TYRE performance takes on a whole new meaning when it’s applied to off-road four-wheel drives. Grip on wet and dry roads is important, but it’s stuff like puncture resistance, off-road grip (in sand and mud and everything in between) and vehicle load ratings that can make or break a 4WD tyre.
However, it can be a tough call when it comes time to replace your 4WD’s standard rubber with something a bit more flash. For starters, how four-wheel drive tyres are classified is confusing. There could be simple, general ways of defining tyre types, such as on-road, off-road and heavy-duty off-road, but it’s not that simple.
Many tyre manufacturers provide some kind of indication of a tyre’s abilities (for example, with star ratings or percentage of use on- and off-road), but because there is no standardised rating system, brands can’t be compared or relied on.
So, 4WD tyres are broadly grouped in three categories: Highway Terrain, All Terrain and Mud Terrain. Then there is the matter of Mud and Snow tyres, as well as Passenger construction and Light Truck construction tyres.
Highway Terrain (H/T) tyres are the lightest in construction and the quietest and the smoothest to drive on. These tyres, with a tread pattern much like a car tyre, are also usually the best for tarmac traction. H/T tyres are fitted to most new 4x4 wagons and some 4x4 utes as standard because they are often cheaper to produce than other tyres.
As most new 4WD owners don’t go off-road, H/T tyres score well in the around-the-block test-drive for their smoothness. So, it’s a no-brainer that H/T tyres are the pick of almost all 4WD manufacturers.
Not only do H/T tyres have the lightest construction and most car-like tread pattern, they have a higher speed rating and lower load rating. They are ‘Passenger-rated’ tyres for this reason. If you have a 4x4 that’s used mainly on paved roads (with the occasional foray onto forest tracks) then it makes sense to use an H/T tyre.
Many OE tyres don’t have great grip, so when it’s time to replace them you usually have a wide choice of better tyres to choose from. It’s also easier to find a direct replacement tyre when your vehicle has an odd tyre size or – in rare cases – is a large diameter tyre in, say, the 19- to 22-inch range.
All Terrain (A/T) tyres are the next step up from H/Ts for off-roading. A/T tyres are usually built stronger and have better tread patterns for hitting the tracks, and they are the best compromise for when you mix urban driving with the occasional off-road trip. Depending on the specific tyre, it may be noisier and offer less on-road grip than an H/T tyre.
A/T tyres take in the middle ground, with typically a lower speed rating than an H/T – often around a T to H rating (see Speed Need, opposite page) – but have a higher load rating. However, not all A/T tyres are built equally tough, because tyre manufacturers design them according to what they believe is important. Some will have a tread pattern and carcass construction close to that of an H/T tyre, while others will have a stronger carcass and chunky tread similar to a Mud Terrain (M/T) tyre.
The way to tell if an A/T tyre is worth buying for off-roading is to check the number of plies (layers in the sidewall and tread area) it has compared with an H/T tyre. The A/T should also have a more pronounced and open tread pattern.
This is where Light Truck (LT) construction comes in – A/T tyres better for off-roading will have the LT designation. LT tyres have a stronger carcass so that they can meet the load requirements of light trucks. Even utes with a one-tonne (or more) payload have them as standard, so the tyres will handle the carrying capacity without popping a tyre. If you look at any new ute, you’ll see that an LT tyre doesn’t necessarily have an off-road tread pattern.
LT tyres can vary in toughness. The best way to tell is to see how many sidewall plies the tyre has – usually four plies in the tread area and two in the sidewalls is a good start. Eight- or 10-ply ratings are best for resisting stone damage and sidewall staking.
A Light Truck All Terrain tyre is the best all-round tyre for a 4x4, as it will do plenty of kilometres as an off-road tourer. As a bonus, in a 4x4 A/T pattern, they tend to have deep tread, so they last longer and have far better mud-clearing properties and puncture resistance than an H/T tyre.
