By Michelin Sport
May 25 2005

Friction coefficient is always worse on a damp track and varies enormously according to surface type. The reason? A film of water between the rubber and the asphalt prevents the molecular adhesion mechanism from working. It is vital, therefore, to break this film. On the other hand, the indentation mechanism, which is vital to grip, will still be operational.

In the wet, microrough road surfaces as detailed in the Monaco GP preview provide optimal grip.

If the depth of water increases (wet surface), micro-indenters might become flooded. Macro-indenters continue to perform their drainage and storage roles, but there remains a risk of high-speed aquaplaning.

Water interferes with grip and tyres must be designed to disperse it quickly and effectively by adjusting the shape of the contact patch and the tread pattern.

Macroroughness drains and stores water but does not break through the residual film of water.

Microroughness creates individual high-pressure points between the surface and the tyre, to help it break through the film of water.

As soon as a tyre starts to slip over the road surface, the macroroughness and microroughness of the surface cause the rubber to move within a whole range of frequencies. This is also described using the word indentation, which emphasises the penetration of road roughness into the tyre tread.