ASK ARB –
tyre pressures

Hi,

I am intending to drive the Outback Way later in the year. The Outback Way website makes this recommendation about tyre pressures:

“Along rough, rocky or corrugated sections of the Outback Way, try to maintain tyres at or near highway pressure so as to keep the tyre walls firm-hard and less susceptible to puncturing by rocks, sticks and sharp objects.”

But the ExplorOz website says the opposite:

“To ease the comfort of the ride and to aid in traction on particularly bad corrugations such as the Gibb River Road or the Development Track to Cape York, it is best to reduce tyre pressures about 4 to 6 PSI lower than what you run on the bitumen. Very rough and stony country such as the Birdsville Track can handle even softer tyre pressures. This may not seem to make sense at first, but if you consider that your tyre is just like a balloon being bounced over sharp objects, then you can see how the higher pressure would make it more prone to ‘popping’.”

One would think that the authors of both sites know what they are talking about, but they can’t both be right. Although the ExplorOz advice seems to be the conventional wisdom on the subject, that doesn’t necessarily make it correct, and the Outback Way advice intuitively sounds better to me. Has anyone done any proper testing of this?

William McCartney
Auckland

Hi William,
Tyre pressures are always a hotly debated campfire subject. Get a dozen 4WDers together and they will all have a different opinion on what works best for what situation. What you will find is that there is no ‘correct’ answer. Why is this? Well, it’s because every vehicle is different in the weight it carries and the tyres fitted. Add to this the varying terrain and driving styles, and what works for one 4WD is not necessarily right for another.

Tyre manufacturers want you to do things by the book for all the right legal reasons, and that usually means that if you are carrying a full load then you run higher tyre pressures. This information tends to be based on sealed road conditions where low pressures and high speeds can cause poor vehicle handling and possible tyre failure. So the information you read on the Outback Way is probably based on that kind of thinking. It’s not wrong information, just not necessarily the best based on experience.

Many 4WD publications, websites, driver trainers and industry experts will give you information based on their vast experiences. When it comes to driving Outback roads, running ‘highway pressures’ has distinct disadvantages. Although higher pressures will help the softer sidewall of your tyres stand straighter, it leaves your tread area more susceptible to impact fractures and cuts from sharp rocks. Reducing pressures even slightly helps the tyre to ‘give’ and roll over sharp objects without the resultant damage. On loose gravel surfaces, high tyre pressures mean a small footprint on the road surface which can result in your vehicle sliding across the surface of the road, especially into corners, whereas
lower pressures spread the tyre footprint, giving more traction and stability as well as increased comfort.

Lower tyre pressures and the resultant larger tyre footprint are used by 4WDers to reduce the chance of punctures and to gain traction on loose surfaces. But in order to do this safely, your speed must be reduced to prevent the tyres overheating. On softer surfaces like mud, snow and sand you will most likely be driving much slower than highway speed, so overheating is not such a problem. On Outback gravel roads you have a choice, to drive at highway speeds on high pressures and risk the chance of impact fractures, harsh ride and instability, or to run lower pressures in more comfort and at slower speeds with more control over the vehicle and less chance of a puncture. Stop at any service
centre along an Outback route and you can bet the advice from the tyre repairer is to slow down and run lower pressures. Mind you, they make a living out of people doing the opposite.

So if we ask the majority of tyre manufacturers, they’ll tell you only to run their recommended pressure for the load you are carrying… which will no doubt be high as your 4WD will be loaded to the hilt for your big adventure.
Legally, it’s the best advice they can give. Many tests have been performed by the likes of off road magazines and industry experts which back up the lower pressures argument, but every vehicle/tyre/load/driving style varies, which means trial and experience with your own vehicle is the only way you will know what is right for you. In essence, however, both websites you quoted are correct.

One tyre distributor that has acknowledged the lower pressures argument is Cooper Tires. Check out www.coopertires.com.au for a free copy of their 4WD Drivers Guide in which they explain the best tyre pressures for off road terrain.

Mark ‘Lowmount’ Lowry
(Manager – Product Development & Evaluation)