M/T tyres (the heaviest and strongest construction, with a blocky, deep tread pattern) have a low speed rating and a high load rating. Their tread pattern is designed for muddy off-road conditions and, while they offer good grip in other off-road situations such as rock shelves, they are not the best for sand driving. On the road, M/T tyres can be noisy and harsh, and they don’t provide the same level of handling or grip as A/T or H/T tyres.
The M/T’s extra rolling resistance also causes increased fuel consumption. It’s no wonder the M/T is favoured for competition trucks. For heavy-duty off-roading, LT M/T tyres are the ultimate choice, as their heavy construction provides good puncture resistance.
Mud and Snow tyres should have better traction in snow, but ‘M+S’ (or M/S) just indicates tread pattern. For tyres to be marked as Mud and Snow, they must have outer tread grooves that lead into the centre of the tread. The ‘M+S’ marking also indicates at least 25 per cent of the tread area is an open tread pattern.
With tyres, it’s a shame you can’t try before you buy, but that’s the same with most consumables. At least with this guide, you’ll be able to go in with eyes wide open.
|Speed rating symbol||Max speed (km/h)|
|Index Number||Max permissible load (kg)|
Three types of size description are used on 4x4 tyres, for example:
265 = section width (in millimetres)
65 = sidewall aspect ratio (as a percentage)
R = tyre construction (in this case, radial)
17 = rim diameter (in inches)
100 = load rating (in this case, 800kg)
T = speed rating (in this case, 190km/h)
9.50 = width (in inches)
R = tyre construction (in this case, radial)
16 = rim diameter (in inches)
LT = Light Truck construction
32 = diameter of tyre mounted on rim (in inches)
11.50 = section width (in inches)
R = tyre construction (in this case, radial)
15 = rim diameter (in inches)
LT = Light Truck construction
IT’S THE LAW
A TYRE placard with available tyre sizes and minimum speed and load ratings has to be fitted to all vehicles to meet Australian Design Rules. These are the correct tyres for your vehicle, although there are a few exceptions (as we will explain). On most four-wheel drives, you’ll find the tyre placard on a door jamb or glovebox lid.
Always make sure load ratings meet or exceed the minimum for your vehicle, because you could be faced with a fine or, even worse, a tyre blow-out. And, if the tyres contributed to the crash, your insurance mightn’t cover you.
Wider rims and tyres are often a good thing, but to stay legal the tyres can’t be wider than the guards. Too-wide tyres can rub against brake lines, the suspension or guard lips, and none of this will end well. The wider tyres spray mud and water everywhere, too. Fit flares and – if needed – trim the guards.
You might have no choice but to fit bigger tyres because of a limited range of sizes, or because you want to increase ground clearance with a taller tyre. You don’t have to stick with the tyre sizes on the placard, but you do need to keep the overall rolling radius to a maximum 50mm over standard to stay legal. The 50mm is for total ride height change, so keep that in mind if you have (or are getting) a lift kit.
Most tyre manufacturers list the overall rolling radius of their tyres, so it’s pretty easy to work out what percentage a certain size is over your four-wheel drive’s standard boots.
You’ve got wriggle room with speed rating, too. The law allows lower-speed-rated tyres for ‘off-road’ vehicles. That minimum is N (140km/h). Don’t be tempted to drive over those rated speeds on unrestricted NT highways, because you’re heading for a tyre blowout and a nasty crash. The tyres aren’t built to go that fast for very long.
BLOWING HOT AIR
TO FIND out what the correct pressure is for your rig, you need to do a tyre pressure increase test.
For this, you’ll need an accurate tyre gauge, preferably analogue (digital types work really well until the battery runs flat just when you don’t have a spare). Spend some good money on a quality gauge and take care of it – if it gets knocked around in the ute tray, it probably won’t be very accurate.
Check the tyre pressures with cold tyres, preferably after the vehicle has been standing overnight (or at least a couple of hours). Make sure the tyres are up to placard tyre pressures. Then, after driving around 50km at around 100km/h, stop and check pressures again. You should see a rise of 4psi from cold pressures for Passenger construction tyres and 5-6psi for Light Truck tyres. If the pressure rise is less, you have over-inflated the tyres; if the pressure increase is more, you have under-inflated the tyres. In either case, it’s a matter of small pressure adjustments until you’re satisfied you’ve got the right pressures.
