The importance of pressure

It’s a sobering thought that up to 50 per cent of your fellow travellers this Christmas/New Year holiday period will probably be running the wrong tyre pressures on their tow car, caravan or camper trailer.

Get off the bitumen, onto some corrugations, or into sand and the rate increases.

Having experienced slowly deflating tyres on two vehicles in recent days -- one due to a screw in the tread and the other due to a tyre bead leakage -- I’m hyper-tuned to the issue.

The tough thing these days is that with the absence of driveway service in fuel stations combined with extended servicing schedules of up to 30,000km on some new cars and 4WDs, the responsibility is yours to ensure that your rubber is up to scratch.

That may mean an exploratory look at the inside walls of your tyres, as they are just as vulnerable to damage as the outer wall.

Everyone knows to set the pressures of their tyres when they are cold, right? You can even get a 2-3psi variation from those in the sun on a hot day versus the others on the shady aside of your tow vehicle or van.

But to what pressure should we set out tyres? There’s no single answer, as it depends on the size of your tyres, the weight of your vehicle and on what surface you plan to travel -- and how fast -- so take the figures below as a guide only, as they are very general and are based on the range of tyres fitted as standard to most medium and larger 4WDs and most ‘off-road’ caravans and camper trailers.

They were prepared from both personal experience, those of 4WD experts and with reference to the excellent 4WD Driver’s Guide produced by Cooper Tires that is available through their tyre stockists, who can be located by visiting www.coopertires.com.au

The starting point for bitumen touring with your tow vehicle is to go by the manufacturer’s recommendation and in the absence of that for a van or trailer, aim to run them at around 36psi for a single-axle vehicle, down to around 32psi per tyre for a tandem axle set.

Some common sense comes into play here. If you have a single-axle caravan with a tare weight of (say) 1600kg, each tyre will be carrying a load of 800kg. If it’s a tandem axle van weighing (say) 2600kg, each tyre will be carrying 650kg, so the inflation doesn’t need to be as high.

Remember over-inflation means your tyres will roll easier on bitumen but will have less ‘give’ to avoid damage from potholes or obstacles left on the road; under-inflation increases rolling resistance, causing heat build-up and potential carcass failure, while the extra ‘bagging’ in the tyre wall exposes them to potential damage when off-road.

While you should generally increase pressures as the load increases -- such as when hitching up a caravan with a ball weight of 150-200kg or more, there is one golden rule: never exceed the maximum pressure stamped on the sidewall of the tyre.

Remember that the higher their pressures, the less the contact area your tyres will have with the road, which means less friction, but also less grip.

THE FOUR PSI RULE

As a general rule, tyres gain up to 4psi over their cold pressure when they are warm after 20-30 minutes of driving. If they gain more than 4psi, there’s too much friction, so you need to add more air; if they don’t increase by 4psi, they are too hard, so you need to bleed off some pressure.

Work in 2psi increments for safety, aiming for a hot running pressure of around 36psi (single axle) and 32psi (tandem). Remember, these pressures are very general and a large, heavy van may require higher pressures. Consult the manufacturer to be sure.

For Light Truck and 4WD vehicles, use 6psi above cold pressures as a guide.

BITUMEN TOURING

For most large 4WDs, tyre pressures will need to be set at 32-38psi for extended bitumen running. When towing, increase the pressure in your rear tyres about 4psi over those on the front to cope with the extra load on the tow ball.

UNSEALED ROADS

On good, smooth gravel you can retain your bitumen running pressures, although if it’s a long stretch you might wish to drop them no more than 2psi all-round for improved stability.

However if you are travelling on a rough unmade surface, or traversing bad corrugations, you should not only reduce speed, but also lower your tyre pressures around 20 per cent on both your tow vehicle and caravan over their bitumen running levels. The lower pressures will give the tyres more flex to absorb the road shocks without impact fracturing and will give you more grip.

It will also give your caravan or camping trailer -- and everything in it -- a much easier ride.

Many experienced travellers we know immediately drop their pressures all round on both tow vehicle and caravan to 25-28psi. Any lower and you increase the risk of sidewall damage.

MUD

This depends very much on what sort of mud you are in and the steepness of the terrain, but if you have a solid base you can run higher pressure than in really sloppy conditions. Work in the 22-28psi range.

SAND

The pressures you run in sand depend very largely on the depth and coarseness of the terrain and also the time of day and the weather. As a general rule, sand is coarser and has more grip when it’s cool, or after rain and gets softer when warm. So your pressures need to be lower in the heat of the day.

Generally, 18psi works well in most 4WD tyres in most conditions, although with bead locks we know of many people who drop their pressures much lower. The further you lower pressures the less steering response you will have and the possibility of rolling a tyre off a rim, or overheating it, increases.

Remember that the sand near the waterline will be firmer (up to a point) than the soft stuff above the tide line.

If towing a camper trailer or caravan on sand, work off the lower end of these tyre pressure recommendations to allow it to ‘float’ over the soft sand.

RE-INFLATION

Don’t forget that the above-recommended pressures are for specific road conditions and once clear of them, your tyres need to be re-inflated as soon as possible to the correct pressures for the terrain you are on. Be mindful that due to lower pressures and the heat of the day, what your tyre gauge tells you could be 4-6psi above the tyre’s correct cold pressure. So when they cool down, they could be too low the next morning.

OK, it’s a hassle to stop and inflate or deflate tyres; the kids are restless; it’s getting dark; your friends are waiting; there’s a cold drink somewhere beckoning. But think how much more inconvenient it would be if you overheated your rubber, had a flat, or rolled a tyre off the rim?

TYRE PRESSURE ESSENTIALS

There are two essentials to managing tyre pressures than no four-wheel-driver or off-road caravanner should leave home without -- a quick deflator and a powerful pump to bring them back up to pressure.

There are many brands on the market, but ARB makes excellent products covering both needs.

DEFLATION

Forget using your car keys or searching for sticks to reduce your pressures -- ARB’s E-Z Tyre Deflator employs a patented design that removes the valve core for rapid tyre deflation, while also giving you an accurate pressure reading through the simple movement of a sliding valve.

It also shows pressure in handy 1psi increments and with its corrosion-resistant brass/stainless-steel construction is built to last in a 4WD/beach environment.

It has a RRP of $67 (Australian East Coast capital cities).

GOING BACK UP

When you need to reinflate your tyres once you rejoin the bitumen or leave the sand, you and your friends and family don’t want to be hanging around waiting for a wheezy pump to bring them back up to pressure. You need a powerful portable pump and ARB’s CKMTP12 (Maximum Performance Portable) DC air compressor is just that.

Made from lightweight, high-grade materials and mounted in a durable carry case, the compressor features a hard anodised cylinder bore and a Teflon impregnated carbon fibre piston seal for reduced friction and a trouble-free life.

Importantly for those with a caravan or camper in tow, it comes with a six-metre long air hose and clips to deliver the maximum power direct from your battery terminals. It is also is supplied with all the necessary fittings to inflate camping or leisure gear, so no more puffing and straining to blow up those floaties or air beds!

ARB’s CKMTP12 has a RRP of $349 (Australian East Coast capital cities).