4WD MODIFICATIONS - TYRES & WHEELS
We couldn’t believe what we were hearing: “All of us are running 50 psi in our tyres.” This was from a tag-along tour leader and the convoy was doing the Canning Stock Route. It wouldn’t have been so bad if all the vehicles were workhorse machines, with light truck tyres, but they were the usual mixed bag of wagons and utes that make up most tag-alongs.
It’s much the same story when we do some driver training and ask people what pressures they are running in their tyres: “Whatever they put in them.” Who the hell are ‘they’?
The starting point for setting the correct tyre pressures for your vehicle is the maker’s tyre placard that’s usually on a door pillar or inside the glove box lid.
The pressures quoted there are for maximum load and speed driving, so the recommended rear axle pressures are nearly always higher than for the front axle. It’s not necessary to have the rear tyres inflated to the maximum pressure listed on the placard for lightly-loaded around-town driving.
For example, a ute that spends most of its time empty or with only a 100 kg in the tray doesn’t need the 50 or 60 psi that its light truck tyres are rated for. Using that much pressure in light-load conditions will give a harsh ride, poor traction on rough surfaces and will wear the tyres in the centre of the tread.
It’s also important to note that the placarded tyre pressures relate to the tyres supplied with the vehicle, or those that are optional fitments. If you’re running non-standard or optional tyres you’ll need to consult the tyre fitter for a recommendation.
If the fitter proves to be clueless, as many of them are, you can experiment by adopting a starting pressure and running for around an hour at cruising speed on bitumen.
The aim is for around a 10-percent increase on each tyre – provided the ambient temperature remains constant. If you get more than that pressure increase you started with too low a pressure and if you get less, you started too high. It’s fiddly, but it works.
Complicating this somewhat is the effect of ambient temperature increase. For example, in the Australian deserts in winter it’s common to have zero temperature overnight and 25 degrees by midday. In those conditions tyre pressures can be expected to rise by an additional 10-percent as the day warms up.
So, if there’s an increase in ambient and you’re driving at highway speeds you can expect a pressure increase up to 20-percent over your cold starting pressure.
Ideal loaded and unloaded starting pressures will be different, particularly at the rear. Sometimes the ideal loaded pressure at the rear will be at least double the ideal pressure when unladen.
Pressure to Match Load and Speed
When you load up the ute or wagon for the Big Trip, you’ll need to up the pressures in proportion to the load. Wagons with a full load of camping gear or the trailer hooked up will probably need maximum pressure.
Don’t ever, ever, ever put more pressure into a tyre than its rating. Recreation vehicle tyres – most of those with low profiles that have /65, /60, /55 and even /45 in the tyre size panel – cannot tolerate high inflation pressures and will crack their casings if over-inflated and then driven on stony roads. You probably won’t feel the casing fail initially, but the pressure drop soon shows up – usually in the form of a ‘zipper’ failure, where the casing splits around the sidewall.
Light truck tyres – denoted by the suffix ‘LT’ after the tyre size – can tolerate higher pressures.
Part of your pre-trip tyre strategy is knowing what to do with tyre pressures when you get into the slow stuff. We all know that pressures need to be dropped for soft sand, but what about gravel roads, mud and stony trails?
For soft sand you can go as low as 16psi without risking a tyre coming off its rim, but that assumes sensible driving, not high-speed or dramatic ‘circle work’. You can drop to the same level for sloppy mud that doesn’t hide ‘stakes’ or debris that can cut into a slack sidewall.
On gravel roads it’s best to drop speed to no more than 70-80km/h and lower pressures by around 10-15 percent. That reduces harshness and lets the tyre casings deform slightly when sharp rocks are encountered. However, if you drop pressures and run at higher speeds – 90-100km/h- you risk overheating the tyres and creating a blowout risk.
Rocky trails are best handled with lower-than-highway pressures – usually in the mid-20psi to low 30psi range. Going lower is OK if the sidewalls don’t bag out very much.
Many people don’t drop their pressures off-road because it’s too inconvenient blowing them up again when it’s time to go back on-road. It takes less time to blow up tyres than it does to demount and replace a flat one, and far less expense than replacing a destroyed one.
A pre-trip check should include a trial run of your tyre pressure gauges – yes, you need two per vehicle – and your compressor and back-up foot pump.
Unless you’re talking a major expedition – Cape York, The Simpson or The Canning – you probably don’t need specific-terrain tyres.
Cape York pre-trippers should make sure their tyres have at least half tread depth and that they’re preferably prominent-block-tread types.
For the Simpson a highway-style tread is ideal. The Canning is alternately stony and sandy, so a compromise tyre pattern is the go, but tyres must be ‘LT’, not recreational or passenger-car types.
Cold Adjustment Only
If you discover that your tyre pressures are too high it’s not a smart move to drop pressure from them while they’re hot from a long highway run. It’s easy to let out too much air from a hot tyre, so wait until they’ve cooled off – preferably next morning.
Your tyre pressure gauges are vital pieces of town and bush travel equipment, so look after them. Because you should be using them every week around town and every day in the bush they should be ready to hand – door pockets are an ideal resting place.
Don’t rely on one of anything when it comes to tyre care. Have a back-up foot pump for your electric air compressor – a ‘cheapie’ will do for back-up purposes – spare valves and caps, a second jack, a tyre plug kit and a second spare tyre and wheel for long trips.