The ability of a tyre to transmit forces to the road surface is defined by its coefficient of static friction, or its coefficient of adhesion. This depends on the condition of the tyre (width, tread depth, air pressure), speed and road surface. Aquaplaning has a particularly adverse effect on adhesive friction. During aquaplaning, the tyre loses its ability to displace water and keep contact with the road, which is necessary for transferring power and steering. The effect of aquaplaning occurs when water pushes a wedge under the bearing surface of the tyre and the tyre lifts off the roadway. The final effect of aquaplaning is dependent on the depth of the water on the road surface, on the speed, on the shape and depth of the tyre tread and the load pressing the tyre onto the road surface. In the event of aquaplaning, the tyres can no longer transmit steering or braking forces to the road surface, resulting in a loss of directional stability.