All about tyre pressure
So, what tyre pressure is right?
There’s a whole lot of factors that go into determining the right tyre pressure, and not many people think of all of them when they should. You need to take into account what you’re driving on,what car you’re driving, what tyres you have and what the conditions are.
On road driving
Tyre pressures for normal driving on the road is pretty easy, but people still manage to get it wrong!
The quickest and best thing to do is to look at one of two sources. First, check the owners manual of your car, or the tyre placard that’s on most cars. This will tell you the factory recommended pressures to run.
The second place to look is the shop that sold you the tyres that you’ve got. They’ll normally tell you what they recommend, or put a sticker on your windscreen to let you know what they recommend for your vehicle and tyre combination.
One thing to remember is that these numbers are not always right. You may need to vary these pressures depending on how much weight you’re carrying (for example, a ute with 50kg in the back will need lower pressure then one carrying a ton of stuff), as well as your own personal comfort. If the pressure is to low, the car will struggle and handle poorly, but too high and it will be overly rigid and unforgiving over even the smallest bumps.
Driving in dirt roads
The reason that most of us own a for-wheel-drive is to get away off the beaten track, and that pretty much always means dirt roads. Driving on dirt roads is actually pretty easy, and doesn’t need too much thought. A general guide is that you’d normally drop your tyre pressure somewhere between 15% and 20%. So if you normally run 40PSI on the road, you’d start off around 30 to 32PSI and see how it feels. Again, you might need to adjust this up or down depending on your car and the condition of the road that you’re on.
Driving off road can be a little more tricky. That’s because there’s more obstacles and more things that you need to roll over to get to where you want to go. Because of this it’s normal to drop your tyre pressure to around 20-24PSI to give you a larger footprint of tred as well as letting your sidewalls flex more so you can get across anything in your way a lot easier.
This takes it to the extreme. While most of us won’t be doing much hard-core rock crawling, there’s still a lot of challenging tracks that we will want to give a go for the fun or the bragging rights!
Normally tyre pressures would be a bit less than normal off-roading, with a lot of people going down to 14-16PSI to get some big flex out of their tyres sidewalls. Just remember that going too low can result in the tyres popping their beads and coming off the rims. While this can mostly be fixed on the tracks, it takes time and can be dangerous if you aren’t sure of what you’re doing.
If you’re running beadlock rims (check your state’s legislation for legality here) you can run really low pressures thanks to the rims locking the beads in. I’ve seen some people running on as low as 4PSI, but that’s on some pretty purpose-built rigs with beadlock rims and huge tyres. Not many of us have that, so remember to keep it a bit higher than this!
Sand and beach driving
Getting your 4WD out on the beach is one of the most popular things that we all love to do. We’ve all seen people forget to drop their pressures and get bogged to the axles less the 5 meters into the sand, and even more that don’t drop the pressure enough and get stuck along the beach when others are just cruising past.
Most vehicles will need to drop their tyre pressure somewhere down to 18-24PSI to start, and see how it works. In really soft sand, you can go down to 12-14PSI to get through it, and other cars that are carrying more of a load will need to keep tyre pressures higher to keep the tyres safe.
What About Rim Sizes?
Given the choice between an 18” rim and a 16” rim, most people will go for the 16” rim. But why? This is something that a lot of people really don’t understand, and a lot of people think they understand, but don’t know the reasons behind it.
The basic reason for this is that the bigger the sidewall, the better it will flex and perform with lower pressures in the tyre. There’s a whole heap more scientific principles behind it too, but that’s the basics. What happens is that when you drop your tyre pressure the sidewall needs to have a lower percentage of flex to get the same footprint with a higher sidewall, so it doesn’t need to deform as much compared to a higher diameter tyre.
Why sidewall size and construction matters
One problem that’s becoming more common these days is tyre sidewall blowing out when you’re off-road. The reason for this isn’t always that the sidewall is weak. The vast majority of times that we’ve seen this is when someone is running large diameter wheels meaning that their tyres have a smaller sidewall.
As an experiment, next time you do some beach or off-road driving, put your hand on the sidewalls of your tyres before you air down. Then do the same after you’ve done some distance with lower pressures. You’ll see that there is a difference in temperature.
What happens is that when the tyres sidewall flexes it creates heat. The smaller the sidewall, the more it needs to flex and the more heat it produces. This heat damages the rubber of the tyres sidewall and weakens it, sometimes to the point of failure (that means the sidewall blowing out and your tyre pretty much exploding). The rate of this happening gets higher with larger sized rims. Thanks to manufacturers putting on 18 or 20” rims from the factory to clear larger brake discs, it’s meant that the tyres fitted have smaller sidewalls, which leads to this problem happening.
There’s two things that can be helpful for this though. The first is to run smaller rim diameters. This isn’t always possible (I’m looking at you, Landrover…), and the second is to run tyres that are designated as LT, or ‘Light Truck’ construction as these normally have stronger sidewalls.
Towing or heavy loads
This is something that most people, even the best of us, either forget or are just a bit too lazy to do. Normally when you are towing, your trailers tyre pressures should be set to match your vehicle. This lets all of the tyres flex the same so that there’s no difference between the way that the tyres travel on the vehicle and the trailer. This is especially important when you’re driving on the beach because sand is a lot less forgiving.
So what is the right pressure?
The quick answer is.. We don’t know!
But there’s a lot more to it. As we’ve detailed above, you should always start off with a base and adjust as needed to get your own set up riding and handling right.
One thing to remember is that you should always have a tyre deflation device of some kind, and I’d also recommend a decent-quality air compressor so you can add air back into your tyres when you get back onto the harder stuff.