Treads and tire wear
The grooves or treads found in most tires are there to improve contact between the tire and the road in wet conditions. Without such grooves, the water on the surface of the road would be unable to escape out to the sides of the wheel as the tire presses down onto the road. This causes a thin layer of water to remain between the road and the tire, which causes a complete loss of friction with the road surface. This causes hydroplaning, obliterating traction required for braking, cornering and hard acceleration. The grooves in the tread provide an escape path for the water. Some tire manufacturers claim that their tread pattern is designed to actively pump water out from under the tire by the action of the tread flexing.
If the road is dry, tire treads actually reduce grip since they reduce the contact area between the rubber and the road. For this reason treadless or 'slick tires' are often used in motor racing when the track is known to be smooth and dry. If it should rain unexpectedly during the race these slick tires can cause a dangerous loss of grip - which is why they are seldom used on conventional road cars.
The contact patch of a tire can be increased, in some circumstances, by lowering the tire pressure. Lowering the tire pressure is a technique used for off-road vehicles in sandy, muddy, or snowy areas, and for drag racing. Decreasing the tire pressure causes the tire to sag so more of the tire is in contact with the surface, giving better traction. It also helps the tire grip small obstacles as the tire conforms more to the shape of the obstacle, and is in contact with the object in more places. However, this increases fuel consumption, tire wear, tire temperature, and raises the likelihood of damage to the wheel if a hard object is struck. Tires that are not designed for variable or lower pressure (such as tires for conventional cars) buckle on hard roads due to their stiffer sidewalls, raising the center of the tire off the surface and reducing, rather than increasing, road contact. The buckling also causes the treads to be squeezed together, reducing their effectiveness. Furthermore, the abnormal stress on an underinflated tire, combined with the resulting temperature increase, can cause the tire to explode while driving.
Another technique to improve traction is to use a softer rubber in the treads so that they mould themselves to the shape of the road surface, though this wears the tire more quickly than harder tires. Since the rubber is softer when warm, race pit crews may even keep tires in warmers to keep new tires at the optimum temperature until they are to be driven on. Soft compound rubber would also improve traction in street vehicles, but it is seldom used because these tires wear out too quickly for normal use.
The depth of the tread grooves is an important part of car safety but that depth gradually reduces due to wear throughout the lifetime of a tire. When the tread on the outer perimeter of the tire wears away, reducing the tread depth, the tire should be replaced. Many countries have laws regulating the minimum tread depth on road vehicles and most modern road tires have built-in tread wear indicators. These take the form of small blocks of rubber moulded into the bottoms of the grooves of the tread at intervals around the tire. When the tread has worn down until the tops of those blocks are level with the top of the tread - then the tire needs to be replaced. If these blocks are not present, a tire tread depth gauge should be used to measure the depth.
In most vehicles, either the front or rear tires will wear faster than the others. Having mismatched tread depths can alter the handling of the car in unacceptable ways - so it is generally advisable to swap the front and rear tires as they wear down to even out the wear patterns. This is called rotating the tires . If the vehicle's suspension is somewhat out of adjustment, it is also possible for the tires to wear more on one side than the other - so it may also be beneficial to rotate the tires from one side of the car to the other - however, careful attention should be paid to the owner's manual since some vehicles require particular tire rotation patterns. Notably, some tires are designed to provide best traction only when spinning in a clockwise or counter-clockwise direction. In such cases one must not rotate the tires from one side of the car to the other because that would put a 'clockwise tire' onto a wheel that turns in a counter-clockwise direction (and vice versa). Such tires typically have an arrow moulded into the sidewall to indicate the preferred direction.
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