By Michelin Sport
May 2 2005

Grip implies contact between two surfaces. On a racing circuit, the first of these is the tyre compound and the second is the track. The level of available grip will be determined by the nature of the track surface and its state of repair, plus its abrasiveness and the possible presence of rain. To produce grip, a tyre needs a partner – the track surface – to stimulate the mechanisms that generate grip: a tyre’s ability to mould itself to surface imperfections and molecular adhesion.

Michelin refers to a tyre’s ability to adapt to variable road surface – when small surface imperfections dig into the tread – as "indentation". Molecular adhesion necessitates direct contact between the rubber and the track surface, so conditions must be dry.

If water is present, it can be dispersed in two ways:
- drainage (through camber, natural gradients or surface porosity)
- storage (where water settles in ‘valleys’ below the level of any surface ripples).

Indentation can only occur when a track surface is reasonably abrasive – ie when it has a number of imperfections that will superficially impact on the tyre tread.

To determine the nature of the "indenters", you need to measure the proportion of tread that is actually in direct contact with the track when compressed on its surface.

One method involves taking an imprint of the contact between the tread and the track. In the case of an F1 tyre, that figure rises to 30-70%.