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and    December 7, 2004: Danger: Smooth Road Ahead

A better explanation for loss of tire grip on damp and wet roads

     Where the rubber meets the road, odd and dangerous things can happen. On wet asphalt at high speeds, for instance, or on a flooded surface at just about any speed, a car tire can hydroplane, skidding along on a thin sheet of water.

     But skidding can happen on roads that are not as wet and at lower speeds, and scientists have never been able to fully explain the loss of friction that occurs under those conditions.

     Now a team of scientists in Germany and Italy has come up with an explanation. Water becomes trapped in the asphalt, and the rubber of the passing tire effectively seals it in place. This, they write in the journal Nature Materials, has the effect of smoothing the road surface, reducing friction.

     Even the smoothest-looking asphalt road has tiny peaks and valleys in the surface. Under dry conditions, the rubber of a tire will deform slightly as it penetrates the valleys and then hits the peaks. These pulsating deformations, multiplied countless times as the tire moves along, create a lot of friction [so the tires have a good grip on the road surface -- DSA].

     When the asphalt is wet, however, the valleys become tiny lakes. The passing tire can't deform into the valleys because the water is there, and it can't push the water out because the rubber hitting the peaks forms a seal.

     So the road, in effect, is smoother - there are fewer deformations and thus less friction.

     The researchers say their calculations can account for the 20 to 30 percent loss of friction that occurs at low speeds (below about 35 miles an hour) on wet but unflooded roads.

This full, important piece is from the New York Times