Understanding 4×4 tyres: How they work and why it matters

In this second instalment of our Unsealed Guide to 4x4 tyres series, we're going to chat about how and why 4x4 tyres work.

By Robert Pepper 10 Min Read

In this second instalment of our Unsealed Guide to 4×4 tyres series, we’re going to chat about how and why 4×4 tyres work.

The purpose of a car tyre is to provide grip so you can accelerate, brake and turn.  It also plays an important part in ride comfort, fuel efficiency, noise and handling.

4×4 tyres have the toughest brief of all tyres as they must work well on every terrain, from ice to rock to mud. It’s also expected that they’ll last a long time, not impact fuel efficiency and not make much noise.  No tyre can be good at all of that, so there are compromises even on the most expensive tyres. That’s why expensive mud-terrains are noisy and increase fuel consumption as well as being, well, expensive. It’s all a matter of compromise.  However, you do find that the cheaper the tyre, the more the overall compromise.

How do tyres grip the surface?

Tyres grip the surface in two ways: adhesion and mechanical.  Adhesion, or molecular grip is stickiness, think of tyres with a coating of Blu-Tack.  The compounds used in the tyre give you adhesion, and that grip would work even on a perfectly smooth and hard surface.

how tyres work
An open pattern tyre offers good offroad grip (like the above BFGoodrich Mud-terrain) but is compromised onroad with relatively poor wear, handling, noise and fuel consumption

The other type of grip is mechanical, and for an extreme example, think of two cogs interlocking in oil – there’s no adhesion; it’s purely mechanical. Tyres work in the same way, as the tyre moulds into the surface of the road, allowing it to grip – think of that Blu-Tack again, but you’re pushing it into bitumen. 

There are two types of mechanical grip: micro and macro.  Micro grip is something like the tyre forming into tiny holes and mounds on bitumen, macro is more rock crawling with maybe fist-sized objects the tyre wraps around. Of course, it’s easier for the tyre to wrap around when aired down, so airing down increases macro-mechanical grip as well as provides a greater contact patch, which helps all grip, including adhesion.

When a tyre ages, its ability to deliver grip, particularly adhesive and micro-mechanical grip, reduces as it no longer has the flexibility to micro-mould to the surfaces. This is especially noticeable in wet conditions.

how tyres work
Tyres using their tread to disperse water at the front part of the tread so the remainder of the contact patch can grip the road. The compound of the tyre helps with adhesive grip.

Does a tyre need tread? 

Maybe not. Circuit racing cars use slick tyres with no tread at all, relying entirely on adhesion and micro-mechanical.  These tyres maximise the amount of rubber in contact with the ground as there’s no tread to get in the way, and they are soft, so they have very good adhesion and micro-mechanical grip.  But slick tyres wouldn’t work offroad as there is no tread to deliver macro-mechanical grip over logs, rocks and the like.  So why don’t road cars run slicks?

The answer is water.  The main purpose of tread on a road car tyre is to disperse water.  When a road is wet there is a thin film of water over it, and maybe not so thin if there’s a puddle.  A tyre at speed will skip over water, similar to how we’ve all skipped a stone over a lake. The stone is heavy enough to sink, but at speed, the water is hard and the rock bounces.  Similarly, stepping into a swimming pool doesn’t hurt as the water parts. But jumping from several metres higher up will hurt and if you go high enough, there’s no effective difference between jumping onto water or concrete.

So, the tyre needs to move the water out of the way before it can grip the road, and that is what the tread does.  The first maybe 30% of the contact patch gets the water out of the way, leaving the last part to actually grip.  So, on wet roads, you have less contact patch and thus grip than on dry roads, and the faster you go, the less contact patch you get. The other reason is less adhesion, as water is a lubricant. The next time you’re on a freeway, look at the amount of water the tyres disperse – it’s pretty impressive.

Yes but what about 4×4 tyres?

The reason the tread blocks are wide apart on 4×4 tyres is for mechanical grip, and there are also tread blocks on the sidewall. Effectively, the tyre becomes a cog so it can offer some grip even when coated in slippery mud, at which point a slick would be useless.  But it is important to keep the tyre free of mud as those tread blocks need to be free to grip. Tyres are designed to eject debris, often using small strips of rubber called sipes, and it is a valid technique to spin tyres to eject mud, but sometimes that can get you into more trouble!

how tyres work
Tyre wrapped around a stone at low pressures; macro-mechanical grip

However, big, open tread patterns have disadvantages, mostly for on-road use.  The tyre balances on big blocks of rubber, which is not good for handling.  The tread blocks trap air as the tyre rotates, are noisy and have aerodynamic resistance.  There’s less rubber in contact with hard surfaces, so wear is greater and adhesion is less. All of this adds up to greater fuel consumption and wear.

Tyre plies get a lot of airtime, and as usual, people want to go for the more the better.  But it’s never that simple.  A ply is a layer of material that makes up the carcass of a tyre, roughly similar to a layer in plywood. The more plies a tyre has, typically the stronger, more load-bearing and more puncture-resistance it will be.  However, much depends on what the ply is made of and the tread which is affixed to the carcass.  More plies also mean more weight and sometimes less flexibility.  I wouldn’t focus on the number of plies; I’d look at the quality of the tyre overall, the manufacturer’s reputation, and what the tyre is designed for.  There are some six-ply tyres weaker than four-ply tyres, for example.

So, how do tyres work in different 4×4 situations?

On dry bitumen, you need adhesion and micro-mechanical grip.  You don’t need big, open tread blocks and in fact, those hurt handling, ride, and fuel economy.  But over rocks the situation changes, that’s where you macro-mechanical grip comes into play, big tread blocks slotting in and over the edges of rocks, similar to cogs interlocking. Adhesion and micro-mechanical grip still play a part, but what with dust and dirt, it’s less important.

Then we come to sand, and here it’s all about reducing rolling resistance, which means flotation, which means low pressures. The actual tread pattern doesn’t make much difference, as any tyre will mould its tread into the sand. It’s all about not sinking in so the rolling resistance doesn’t increase. 

On icy surfaces, you’re looking mostly at adhesion as ice is hard, so your macro-mechanical isn’t going to work, and it’s smooth, so your micro-mechanical won’t be much use either.  This is why winter tyres have special compounds for adhesive grip.  You can always put studs on, which create a macro-mechanical grip, which is very effective on ice.

Knowing your tyre grip helps you as a driver, something we’ll cover in this series.

Soft surfaces require flotation to reduce rolling resistance. Grip is mostly micro-mechanical as the tyre can mould its tread into the surface, and the more tread in contact with the surface, the better!
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Robert Pepper is an independent automotive journalist specialising in 4x4s, camping, towing, fast cars, and tech. Robert’s mission is to make these high-risk activities safer through education informed by his own experience and a commitment to inclusivity. He has written four books and hundreds of articles for outlets in Australia and around the world, and designed and delivered driver training courses in all aspects of offroading, towing, and car control. In order to maintain independence Robert’s current outlet is his own YouTube channel and website.
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