One of the first things you do when you come across someone stuck on a track or on the beach is ask them what their tyre pressures are. If they haven’t been around the off-road scene too long, and are unaware that dropping some air out of their tyres at the start of the track is one of the first things to do once you’re off the blacktop, then nine times out of ten letting a bit out is all it takes to get them going again – and this in fact is as effective as carrying correct 4WD recovery gear!

So why are our trucks more capable with less air in the tyres? We all know that driving at 40psi will not only cause you to lose traction that much easier, but also stiffen up the ride considerably when driving over rocky landscapes. That’s because as the sidewall cannot absorb as much impact from the undulating ground as it can when running at lower pressures. Having less air will cause the tyre to bag out and mould to the terrain, increasing the ratio of rubber to ground, giving you the maximum amount of grip and preventing you from having to reach for your snatch strap or your 4WD winch as often.

For those of us who live to lock in the hubs, airing down with a 4wd tyre deflator is as second nature as never passing up a beer from a mate’s esky. But it’s so ingrained into our thinking that we don’t often stop to question just why we do it. Yes, the contact patch theory is well established, but what else should we be considering? How do you know what pressure is right? Do different terrains need different pressures? Do you ever need to add pressure to drive safely? What would happen if you needed traction in a hurry; could you simply dump all of the air out of your rubber and drive out of whatever situation you’ve dug yourself into?

STATIC LOAD RADIUS

Most of the time we’re dropping air out of our tyres once we hit the hard stuff, but should this always be the case?  Let’s not forget that your tyre’s primary job is to support the weight of your vehicle and its contents. If too much pressure is dropped from the tyre it can ‘pop a bead’ and become separated from the wheel, which will have you going nowhere quickly, and will end up with you having to get the portable 12v air compressor out more often. One method of ensuring you’re always running at the correct pressure is to measure the static load radius. This is done by measuring the height between the ground and the centre of your wheel hub when the vehicle is stopped. This is an important measurement, particularly for heavily laden touring vehicles. Let’s say that you’re meeting some friends at the beach and taking them for a drive up the dunes in your 4WD. You know that 18psi is spot-on for your truck and works very well for you when you’re driving on the sand on your own. You measure your static load radius to be 15in at this pressure. Once you’ve picked up your mates and they’ve loaded their gear up and climbed in, you’ve notice that the truck is sitting a bit lower than normal. You remeasure your static load radius at 14in, signifying that the extra weight your vehicle is now carrying has increased the load on your tyres, which in turn makes damaging them more likely. This is a prime example of actually needing to add air to aid your off-road capability! If you were to drive for an extended period on sand, say if you were attempting a desert crossing, and your static load radius is lower than it should be, then you could overheat and damage the sidewalls or roll the tyre off the rim during cornering. Being stuck halfway to Karratha with a couple of buggered tyres is nobody’s idea of a fun trip, and it’s not something that even a quality 4×4 recovery kit can help with, unfortunately!

THE 4PSI RULE

With all the differences in vehicle weights, tyre sizes and ratings, carrying capacities and even how we individually choose to load up our trucks, how are you meant to know what pressures to run? The answer is actually a lot simpler than you may think. It is called the 4psi rule and it can be used across any vehicle, in any temperature on any terrain, and as such it’s one of the 4WDers most valuable tools. Inflate your tyres with your 12v air compressor to the pressures written on your tyre placard, go for a drive for at least an hour and then pull over and check the pressures again. If they have gone up by more than 4psi then the starting pressure was too low and you will need to add a couple of psi per tyre next time. If they have gone up by less than 4psi then your starting pressures were too high and you should drop a couple of psi out. These new pressures should be your starting point the next time you’re going for an hour’s drive, however make sure that your tyres are cold before you start off again. The good news is that once you’ve figured your optimal pressures out, that’s it – you’re set. All you’ve got to do then is use your portable air compressor and your tyre deflator to make sure you’re always at that correct pressure!

WHAT CAN YOU DO IN CASE OF AN EMERGENCY?

After discovering the advantages of lower pressures, we aired back up to 40psi and proceeded to do something a bit counter-intuitive. We backed down to the edge of the high water mark on the rising tide and got ourselves thoroughly bogged. With water getting disturbingly close to our rear bar we wanted to see if dumping the air down to 4psi with our Kwiky tyre deflator would allow us to drive straight out of our predicament without too many dramas. It should be noted that 4psi is a very low pressure and the probability of rolling a bead is incredibly high, but in an emergency situation it may just be enough to get you out of strife. Airing back up with a Thumper max air compressor as soon as it’s safe to do so should be considered mandatory. As it was though, the increased contact patch worked perfectly in this case, allowing us to drive straight out.