Why do tires have grooves in the tread and How do car tires grip a wet road?

If roads were always dry, tire manufacture would be a simple operation: all tires would be completely smooth, like racing tires; for the greater the amount of tire in contact with the road, the greater the amount of traction.

Our changeable climate creates another set of problems, however, for smooth tires (especially wide ones) speeding along a wet road actually begin to float. With traction drastically reduced, a car may hydroplane at speeds of 45 or 50 miles per hour.

Since no one wants to change tires each morning according to fallible weather reports, tire companies try to produce the best all weather tire, with rubber soft enough for good traction but sufficiently hard to last.

While wide treads provide contact with the road surface, an intricate design of tread grooves and/or slots that are cut into the tire channel water out from between the tread and the road as the tire rotates. Tires used to have five to seven straight, continuous ribs, but today slots are cut across the tread pattern, which shoot water out to either side. These slots and grooves are deeper than those on older tires: 3/8 inch is standard, with 1/16 inch being the legal limit to which a tire may be worn down.

It is essential during a heavy rainstorm that a tire’s grooves remain open, and methods of assuring this vary from one type and brand to another. A bias belted tire, for example, made of crisscrossed plies, has two or more circumferential belts of fiberglass or steel wire.

These belts, added to the body beneath the tread, are an improvement on the standard bias tire. Perhaps the most efficient tire, in terms of both traction and tread wear, is the radial tire. The name describes the body cords that run radially from side to side, at an angle of 90 degrees to the direction of travel.

Two or four belts made of polyester, steel, fiberglass, or aramid (which is lighter than steel but more costly) encircle the tire to stabilize it and hold the treads open.