Tyres: Moulded around the way you ride
FOR things which started life dripping out of a tree in some distant, tropical rainforest, tyres have achieved a pretty significant role in all our lives.
We all moan when they burst, we all moan when we have to shell out a couple of hundred quid to replace them, but just think what life would be like without them. They’re the unsung heroes of motorcycling. We all go mad about motors, get turned on by tuning and even think suspension is sexy. But mention that you’re into rubber and people give you strange looks.
Maybe it’s because they just don’t understand. Most of us know an engine is made by casting and machining, but how exactly did they turn sap into something which means you can crank it over at 100mph in confidence?
Just 10 years ago, we were all riding around on rubber which had been developed more by trial and error than sound science. But as technology improved, and as computers became able to replicate every aspect of a bike’s tyre needs, manufacturers honed their production skills until they came up with the products we all use today.
Tyre development continues in much the same way as motorcycle development, which means that while there are new models introduced every few years, changes are constantly being made to current versions. For example, the first Dunlop D204 was different in detail to the final version, which was eventually replaced by the current D207. In addition, tyre design is so crucial to the handling of a motorcycle that there are usually different versions of the same tyre for different types of bike – the Metzeler MEZ4s fitted to the Ducati ST4 are different in construction to the MEZ4s designed for Suzuki’s TL1000R.
The crux of the matter is the carcass, the structure of the tyre underneath the rubber tread. The aim of a tyre designer is to control as closely as possible the way the tyre deforms as it’s used. With no weight on it, a tyre would be completely round. But when fitted to a bike and ridden down the road, the bottom of the tyre is deformed into a flat, roughly oval shape. With a very stiff carcass, this deformation would be small, so the amount of rubber in contact with the road would also be small. The benefit of this is that a small contact patch generates less heat, so it can cope with higher speeds. A more flexible carcass, on the other hand, has a much larger contact patch, but it generates more heat, causing the rubber to deteriorate more rapidly. So the carcass has to be a compromise between rigidity, stability and longevity.
In the last few years, computers have made it possible to simulate the behaviour of a carcass quite accurately, so designers can experiment with different constructions without the need to physically make them.
Most modern tyres are described as radials. This means that beneath the tread and around the circumference of the carcass is a belt designed to prevent the tyre expanding under the effect of centrifugal forces at high speeds (this is undesirable because it changes the tyre’s profile, which in turn upsets the way the bike handles).
This belt is formed from strands of aramid (usually known by the brand name Kevlar) wrapped around the circumference of the carcass. Other firms, including Metzeler, use steel.
The main body of the carcass has its cords running across from one side to the other, usually at or close to right angles with the wheel rim. These are set into a tough rubberised material and are used in layers, or plies, to give the tyre its basic strength. The number of layers will often vary, according to the weight and power of the bike the tyre is intended for – heavier machines use three-ply tyres, lighter bikes have with one or two-ply tyres. The aim is to provide enough sidewall stiffness to stop the bike squirming in corners, and at the same time have enough compliance for a high ride quality.
The carcass is constructed in a continuous length, which is churned out of a machine like a piece of spaghetti. Then it’s chopped up and wrapped around a mould where the ends are glued together before the outer belt of aramid or steel strands is put in place. The ends are cut at an angle so there is a large surface area for the glue, but this type of joint is much stronger in one direction than the other. This is the reason tyres have directional arrows on the sidewalls. The front tyre has to deal mostly with braking forces from one direction, while the main forces on the rear tyre come mostly from acceleration in the other direction.
Once the carcass is constructed, it’s placed into a mould where tread is applied. This is injected into the mould in liquid form through tiny tubes, which leave evidence as the bristles found around new tyres. Nowadays there is no natural rubber in the tread – it’s entirely synthetic and developed to offer the required balance of grip and wear. These two characteristics are generally contradictory, better grip meaning worse wear, but as the chemical cocktails have become more complex, both have improved.
The design of the tread pattern owes a surprising amount to aesthetic considerations – many riders will choose between similar-performing tyres on looks, so a lot of effort goes into its style. The other important function of tread is clearing water, and the tread pattern must also be designed to move water rapidly from the centre of the tyre to the edges. Tread also determines the movement of the outer layer of rubber, and in turn the heat generated as well as the stability of the machine, and it’s here that some of the biggest advances have been made in computer simulation of tyre performance. Where once tyre manufacturers had to cut a tread then try it out, today the whole operation is carried out on computers.
Virtually the whole process, apart from moving the tyres from one area to another, is automated. The first person to touch your new tyre may well be the man fitting it to your wheel.