D Car — November 2003

Unlike engines, tires do not always get the same attention when it comes to maintenance. But they are the car’s only point of contact with the road while traveling at high speed, so they deserve proper care. The most important part of tire maintenance is keeping the inflation pressure at the correct level. The correct pressure for your car is shown on a label near the front door…

…when you drive on tires with too little air, they wear out faster and use more fuel. That is not the only drawback: cornering and braking performance also get worse. If you drive at high speed with underinflated tires, they can develop a standing-wave condition (see the diagram on the left), which in the worst case can lead to a tire blowout. What about the harm caused by overinflating a tire? First, the tire flexes too little over bumps in the road, making the ride uncomfortable. The center of the tread also becomes the only part in contact with the road, so that area wears abnormally fast…

…the pressure has dropped below the recommended level. Both overinflation and underinflation are bad! What problems occur if tire pressure is below or above the recommended level? First, when pressure is too low, the tire deforms more under the same load than it would at the proper pressure. Contact pressure in the center of the tire (the pressure between the tire and the ground) decreases, while pressure at the shoulders increases. In other words, the contact pressure differs between the center and shoulder areas. As a result, the rubber in the contact patch flexes more and wears faster. With less air in the tire, it also flexes more, increasing fuel consumption…

Typical recommended passenger-car tire pressure is around 180–200 kPa. When carrying a heavy load or driving at high speed, it is best to raise this by about ten percent. However, slightly high pressure causes fewer problems than low pressure. In practice, it may be simpler to keep the tires a little overinflated and accept a slightly firmer ride than to adjust the pressure for every driving condition.

Tires have grooves (tread) cut into their surface, but the tread pattern is not decorative: it channels water away from between the tire and the road. For passenger cars that are driven only in dry weather, a racing-style slick tire with no grooves would actually be advantageous because it provides more contact area. Once the tread surface has worn down…

…and the grooves become shallow, the tire’s ability to drain water naturally decreases, making it more likely to skid on rainy days. A wear indicator is designed so that once the grooves reach a certain minimum depth, a smooth, groove-free section appears in the tread. When this indicator becomes visible, it means the tire must be replaced with a new one for safety. Small triangular marks are molded into the tire sidewall, and the corresponding part of the tread…