

Lowering your tyre pressures in these conditions will not only improve your vehicle’s ride quality, but also improve tyre flexibility, which can reduce the risk of chipping in the tread area. See the side wall is also factored as part of your suspension.

Rocks

Reducing your tyre pressure in rocky terrain has several benefits. Lower pressure allows the tyres to flex more easily so they can conform to the terrain, which improves traction and reduces the chance of damage from impacts, and it gives the tyres a longer footprint, which also aids traction.

So how low should you go? If you’ve engaged low-range to tackle rocky terrain and you’re driving at slow speeds, you can reduce your tyre pressures to as low as 22psi, but care has to be taken when driving at these pressures. It’s important to maintain controlled throttle and steering movements when driving with low tyre pressures to avert the risk of the tyres slipping on their rims, which could result in deflation or a tyre coming off the rim altogether. And don’t drive too fast, because excessive speed will generate too much heat, which can damage the tyres. If conditions improve and your speed picks up, inflate the tyres to around 28psi if the track is still rocky.

Finally, keep an eye out for nasty looking rocks and other sharp objects that could damage your tyres’ sidewalls, which will have ‘bagged-out’ due to the lower pressures.

Mud

Setting the correct air pressure for mud driving very much depends on one thing: the mud. If the mud is all slippery and slimy up top, but there’s a firm base underneath the surface, then relatively high air pressures are ideal. But if it’s all deep and gooey, with no firm base, then low air pressures will give the best result.

By running at close to road pressures of around 28psi, the tyres can cut through the top layer of mud and gain purchase on the firm base underneath, hopefully gaining enough traction to get you through the muddy sections. This is why it’s usually a good idea to stick to existing tracks when crossing a claypan, for example, as the tyres from the vehicles that have already been through will have cut through to that hard base layer under the surface.

When the mud seems bottomless, however, air pressure as low as 18psi will give your tyres a better chance of gaining purchase thanks to a longer tyre footprint. Once again, take care when driving with very low pressures as there’s always the chance of a tyre slipping on the rim, and mud it can work its way in between the bead and the rim, resulting in tyre deflation.

Sand Driving

Dropping tyres pressures for sand driving is a given, but how low will depend on several factors, including how soft the sand is, how coarse it is, whether it’s wet or dry, and how hot the ambient temperature.

As mentioned, lowering air pressure increases the tyre’s footprint, which in the case of sand helps it to float over the surface rather than dig into it. If the sand is particularly soft, which is usually the case if its fine and dry, you can drop your tyres down to as low as 16psi without fear of pealing one off a rim, so long as you don’t drive too fast and you don’t make any sudden steering movements. If the sand is coarse and damp, it will usually be a little harder, in which case you won’t have to lower pressures as much.

As well as reducing the chance of getting bogged, lowering your tyre pressures in sand will reduce the strain on your vehicle’s engine and driveline components, and will minimise the chance of track damage.

Finally, don’t forget to reinflate your tyres to the placard recommended pressures once you’re back on the road…and while you’re at it have a good look over each tyre to check for signs of damage, and repair or replace if necessary. Also check the valve stems for leaks and make sure the dust caps are in place.

When off road, we need to increase the surface area of the tyres to stop getting stuck

Tyre DeflatorsTyre deflators. are fantastic little devices and awesome when a set pressure is going to be required.

Take time to set the pressure on one tyre and adjust the deflators so they will not release air below the pressure and then then screw onto each valve.

When finished, put back into your accessories bag and they are ready for your next adventure

There is nothing to wear out and they are a fabulous investment. deflator and gauge

Deflator and gauge is a fantastic unit, as we saw on our program, just follow these three quick simple steps.

  1. Screw onto the value,
  2. Unscrew the valve stem and
  3. Pull back the sleeve

When you have reach the desired pressure, screw in the valve until seated, check to see the valve does not leak with a little spit on your fingers rubbed over the valve and away you go.

 

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2 responses to “Tyre pressures and 4WD operation”

  1. G hope says:

    What do you use to reinflate the tyres?

    • Total Driver says:

      Hi
      Thanks for commenting. There are a few options.
      1. Have your own compressor onboard. I like the twin head Thumper compressors, as it does not take all day to inflate.
      2. Drive cautiously to a servo, or car wash and inflate there.
      3. The vehicle may have a factory compressor, such as Discovery 2 -6. You can be creative, tap into the tank, install a suitable fitting and inflate from there.

      For me, I have a twin Thumper mounted permanently in the vehicle, I turn on and off with a switch and that makes the inflation process manageable
      Cheers
      Gene

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