*INFO* What makes tyres grip?

RULE 1
All rubber grips the same doesn't it?

No! A tyre's ability to grip is dictated by the coefficient of friction of the rubber compound in it, multiplied by the force acting (weight) to press it against the road surface (Armonton's Law).
A higher coefficient of friction = a softer compound = more deflection within the compound before breakaway = more grip (if weight is adequate)


RULE 2
The average motorist would assume that having a wider tyre would lead to more grip because there is more tyre in contact with the road?

Not so, the size of the contact patch on the road is related to:

- the weight on the wheel.
- the tyre pressure.

This relationship can be illustrated by considering a tyre with a load on it of 900lbs and an inflation pressure of 30psi. Because each square inch of tyre in contact with the ground can support a weight of 30lbs, the size of the contact patch will be 900 divided by 30 = 30sq.inches. If the inflation pressure were to be increased to say 90psi, the contact patch would shrink to 10sq.inches.

This translates into a scenario where, with identical tyre pressures and loads, a narrow tyre will have exactly the same size of contact patch as a wide tyre. The critical difference being that the wide tyre has wider short patch whereas the narrow tyre will have a longer narrow patch.
This relationship is true of all tyres except runflats or tyres with very stiff carcass walls.

So then, if the contact patch is the same size why doesn't a narrow tyre grip as well as a wide one?
The answer has a lot to do with temperature.

Without going through a lot of detailed maths, each part of a narrow tyres tread is deflected (bent flat on to the road) further and for longer than a wide tyre under the same load and inflation pressure. This results in it getting considerably hotter which in turn effects the co-efficient of friction. In fact, a 155-width tyre will get 3.2 times hotter than a 225-width tyre under identical conditions even though the 225-width tyre is only 45% wider! As with width, so the same is true of a larger diameter tyre. The greater the diameter the less the deflection and the less contact time each part of the tyre spends on the road surface leading to reduced temperatures.
Because of the extra heat and stress, manufacturers tend to make narrow tyres out of harder (lower co-efficient of friction) rubber to help prevent overheating. Narrow tyres are more prone to lower grip performance from being, at first too cold and then getting too hot too quickly - in short their performance is much more unstable than a wider tyre.

Tyre wall roll also has an important part to play in handling and tyre temperature as well. This is controlled by wall height, inflation pressure and manufacturing characteristics (ie crossplys have stiffer side walls).
As well as temperature problems, a narrow tyre is unable to provide the lateral stability that a wide tyre can.


The pressure placed on a surface by a tyre corresponds very closely to the inflation pressure.
Using the previous example, the tyre will be exerting a higher point load pressure at 90psi than at 30 psi, even though the overall weight remains identical. This is important in as much that, in order to get maximum grip from a tyre, you must have a load that will "hold" it on to the surface up to and over the point where the rubber compound is incapable of resisting the shear forces acting upon it. Inflation pressure is one way that handling characteristics can be changed quite dramatically particularly as it not only effects the point loading and contact patch shape stability but also influences side wall stability and the resistance to sudden breakaway.


What about tread then? Tread patterns in road tyres are there to encourage the movement of water out from underneath a tyre so that the rubber can effectively be pressed onto the road surface. Without tread, tyres can actually "float" (aquaplane) over a wet surface, in which case they are not in contact with the road surface at all causing them to lose their grip. Because of this danger all road legal tyres have to have a least 17% tread.
In general terms the deeper and more extensive the tread and the higher the inflation pressure, the more resistant the tyre will be to aquaplaning.

Temperature: In general terms, the rubber compound in tyres becomes more flexible & more easily deflected ("softer") as the temperature rises until it reaches the point where a degree of melting ("stickiness")within the compound takes place. This obviously improves the co-efficient of friction levels making the tyre grip better to start with. However, as temperatures continue to rise, a point is reached where the tyre gets so hot that it "goes off" i.e. it stops behaving as a solid and starts to behave more like a liquid with obvious implications for grip and tyre life.

Producing a tyre with a compound that warms up quickly but doesn't go off easily is a balancing act that is impossible to achieve over a wide set of circumstances without some compromise being made to the ultimate levels of grip.







SUMMING UP.

So it appears that for the best grip you need:

- a high coefficient of friction rubber compound

- enough load

- a wide tyre

- a small tyre wall height

- a large rolling diameter

- as little tread as possible in the dry

- more, deeper tread the wetter it is

- finally an even, smooth, grippy & warm road surface!




Is that it then?

No, obviously not! A tyre fitted to any road going car is going to encounter a huge variation in working conditions. For this reason a tyre capable of delivering adequate grip under all conditions is always going to be something of a compromise. This is especially true when factors such as ride smoothness and the vehicles ability to deal with bumpy and rough surfaces are taken into account.
Fortunately for us, tyre & vehicle manufacturers have spent a lot of time and money researching these complexities on our behalf. Modern tyres represent the very best of all worlds and there is a lot of choice out there. If you are after extra grip then consider first moving to a competition type alloy rim a couple or so sizes bigger than std. (you can go even bigger if you have plenty of power and weight and space to fit them!) This will afford you to the widest choice of wider, lower profile tyres. For further improvements in grip, forget the expense and head for the softer, usually top of the range, competition orientated, premium brand tyres - some things are worth paying for! Oh yes and don't forget to improve the suspension at the same time. Stiffer lower suspension set-ups are a must if you want to get the most out of your tyres.

The rest is up to you. Trial and error!