4WD tyre pressures are probably the most critical factor you control when you get off the bitumen road. Whether its gravel roads that are full of corrugations, big mud holes or a soft beach you are travelling on, having your tyres at the right pressure for the terrain you are driving on is crucial. In a previous post, we explored 6 reasons tyre pressures are critical for 4WDing.
The thing is though, the air pressure in your tyre varies considerably, depending on what you are driving on, how long you've been driving and how quickly you are going. The bouncing and rotation of a tyre causes the air inside to warm up, and as a result the air pressure increases whilst the tyre is warm.
The question then, is do you check your tyre pressures when they are hot, or when they are cold?
You might not think it makes much of a difference, but it is certainly something worth considering.
How much difference does it make?
We left Derby a few weeks ago, and headed onto the Gibb River road, after doing several hours of highway driving (at 110km/h). When the Gibb River Road turned to gravel, we pulled over and let the tyres down. From the original 40 PSI that the tyres were originally pumped up to (cold, when we left Perth), they were reading around 45. I dropped all 4 tyres down to 30 PSI, and we were on our way. A few days later, I checked the tyre pressures early one morning, when they were cold. The rears were 24/25 and the fronts were around 27 PSI. That's a pretty serious difference!
It''s also a pretty clear indication of the weight distribution in our 80 series too; there was a lot more weight in the rear of the vehicle, which is why the tyres were actually at a much lower tyre pressure cold. A lot of 4WD's do not have the weight evenly distributed, which is why people run different tyre pressures from the front to the rear.
What factors change the air pressure?
In most case, the quicker you drive, the hotter the air in your tyres will get, and thus the higher the pressure will go.
The more flexing your tyre has to do, the quicker the air will heat up too. Corrugated roads for example make your tyres work very hard as they are continually bouncing up and down, absorbing the shock of every corrugation. Driving on a soft beach in the middle of summer, where the sand is 45 degrees will quickly warm your tyres up too!
The more weight you carry in your 4WD, the more pressure your tyres should have in them. More weight results in a greater temperature in your tyres, and thus a higher air pressure.
The footprint size that your tyre has on the ground plays a very important role in the temperature your tyres will reach. The bigger the footprint (wider and taller tyres) the less air you need in each tyre as the weight of your vehicle is spread over a larger surface area. As a result, the bigger the tyre, the less heat will be generated, and thus the less the air pressure will go up.
However, bigger tyres are not always better. Have a read of Fitting bigger tyres to your 4WD to see both sides of the coin!
The 4PSI rule
The 4PSI rule has been around for many years. It's used to find the correct tyre pressure for any given terrain you are driving on. Sometimes the perfect pressure is not always possible, but an understanding of the rule is very helpful when choosing the right pressures.
The way it works is this; check your tyre pressures when the car hasn’t been driven for several hours (the air in your tyres will be 'cold'). Then, drive as you would normally for the terrain you are on. After an hour, pull over and check the tyre pressures again. If they have gone up by 4PSI, you are bang on the money with the right tyre pressure for your given speed, weight and terrain.
If they’ve gone up by more than 4PSI, your tyre pressures are too low and should be increased a little. Likewise, if the pressure has not gone up by 4PSI, you need to let some air out. Some people suggest for 4WD's it should be the 6PSI rule, but if you are in between the two you are doing well!
However, bear in mind that your tyre pressures should reflect the speed you are driving at, the terrain you are on and the need for puncture resistance. In some cases, its better to have your tyres a little lower than what the 4PSI rule suggests, if it means your chances of getting a puncture are lessened.
Consider the heat
Hopefully the above post has given you something to think about next time you check your tyre pressures. Have a think about whether the tyres are hot or cold, and how much of a difference it could make to your 4WDing.
If you arrive at a beach after an hour of quick driving on the bitumen and drop your tyres down to 16 PSI, the following morning they are going to be more like 12 PSI.
Likewise, if a pressure is suggested for a particular road and you let them down in the morning before driving off, the tyres are going to warm up and your pressures will rise, to a level that you may not be happy with. I've seen my tyre pressures go from 16PSI up to nearly 20 from purely driving on a beach. That 4 PSI can make all the difference on soft sand. Choosing the best Beach driving tyre pressures is easy, when you understand what influences them!
So, do you check your tyre pressures when the tyres are cold, or when you’ve been driving for a while? I find majority of the time when you check your tyre pressures the tyres are warm. As such, I try to make allowance for it due to the huge variation, and would recommend you do too!