The sidewalls of radial ply tires bulge where the tire meets the road, making it difficult to estimate inflation pressure visually. It needs to be checked with an accurate tire gauge.
Using correct inflation pressures extends tire life, and is vital for safety.
Sidewalls of an under-inflated tire flex too far, which pushes the center section of the tread up and away from the road surface. This causes wear at the shoulders of the tire.
In an over-inflated tire, the sidewalls are straightened, which pulls the edges of the tread away from the road, and causes wear at the center of the tread.
A tube-type tire uses an inner tube which provides an air-tight container inside the tire.
A tubeless tire is lined with a soft rubber layer to form an air-tight seal. This inner liner also seals against small penetrations, letting air escape only relatively slowly. When a tubeless tire is fitted, an air-tight valve assembly is used. It can be a tight fit into the rim, or be held with a nut and sealing washers.
A tire and wheel assembly must be balanced. As the wheel rotates, centrifugal force acts outwards. Any part heavier than the rest will vibrate vertically, with the heavy area slapping the road surface with each turn of the wheel. This is called static unbalance.
Dynamic unbalance causes the wheel assembly to turn inwards, and then outwards, with each half revolution. As speed rises, rapid side movement of the front wheels causes a sideways-vibration, or wheel wobble effect, at the front of the vehicle.
These conditions must be corrected to prevent cupping or dishing of the tread, and reduced tread life.
Tread life can also be reduced by incorrect wheel alignment. The feathered edge of this tire indicates an incorrect toe-setting. And wear on the one shoulder of this tire could be due to incorrect camber setting.
Most passenger car tires have tread-wear indicators molded into the tread pattern. They generally provide an indication when the depth of a tire groove falls to 1-and-a-half millimeters.
Control of a vehicle in any weather conditions depends finally on frictional forces generated between the tires and road surface.
On a dry road, a smooth rubber surface can provide a high coefficient of friction, sufficient to maintain a degree of control during braking, accelerating, and cornering.
In wet conditions, the coefficient of friction between a smooth tire and the road surface falls to an extremely low value.
Grooves in the tread pattern clear water away from the contact patch area. This allows a relatively “dry area” to be formed, and for road adhesion to be maintained.