Under-inflation is the biggest killer of tyres, increasing irregular tread wear and the likelihood of damaging the carcass. Under-inflation typically happens because no one checks pressures, so they lose air. Make it part of your routine to check pressures at least once a day; better still, get a tyre pressure monitoring system so you can keep an eye on pressures as you drive.
Over-inflation doesn’t help tyre life, either, as it increases the risk of punctures, irregular tyre wear and decreased ride quality. Maximum tyre pressure is marked on the tyre sidewall – if you’ve exceed this, you’re asking for trouble.
EVEN though the minimum legal tread depth is 1.5mm, grip reduces when tyres get down to 3mm. You’re more likely to get a puncture when this happens, especially when off-road.
If you’re about to go on a big trip, then a fresh set of tyres is ultimately a cheap form of insurance – even though most 4x4 tyres aren’t exactly cheap. You’ll have the better tyre grip and a much better chance of avoiding punctures.
Most tyre manufacturers recommend that any tyre more than six years old should be replaced, irrespective of how much tread it has. So that never-used spare sitting on the back of your 4WD for the past 10 years isn’t going to be much use. Rubber degrades (as does the tyre’s steel reinforcement), especially when out in the elements, so the risk of a blow-out is very high after about six years. Rubber hardens on old tyres, too, which you will find out the hard way the next time you drive in the rain.
Most tyres have a four-digit production date stamp on the sidewall. The first two digits represent the week of the year they were produced and the last two digits represent the calendar year. So the figure ‘4210’ on a tyre means that it was made in the 42nd week of 2010. If the sidewalls or tread area has small cracks showing, then the tyres should be thrown away.
MICHELIN’S airless tyre, the Tweel concept, lobbed 11 years ago and was a modern interpretation of the old buggy wheel. Except this time, the ‘spokes’ were a lattice-like interwoven structure that absorbed road shocks like a pneumatic tyre. Michelin has gone quiet on the Tweel, but Bridgestone showed something similar with its airless tyre, the Air Free Concept, at the 2013 Tokyo Motor Show.
The Air Free Concept tyre has a spoke structure made from reusable thermoplastic resin that stretches along the inner sides of the tyres to support the vehicle’s weight. Along with the rubber tread, the materials used in the tyres are all recyclable.
Both the Michelin and Bridgestone tyres have a problem with the lattice-like absorbent part of the tyre: when filled with mud (especially when dried) or snow, they become like Fred Flintstone’s chariot wheels – there is no give in the tyre. The other problem is that, in Bridgestone’s case, the tyres only support a vehicle weighing 410kg and have a maximum 60km/h speed. The airless tyre has a way to go yet before it’s a production reality.
The closest thing to a puncture-free (air-filled) tyre is Continental’s ContiSeal tyre, which has a sticky, viscous layer from shoulder to shoulder along the inner carcass. It’s not designed or intended to act as a permanent puncture repair; it’s more like an automatic plug repair kit.
If an object up to 5mm in diameter penetrates the tread, the ContiSeal layer envelopes the puncturing object and gives a near instantaneous seal. If the puncturing object becomes dislodged from the tyre, the material is designed to seal most holes up to 5mm in diameter. Continental claims there is no tyre performance detriment, but it’s currently not offered in its 4WD tyre range.
STOP THE LEAK
TO REPAIR a tyre puncture out in the bush with a wheel still fitted to the vehicle, you’ll need a tyre plug puncture repair kit. This should include at least 10 self-vulcanising plugs, a probe tool, an insertion tool, a razor blade, lube, and a 12-volt tyre compressor to re-inflate the tyre.
Only the tread area should be plugged. If the sidewall is punctured or torn, don’t try to plug it unless you have no spare tyre and it’s to get you out of the bush. If you have no choice but to do this, drive slowly until you reach town and get the tyre replaced immediately.
For a simple guide to get the most out of your 4x4 tyres, check out our Five tips for longer lasting tyres.