Fitting all-terrain tyres

In Technical by Glyn DemmerLeave a Comment

The other day a friend chatted to me about the fitment of all-terrain tyres and queried why they were not fitted to new vehicles as opposed to the highway terrain tyres fitted as original equipment. Manufacturers will occasionally fit them onto limited-edition vehicles but this is not the norm. The reason to fit a highway tyre is not only cost but also the fact that many 4×4 owners spend up to 90% of their time on tar. Rarely venturing off-road.

All-terrain tyres look seriously cool and allow you to take full advantage of the traction provided by your four-wheel-drive system offroad and on gravel. But they can be a compromise, special tyres can outperform them in specific circumstances such as snow, ice and mud. But across all surfaces, an all-terrain tyre will deliver good performance. They will have a stronger bead ( steel rope-like part that runs around the inner perimeter of the tyre and holds it onto the rim ). The plies ( inner belt-like structures under the rubber ). Steel belt plies in modern tyres make them more durable and puncture-resistant. The tread is also thicker, designed to handle rough terrain, grip loose surfaces and disperse mud, water and snow efficiently.

This is where the trade-off comes in, the all-terrain tyre will be heavier than its highway counterpart and not as efficient on tar and in the wet. Braking will be slightly compromised. They will also be noisier on-road and cornering will be affected due to the thicker sidewalls’ inability to flex easily.

Then we get to sizes, larger tyres add to your vehicle’s stance and they roll easier over obstacles. The sacrifice will be fuel economy and a possible change to your speedometer. Oversize tyres can also affect gear ratios so it’s important to fit a tyre that is within the vehicle’s manufacturer’s specifications. Never compromise your warranty as your whole drivetrain could be affected. If you decide to go the route of fitting a lift kit to accommodate a larger tyre once again ensure that it is warranty compliant. Another disadvantage is the fact that the front wheels may foul when turning a full lock!

So my suggestion is to stick to the tyre size your vehicle left the showroom with or check the specifications to see if you could go a little wider which gives you a slightly larger footprint.

So now that we have got the size over with let us look at the load rating or load index. This is the maximum load a tyre can take at a specified speed, which is indicated on the as a speed rating. Both figures appear on the sidewall of the tyre. Then look at your gross vehicle mass to see if the numbers balance, especially if you are going to load the vehicle for overland trips. The wrong specification could cause a tyre to overheat and fail.

Modern all-terrain tyre empty light truck technology, which is shown on the sidewall as LT and will appear on the sidewall before the tyre size.

As load capacity increases so will the tyre pressure, you will feel this on the road as the tyre will deform less and transmit movement through the suspension and affect ride quality.

So whilst we think of a tyre as being “rubber” tyre manufacturers mix a variety of other items into the blend, you have steel plies, steel beads, carbon black and more recently silica which improves grip and cut/chip resistance.

The tread pattern is also important and manufacturers experiment with the voids ( the gaps between the tread blocks) and sipes ( the lines inside the tread blocks ). The noise you hear from an all-terrain tyre is caused by the passage of air passing through the voids.

With thicker rubber compared to a highway tyre and larger voids and fewer sipes, you have a tyre that is more durable and the larger voids allow the tyres to eject small stones easily. They also improve performance in mud.

The high pressures require a stronger bead which seats the tyre more securely on the rim. This allows you to run at lower pressures off-road without debating a tyre ( running it off the rim ). So it is good to upgrade to an all-terrain tyre if you travel a lot on gravel and go off-road occasionally. A newish trend is to have a certain amount of tread on the sidewall, this gives additional grip when the going gets rough.

If you are thinking of upgrading to all-terrain tyres when purchasing your 4×4 discuss a refund for the tyres that are on the vehicle with the dealers. Many dealers are happy to assist. Ensure that the tyres are professionally fitted, balanced and aligned. Have them checked after a trip and rotate them according to your manufacturer’s schedule.

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