Grooves or treads found in most tires are there to improve contact between
the tire & 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 & the tire's surface, which causes a severe loss of grip. At
higher speeds, this can cause hydroplaning, substantially reducing traction
during braking, cornering & hard acceleration. The grooves in the tread provide
an escape path for the water - & it is even claimed by some tire manufacturers
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 & the road. For this reason treadless or 'slick'
tires are often used in motor racing when the track is known to be smooth & 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. Another technique to improve traction is to use a softer rubber in the
treads so that they mold themselves to the shape of the road surface. Since the
rubber is softer when warm, race pit crews may even employ tire heaters to keep
new tires at the optimum temperature until they are ready to be driven on. Soft
compound rubber would also improve traction in street cars but it is seldom used
because these tires wear out too quickly for normal use.
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 & most modern road tires have built-in tread wear
indicators. These take the form of small blocks of rubber molded 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 cars, 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 & 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 (& vice versa). Such tires typically have an arrow
molded into the sidewall to indicate the preferred direction.
