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Allan Whiting27 Dec 2010
ADVICE

Replacing your 4WD's tyres

The absolute starting point for 4WD modifications is the tyres, because no new 4WD leaves the factory running on rubber that's optimum for Australian bush driving

"The tyres fitted to Triton as standard equipment are designed primarily for use on sealed roads. If your Triton is to be driven primarily on unsealed roads please consult your Mitsubishi dealer for advice on suitable tyre choice."

This warning appears in the 2010 Mitsubishi Triton specifications and features brochure and a similar warning should be printed on every 4WD maker’s spec sheets and in their driver’s handbooks. But, of course, it isn't.

Most 4WD owners find out about the deficiencies of their vehicles’ standard tyres the hard way: with a series of flats.

The fact is that all new 4WDs -- softroaders and 'real' 4WDs alike -- arrive at dealerships rolling on rubber primarily designed for sealed-road driving.
On-road braking and handling, noise and fuel consumption, long tread life and, increasingly, ‘cool’ appearance, are the deciding factors for the majority of 4WD buyers when it comes to tyres.

Back in the 1980s the ‘look’ was fat donuts on relatively small, highly dished chrome wheels. These tyres were generally OK for off-roading, but decidedly ‘wobbly’ in fast corners and under heavy braking.

The current fashion is for ultra-low profile tyres with almost no sidewall, mounted on the largest-diameter rims that will fit in the wheel wells. These tyres are universally disastrous off-road.

The happy medium for those of us with a compromise need for reasonable on-road behaviour, good tread life and fuel consumption, along with some off-road ability is a tyre shape somewhere between these two extremes. Sidewall profiles in the 60 to75 range are the most common.

The single greatest enemy of 4WD tyres is the classic Australian gravel road. A road topped with sharp stones more than golf-ball size is a tyre-puncture risk.

Most punctures start with a stone cut through the tread area that causes a slow leak. This gradual pressure drop isn’t picked up by the driver, who is busy dealing with washaways, corrugations, potholes and wandering wildlife.

As the pressure drops further the tyre flexes excessively and gets hot. Very hot.  The driver notices only when the tyre goes ‘bang’ and tears to pieces.
Another common bush puncture is sidewall damage, caused by the tyre being ‘pinched’ between rim and a rock, or cut by a tree root as the vehicle drives past.

The standard tyres fitted to new 4WDs succumb to this bush damage readily and are therefore best replaced by tyres rated as Light Truck (LT).

An LT tyre is more heavily made than a passenger-car-rated tyre, using heavier carcass belting and thicker steel under-tread belts, and can run at higher pressures.

If a tyre doesn’t have this heavier construction it isn’t ideal for bush driving – on or off road.

BIGGER NOT BETTER
Sadly, there are currently no LT tyres to suit wheels with a diameter of 18-inch or larger. The most common LT sizes are for 15-, 16- and 17-inch wheels.
This fact should determine what 4WD you buy: if it doesn’t come with 15-17-inch wheels, or can’t be retro-fitted with them, it’s not the vehicle to buy.

An example is the current Land Rover Discovery 4 range. The brilliantly performing 3.0-litre diesel model has a large brake package that cannot fit inside any wheel smaller in diameter than 19-inch. So there’s no LT tyre available for this vehicle.

Discovery 4 buyers can opt for the 2.7-litre diesel model that has smaller brakes and can run on 17s, or they can buy a used Disco 3 2.7-litre.

Fortunately, most softroader and real 4WDs run on 17s or less, so there’s plenty of tyre choice available, but make sure you don’t choose any tyre with a profile lower than 60, such as a 265/60R17. Better is a profile of 65, 70 or even 75.

Tyre size and type are your next decisions. Size depends on what can be legally or physically fitted to your vehicle.

There is a code of practice, set out in Vehicle Standards Bulletin, VSB14, which attempts to nationalise a ruling for larger diameter and width tyres, but there’s presently a catch in the form of a clause that prohibits tyre size changes to any vehicle that’s equipped with electronic stability control.

The Australian Automotive Aftermarket Association is lobbying governments and carrying out vehicle testing, with the aim of having this clause removed.
In the meantime, it’s up to the States and Territories to limit what can and cannot be done with replacement tyres.

The most conservative is NSW, which currently limits diameter changes to 15mm over the maker’s largest optional tyre diameter for that vehicle model and a 25mm increase in wheel track (the distance across the footprint of the vehicle, measured at the tyre tread centres).

If you’re in doubt about replacement tyre legalities, check with the current situation in your jurisdiction.

WHICH TREAD?
Modern 4WDs have generous rubber sizes, so finding an LT replacement tyre that’s legal and fits shouldn’t be too difficult. The tricky bit is sorting out what tread pattern you should buy.

‘Mud’ pattern tyres look aggressive and project an image that many 4WD owners fancy. However, separate-block-pattern tyres are often noisy and have much higher rolling resistance than highway-pattern tyres, so they increase fuel consumption; sometimes by as much as five per cent.

Also, ‘mud’ tyres tend to wear unevenly, in a classic ‘heel and toe’ manner that can cause annoying vibration when the tyres are worn.

If you’re not regularly driving in muddy or rocky country you probably don’t need tyres with such an aggressive pattern.

All-terrain (AT) patterns are most people’s choice, because they’re a compromise between on-road ability, with relatively low noise and fuel consumption issues and reasonable off-road grip on most surfaces.

Pure on-highway patterns clog up instantly on wet, soft ground, giving little grip, but work fine on sand, so if your main off-road pursuit is beach driving they’re a good choice.

The advent of electronic traction control has taken some of the off-road traction responsibility off 4WD tyres.

With classic ‘open’ diffs front and rear, yesterday’s 4WD needed strong tyre grip to get anywhere off-road.

Nowdays, it’s often possible to get by with less grip, because traction control will intervene to prevent total wheelspin in most loose and slippery terrain conditions.

This means it’s possible to use less aggressive tyre patterns for the same off-road results.

HANDLING PRESSURE
No matter what tyre choice you make the next important step is pressure monitoring. You can get a tyre pressure monitoring system for a few hundred bucks and this is the best preventive measure you can take.

As soon as a tyre develops a slow leak the system will detect it and warn you with an alarm and a flashing light. This makes it possible to stop and put a temporary plug in the tyre before it become irreparably damaged by pressure drop or blows out, risking an accident.

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Written byAllan Whiting
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