Choosing off-road tyres – Why your choice of rubber needs to be rethought
When it comes to choosing tyres for your off-roader, do you really need one set for the road and one for the bush?
BACK IN THE DAY, everyone ran skinny, narrow tyres on their Series Land Rovers, G60s and FJ40s. There was pretty much just the one tread pattern, “open”.
Then the bigger-tyre craze started, and 31s became small, 33s so-so, 35s were where it’s at and 37s the way to roll. In recent years the giant-tyre fixation has died down a bit, for a few reasons I think. Firstly it’s becoming harder and hard to modify modern cars, and the law is cracking down. But another reason is that there’s less and less need for bigger tyres, for two reasons.
Reason number one is car capability. You can have whatever opinion you like about modern electronic vehicles, but you’d need to be some kind of denialist to say they aren’t more capable than their forebears. The Discovery 4, for example, kills the Discovery 1 for offroad capability. A big part of the difference is traction control which is slowly making cross-axle locker redundant, but there’s also better gearing, sophisticated autos and tractable engines. In short, today’s cars manage to get places with all-terrains that older vehicles needed muddies.
Reason number two is the tyres themselves. We want our tyres to grip in all conditions from dry bitumen to ice to rocks, resist punctures, carry loads, cost little and for a small but increasing number of us, offer fuel economy and low weight. Happily, modern tyre technology is helping us have it all. Today’s all-terrains grip better offroad than yesterday’s mud-terrains, and today’s mud-terrains are as good on road as yesterday’s all-terrains.
You might also argue that today’s 4×4 tracks are easier than ever, thanks to enthusiastic grading. There’s a bit of merit to that argument, although we do tend to look back on the old days with fondness that’s not always deserved. And if your vehicle is more capable then you’ll cruise over problems that would have stopped you before.
All this progress means that:
- a standard vehicle doesn’t need a tyre as good as an older vehicle to cover the same ground.
- the all-terrain of today outperforms the mud tyre of yesteryear
- ever-better road handling this means that newer vehicles can still ride and handle well enough with offroad tyres.
So the days of needing to swap tyres from offroad to on are pretty much over, but why are modern tyres so much better? Let’s take a look at Cooper Tires’ S/T Maxx to find out, which is pretty representative of modern tyre design.
What we need first from a tyre is strength, and that’s getting better over the years as people are reporting fewer and fewer punctures – this is why roadcars now have no spares at all in many cases.
Strength in a tyre is designed in several ways. Tyres are made up of cords which run across the sidewall and belt, and the strength of a cord is generally determined by its tensile strength (pulling force to break). Strong cords are good, but there’s also the cord density, expressed in “EPI” or Ends Per Inch. Imagine cutting a cord – how many ends could you fit across an inch. What you want in a good, strong tyre is cords with high tensile strength, and lots of them, so a high EPI. This is because lots of small cords have three advantages over smaller number of large cords – firstly the small ones forma layer more difficult to penetrate than and secondly they are more flexible, and flexibility is critical for a tyre as that is largely how it grips, and thirdly it means a lighter construction for the same weight.
The S/T Maxx benefits from advances in cord textile manufacturing so its cords are now thinner for the same strength as previous tyres. This means a greater EPI can be used, and the tyre is lighter, so what Cooper have done is move the S/T range from two-ply to three-ply for greater strength. The third ply is designed to distribute impact energy over a greater area of the tire’s sidewall, reducing the likelihood for damage or puncture.
Tyres also have steel belt wires, and here again the Maxx’s belt wire strength is about 12% higher than the S/T’s, while at the same time being about 12% lighter. Today’s 1mm steel is as strong as yesteryear’s 10mm steel, thanks improved manufacturing techniques.
The move to a three-ply tyre has made the overall tyre heavier than a two-ply, but not as much as would have been the case if the three-ply was made from heavier materials. Importantly, the new belt wires reduce weight in the tread areas which is critical because the tread area is at the edge of the tyre, travels fastest and therefore uses most energy to speed up or slow down. This also means there’s less stress on the drivetrain components, and that’s always good for longevity.
The compound of the tyre has changed for the better too. Tyres aren’t just rubber – they also contain significant carbon black which improves abrasion resistance and helps dissipate heat which is the enemy of the tyre carcass. However, carbon black is not great for grip or rolling resistance. For that, you want silica which scores well on both points. Again, improvements in manufacturing have seen silica be produced cleanly and then chemically bonded into the tyre compound, so the tyres have better grip (especially in the wet), less chipping offroad and last longer. Cooper claim the Maxx has 20-25% lower rolling resistance than the predecessor S/T-C tyre. That equates to better fuel economy of course, but also less energy required to turn the wheels which means better progress offroad.
To see the importance of the compound we can look at winter tyres for a clue. These stop around 30% quicker than standard tyres in icy conditions, and that’s got little to do with the tread and more to do with the compound. Tyre manufacturers are well aware we might often rate tyres by how aggressive they look, but in reality there’s more to tyres than meets the eye.