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Tyres & Tyre Choice

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Whether it be Avon, Pirelli, Toyo or Yokohama the choice of tyres that you make is the most critical aspect of your suspension design and set up. Whether the vehicle is a track day hot hatch project that must run on the road or a single seater Formula car used for Sprints or Racing  the choice of tyres dictates the basis for the set up, design and improvement of your cars suspension.

When you round a corner your tyres move across the surface with a slip angle. That is is the angle between a rolling wheel's actual direction of travel and the direction towards which it is pointing. It is not possible to round a corner in a car without generating a slip angle. The cornering force increases in a linear fashion to begin with for the first few degrees of slip angle, then increases non-linearly to a maximum before beginning to decrease. You will be familiar with this maximum point as the feeling of “no grip” that is often experienced on a circuit. The key to rounding a corner at maximum speed is to optimise the slip angle. If you enter the corner too quickly and too large a slip angle is generated the car will slide.

The G forces that you feel while rounding the bend are known as centripetal force. This is provided by the frictional force generated between the tyres and the track.

When a car rounds a bend the centripetal force generated by the tyres means that weight is transferred to the tyres on the outside of the corner as indicated below.

Car Overhead Right

It is the amount of this weight transfer that dictates the vertical load on the tyre. Any given tyre will offer more grip with a greater vertical load up to a point. However go beyond this point and it is a law of diminishing returns. This is known as tyre load sensitivity which is the falling rate of its lateral or longitudinal grip (e.g braking and accelerating) when placed under an increasing vertical load for a given slip angle. This means that as the vertical load on a tyre increases its lateral and longitudinal grip increase by a smaller gradient.

Now some weight transfer is inevitable and to a degree desireable (this is discussed fully in chapter 2) but once the tyre is overloaded the car will start to lose grip.

I expect you're wondering what all this has to do with tyre choice? The majority of road tyres are a compromise so that the manufacturer can offer a hard wearing, low-cost products that can be used in a range of environments and temperatures. If you are forced to use a road tyre it is useful to look at the treadwear figure. 100 is the index. So a figure of 200 means the tyre will last twice as long as the index. A typical road legal trackday tyre will be 60. A hard road tyre over 400!

If we start to look at track use only tyres, then design parameters are much more focussed. The key requirement for a track tyre, for dry use, is to generate the maximum amount of grip at much higher vertical loads.

Due to the fact that a track tyre (especially slick tyres) are designed to generate much higher levels of grip, they will also involve much greater centripetal force and in turn more weight transfer.

So fundamental is the choice of tyre to the success of a track day project or a Formula car very often in race series they will be controlled. This at least gives a level playing field ( except perhaps the fact that some privateers may well be able to afford more regular changes!). If you aren't forced to run a control tyre and it makes sense to choose the best tyre to help your project meet its objectives.

Some tyre manufacturers are able to provide load data versus lateral force. This is very useful when designing or modifying suspension and chassis. Essentially if you know what the tyre is capable of, then the suspension can be built and designed around it.

So before moving onto the next section if you aren't already sure which tyre you are going to run then see if any of the suppliers of the shortlist of tyres you have selected can provide vertical versus lateral load data for their product. 

The graph below shows the varying lateral force generated by a tyre at different vertical loadings. For example as you can see the 450kg vertical loading can generate a much greater lateral force for a given slip angle than the 300kg vertical loading BUT the lateral force diminishes more quickly as the limits of the tyre are approached.  

TYRE LOAD GRAPH 

Armed with this data it is much easier to work out what kind of spring and antiroll bar rates will be required to maximise cornering force.

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