Car tyres are often taken for granted. You turn the steering wheel, your car takes the corner, you press the brake, your car stops. This is fine but knowing how car tyres work and the characteristics they generate in terms of handling and roadholding is both interesting and potentially helpful to your driving.
Every car tyre has an area of tread, about the size of an adult’s footprint, in contact with the road surface. This is called, somewhat predictably, its contact patch. Imagine the tyres’ four contact patches travelling along a road in a straight line. These four areas of tread are what keep your car on the road, allowing you to accelerate, steer and brake with confidence.
Now, what happens to car tyres in corners? Imagine the wheels following the arc that the corner represents. The wheels will be following the arc faithfully, but there’s a hidden force at work. Imagine you’re Superman and have X-ray vision. Looking down at the top of a wheel and tyre combination, you’d see that the contact patch will be following a tighter curve than the wheel is following.
So, what’s happening? Why the difference? It’s there because sideways force put on the tyre by the weight of the car is deforming the car tyre’s carcass. Now, imagine a straight line drawn through the centre of the wheel and another drawn through the centre of the contact patch. There will be a difference between the two – this difference is called the slip angle.
As slip angles increase, the car tyre’s grip increases, up to a point. When the forces involved head towards the maximum level of grip the car tyres offer, one of three states will apply. Say the slip angles are equal at both ends of the car. In this case, the grip at each end of the car will be the same. When the grip level’s limit is reached, the car will go into a classic, four-wheel drift. The car’s handling will be ‘neutral’. Racing cars are set up to give neutral handling.
Supposing the slip angles of the car tyres at the front are greater than those at the rear. Then, the driver will need to apply more steering input to make the car follow the chosen curve. This is called ‘understeer’.
When it reaches the limit of effective grip of its car tyres, an understeering car will slide off the track forwards – it simply won’t be able to corner tightly enough. In practice, most road cars understeer. Why? Because understeer is a generally controllable condition – it’ll help scrub off excess speed when a corner is taken too enthusiastically.
The third state occurs when the rear car tyres’ slip angles are greater than those of the front tyres. The rear tyres will be giving less grip than the front ones. At the limit, the car’s tail will slide towards the outside of the curve. When you see a Formula One car or Touring Car spin off a track, it’s gone beyond oversteer.
This explanation is necessarily basic. In fact, very many parameters affect how car tyres respond to the forces imposed upon them. However, the physics are just as basic, and give car designers benchmarks from which to work.
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Merityre.co.uk are one of the leading UK independent suppliers of car tyres. Why not visit their website for an online tyre quote or contact your nearest fitting centre.
