Tyre pressures when off-road: what to do
Telling you what tyre pressures to use is like asking “How long is a piece of string?”
Tyre pressures are measured in PSI (Pounds per Square Inch) and the right tyre pressures will depend on a lot of things:
- temperature
- terrain
- weight
- speed
- type of rims
What is the temperature of the day?
Tyre pressures change with temperature, meaning when the tyre has not been driven anywhere for some time. Let’s say overnight, the tyre is at its lowest pressure. As the temperature of the day heats the outside of the tyre, the measurement of pressure will increase. As you drive along the road the tyre will heat up and it will measure higher. For every increase of 5 degrees Celsius (10 degrees F), tyre pressure will increase by approximately 1PSI.
Tyre pressures should always be measured when the tyres are cold.
What type of terrain are you driving on?
If you are driving on soft terrain (ie: Sand or Mud) lowering the tyre pressures will increase the area between the tyre and the ground. This is called the contact patch.
This means that your car is lighter per square inch and is more likely to float over the soft sand or mud rather than sinking into it. Lowering your tyre pressure also reduces your ground clearance, this is caused by the sidewall of your tyre bulging out. You may decide to keep a higher tyre pressure for some situations. As tyre pressures get lower, your maximum speed should be slower. Cornering too fast with low tyre pressure can cause the tyre to roll off the rim. See Additional Information below for “how to put the tyre back on the rim”.
If you are driving on rocky terrain you may choose to lower the tyre pressure to absorb some of the shock when bumping into the next rock. Be careful, as too low can pinch the tyre between the rock and the rim which can cause a puncture. It can also push the tyre sideways causing the tyre to roll off the rim. This is an easy fix with the right tools, see Additional Information below.
You may decide to keep higher tyre pressure so that you keep the greatest ground clearance.
How heavy is your 4WD at that point in time?
The heavier your 4WD (including your load), the larger the contact patch will be when comparing the same PSI. This means that at low pressure driving you will need more PSI in your tyres so that you are not pinching the tyre between the rim and the ground. I have included details about how to calculate your vehicle weight using the PSI of your tyres and the size of the contact patch in the Additional Information section.
What speed are you driving at?
The lower the tyre pressure the slower and more careful you will need to drive, especially when cornering. If you turn the steering wheel hard and fast with low tyre pressures, you may end up rolling the tyre off the rim.
The tyres will heat up more as they will flex more at lower pressures.
Some options lock the bead of the tyre to the rim so that even at really low pressures the tyre cannot roll off the rim. It is caused by the forces of the internal air pressure being less than the force of the tyre being pushed sideways, during the turn.
Can I prevent the tyre from rolling off the rim?
Some rims have bead lockers; this squeezes the tyre between the rim and the bead lock, meaning that even at very low tyre pressures, the tyre is unable to release from the bead. The change in tyre pressure does not change the pressure between the rim and the bead lock.
What tyre pressure do I need when on the road?
If you are using the tyre size that matches your tyre placard on your vehicle, then the best option is to use the tyre pressure that the placard shows.
If you are using a different tyre size, then the load rating may differ from what is shown on the placard.
There are a few things that you can do to find a suitable tyre pressure.
A quick test
Go to an empty carpark or a quiet back street
Draw a 2-3cm band of chalk across the top of the tread the full width of the tyre.
Then drive straight for about 50m, do not turn the steering wheel.
When driving the chalk will rub against the road and wear off.
Looking at the way the chalk wears off will tell you whether or not your tyre pressure is correct.
If your tyre pressure is correct.
The chalk will wear evenly across the tyre; this is a good tyre pressure for the load of the vehicle.
What happens when the tyre pressure is too low?
You will find the chalk has worn away from the outside edges of the tread and chalk remains in the centre of the tread.
What happens when the tyre pressure is too high?
You will find the chalk has worn away in the centre of the tread and the chalk remains on the outside edges of the tread.
What does it mean if the chalk remains on only one side of the tread?
This can be that you accidentally turned the steering wheel while trying to drive straight. It is most likely to be that your vehicle needs a wheel alignment. Once aligned correctly repeat the above steps to find the correct tyre pressures for your vehicle when driving on the road.
My example
The short answer is: There is no magical number for all times.
Airing down your tyres will take some trial and error.
This is where I never say that I failed.
FAIL = First Attempt Is Learning.
As long as you learn from your mistakes, and your mistakes but also learn from the mistakes of others, you will start to understand the best tyre pressures for your vehicle in each situation.
Remember: the more the tyre flexes the more it will heat up. The more it heats up the faster the tyre will wear out.
What I did
When airing down, I always remove the valve from my tyre. This allows the air to be released faster and I listen to the air while it is being released. You will hear the pitch of the sound change as the pressure reduces. Different tyres will have the pitch change at different pressures, so it is about recognising the sound you need for your vehicle.
I also watch the amount of bulge on the side wall, it is hard to say how much but I have learnt what I am looking for.
I had just purchased a new set of tyres and my first chance to test them out was our trip to K’gari.
When on K’gari, I had let my tyre pressure down to a point where the bulge of the tyre looked right to me for about 15 PSI. Little did I know at the time that these new tyre wide walls had more flex at a higher pressure than my previous tyres. Without measuring the pressure I thought I had around 15 PSI in the tyres, which is what I normally run when beach driving.
On my last day on the island, I was coming back over Ngkala rocks, the sand was softer trying to come up the hill and my car was struggling. I was a little stubborn and said “The PSI was 15 and I did not want to go down any lower because of the bulge of the side wall”.
I should have checked my tyre pressure. I would have discovered my tyres were nowhere near what I thought they had been. When I was airing back up, I discovered that the tyre pressure was 25 PSI. This means that with the new tyres, my beach driving should be closer to 20 PSI for the load that I had during the trip.
First Attempt Is learning.
Additional information for tyre pressures
Getting the tyre back on the rim.
If you have an air compressor available and some lubricant to spray between the tyre and the rim it can be very easy.
Just use your jack and raise that tyre off the ground, or remove it completely from the car.
Clean any debris from between the tyre and the rim, you do not want dirt, dust, mud, grit (or anything else) in between the tyre and the rim.
Spray the lubricant between the tyre and rim. Water will work as a lubricant if nothing else is available. Soapy water or even a spray deodorant will work too.
Start your air compressor and using the tread of the tyre (not the sidewall) pull or push the tyre to close the gap between the tyre and rim. Keep your fingers away from the gap!
Once there is no gap the air pressure inside the tyre can start to increase.
Move to a safe distance away from the tyre.
The increasing pressure will push the tyre onto the bead.
You will hear a load pop as the tyre moves into place, this can be dangerous as any excess lubricant or any small debris can be pushed out from the gap at a very fast speed.
Find the weight of your vehicle using tyre pressures.
If we know the tyre pressure in each tyre, we can actually find the entire weight of the car. By calculating the square inches that the contact patch of each tyre makes on the ground and multiply that by the PSI.
If you have ever parked a car on your lawn for more than a couple of days, you will see the contact patch of each tyre as a patch of grass that has died.
If you get out a tape measure, measure (inches is easiest) the width and breadth of the dead grass and multiply that by the PSI of that tyre.
Do the same for each tyre and add them all up. This will give you the weight of your vehicle in pounds.
To calculate and convert this to kg; divide pounds by 2.2 to get kg’s.
This will give you a rough value.
RACQ website also provides a guide on how to check your tyre pressures.