Aquaplaning

Aquaplaning or hydroplaning by the tires of a road vehicle, aircraft or other wheeled vehicle occurs when a layer of water builds between the wheels of the vehicle and the road surface, leading to a loss of traction that prevents the vehicle from responding to control inputs. If it occurs to all wheels simultaneously, the vehicle becomes, in effect, an uncontrolled sled. Aquaplaning is a different phenomenon from when water on the surface of the roadway merely acts as a lubricant. Traction is diminished on wet pavement even when aquaplaning is not occurring.

Every vehicle function that changes direction or speed relies on friction between the tires and the road surface. The grooves of a rubber tire are designed to disperse water from beneath the tire, providing high friction even in wet conditions. Aquaplaning occurs when a tire encounters more water than it can dissipate. Water pressure in front of the wheel forces a wedge of water under the leading edge of the tire, causing it to lift from the road. The tire then skates on a sheet of water with little, if any, direct road contact, and loss of control results. If multiple tires aquaplane, the vehicle may lose directional control and slide until it either collides with an obstacle, or slows enough that one or more tires contact the road again and friction is regained.

The risk of aquaplaning increases with the depth of standing water and the sensitivity of a vehicle to that water depth.

There is no precise equation to determine the speed at which a vehicle will aquaplane. Existing efforts have derived rules of thumb from empirical testing. In general, cars aquaplane at speeds above 53 mph (85 km/h), where water ponds to a depth of at least 1/10 of an inch (2.5 mm) over a distance of 30 feet (9 meters) or more.

Motorcycles benefit from narrow tires with round, canoe-shaped contact patches. Narrow tires are less vulnerable to aquaplaning because vehicle weight is distributed over a smaller area, and rounded tires more easily push water aside. These advantages diminish on lighter motorcycles with naturally wide tires, like those in the supersport class. Further, wet conditions reduce the lateral force that any tire can accommodate before sliding. While a slide in a four-wheeled vehicle may be corrected, the same slide on a motorcycle will generally cause the rider to fall. Thus, despite the relative lack of aquaplaning danger in wet conditions, motorcycle riders must be even more cautious because overall traction is reduced by wet roadways.

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Wikipedia