MotoGP Blog

MotoGP tyres

January 4th, 2008

There are three components that come together to provide race-winning performance in MotoGP; the rider, the bike and the tyres. In this article we will examine what goes into making a MotoGP tyre and how and why the tyres are so critical.

Tyres are, put simply, the way the motorcycle engine’s power is transformed into forward motion. They are the interface between the engine and the tarmac, and allow the bike to accelerate, brake and turn corners. If you have ever ridden a motorcycle with worn or incorrectly inflated tyres, you will be very aware of the difference this makes to the handling of the machine and your own confidence in what it will do, and how large a difference to the feel of the motorcycle a comparatively small physical difference in the tyres can make. This is the reason tyre companies are so reliant on data gleaned from races and testing and even go to the extent of taking casts of the tarmac to simulate the profile of the tarmac in the laboratory.

The main aspects of tyres that are crucial to their performance are:

Construction: the way in which the structure of the tyre is put together.
Compound: the nature of the rubber which is in contact with the tarmac.
Profile: the external shape of the tyre.

We’ll have a look at each of these in turn:

Construction

The construction of the tyre is formed of two parts: the carcass and the plies to which the compound is attached.

The carcass provides the tyre strength, flexibility and shock-absorbing characteristics. The carcass is the elementary building block on which the rest of the tyre is built and a radial carcass (in which the material is arranged at 90° to the rotation of the tyre) offers more flexibility than a more traditional cross-ply carcass and so offers improved damping from the tyre, and is the construction used in MotoGP. This diagram from Avon Tyres illustrates the construction and the plies on a motorcycle tyre.

tyre-construction

The plies are belts of fibres which may be arranged at various angles to the direction progression along the road, from 0° (around the circumference of the tyre) to 90° (across the direction of motion). Combinations of layers of these plies affect the feel and behaviour of the tyres during riding - some combinations will be more stable when cornering but unstable when travelling in a straight line at high speed for instance. Also, the ability of the tyre to heat up, retain heat or likelihood of overheating are all affected by the arrangement of the plies beneath the compound. The plies are usually made of nylon or aramid fibres, aramid being very strong and stable even when heated. The strength of aramids can be characterised by their use in body armour - they are what give Kevlar its protective properties.

Compound

The compound of the tyre is key to the grip the tyre can offer. Much of the experimentation of the tyre companies is about finding the right compound for the particular asphalt and conditions on the day. Compounds are composed of over a hundred different elements but the three main components are synthetic rubber, oil and sulphur. The relative amounts of these ingredients in the compound changes the properties of the compound and how it reacts with the road surface.

Tyres intended for wet weather riding - intermediates or full wets - will have a further major compound added to them - silica. This improves the grip of the compound considerably in the wet and helps the rider achieve amazing lean angles on a wet track.

A tyre need not be composed of a single compound. It is now quite common for a tyre to have two or three compounds across it’s width. The centre compound will be used when the bike is upright on a straight and so will be hard wearing and able to cope with high temperatures. The compound wither side will be softer and provide more grip in the corners and will not need to cope with high temperatures for long periods of time. The compound on either side of the tyre may vary too. For example, if a circuit has a bias towards right-hand corners or there is a very long right-hander on the track, this may lead the tyre company to use a harder-wearing compound on the right side of the tyre. Compounds are a very closely-guarded secret in the competition between the tyre companies, and are where much of the research budget is spent.

This is one area where race technology has filtered down to road bikes, as variable-compound road tyres are commonplace.

Profile

The profile of a tyre, visible when viewing a cross-section of the tyre cut at 90° to the rotation of the tyre, influences the handling of the motorcycle massively, especially the front tyre. A “V” shaped cross-section will lead to a machine which is unstable and nervous on the straights but solid when cornering as a larger contact patch is in contact with the road. A “U” shaped profile to the front tyre will make for a more stable bike in the straights but one which is more reluctant to turn.

The diameter of the rim the tyre is fitted to is also a factor in the profile. The 2007 MotoGP season saw some riders using 16 inch fronts, which are smaller than the more conventional 16.5 or 17 inch diameter rims. The riders choose these rims as they make the bike easier to turn and more agile in corners. The trade of is that they are less stable in the straights and will “fight” the rider more.

In summary

Tyres are still something of a black art, with manufacturer secrets jealously guarded and competition intense, and a seemingly never-ending cycle of testing and re-testing of combinations of construction and tarmac and compound. How much more there is still to learn can be illustrated by the discovery of Gary McCoy. He revolutionised the understanding of the science of tyre grip in MotoGP when he burst into the 500cc category. He ran with 16.5 inch rears, half an inch smaller than the convention then, and span the tyre up going through corners. What he discovered, apart from spectacular footage, was that he was able to gain more grip and more corner exit speed. By spinning the tyre he was heating up the only surface of the tyre without overheating the carcass, so his tyres were actually able to last as long or longer than his rivals while undergoing what looked like torture.

The advent of traction control has meant that McCoy’s trick is no longer the fastest way around a corner. It will, however, always be the most spectacular, and there will always be more to learn about tyres.

References