What is a Tyre?
Simply put, a tyre is a flexible container of compressed air. In reality, a tyre’s duties are many. It supports a vehicle’s load, it propels a vehicle: forward, backward, and side-to-side. It stops a vehicle. Finally, it also cushions the load from road imperfections. Today’s tyres comprise between 19-25 different components. Contrary to popular opinion, tyres are built from the inside out rather than the outside in. The heart of every tyre is an inner liner. Its job is to give the tyre shape and hold in the air. Wrapped around the inner liner are fabric belts. Fastened to the bottom of the fabric belts is the bead, which holds the tyre to the wheel.
On top of the fabric belts are steel belts. These belts have two jobs. One is to give the tyre stability, and the other is to make the tread pattern as flat as possible. (A flatter tread means more contact with the road.) On top of the belts is the tread of the tyre. There are different tread patterns for different types of tyres. Finally, on the side of the tyre, appropriately enough, is the sidewall. This is what gives a tyre its stiffness or ride characteristics. A taller/softer sidewall will absorb more bumps, while a shorter/stiffer sidewall will provide better cornering ability and sharper steering response
Why are Tyres Important?
Many consumers don’t know enough about tyres to make an educated purchase. For some, the choice comes down to price and/or availability. Others buy tyres based on appearance or reputation. Educated consumers know to purchase tyres based on safety, quietness, road-holding ability, and wear.
Regardless of tyre choice, it is necessary that consumers understand why tyres are so important. Tyres are a vehicle’s only contact with the road. Even the brawniest engine, strongest brakes, or most advanced anti-skid system, is at the mercy of the tyre’s grip on the road. Every move a driver makes with the steering wheel or brake or gas pedal is transmitted to the road through the four notepad-sized contact patches of the tyres.
Since a vehicle doesn’t have the ability to change tyres like a person changes shoes, consumers must select a tyre that will work year ’round and in every conceivable driving situation. You wouldn’t wear high heels to go hiking, and you wouldn’t wear ski boots to dance a ballet. However, consumers often ask their cars’ tyres to do many things, and many times a vehicle’s tyres aren’t suited to the task.
Selecting the right tyres for your driving needs is as important as selecting the right vehicle. Tyres that are worn won’t be able to displace enough water to prevent hydroplaning. Performance tyres won’t be able to get much grip in snow. All-season tyres can’t manage the heat creased in high-speed driving. Passenger-car tyres might not be able to carry the heavy load found in a contractor’s bakkie.
One Size Does Not Fit All
Perhaps the biggest mistake a consumer can make when replacing tyres is not using the correct size. On the sidewall of your original-equipment tyre (and all tyres) is a code that tells the tyre’s size and capabilities. Here’s a sample tyre-size code and a description of what that code means: P195/60R16 63H M+S
- P = Type of tyre
- 195 = width of the tyre across the tread in millimeters
- 60 = Aspect ratio of the sidewall compared to the width
- R = Radial construction
- 16 = Diameter of the rim in inches
- 63 = tyre’s load rating
- H = tyre’s speed rating
- M+S = tyre is suitable for all-season driving
If the tyre-size code starts with LT instead of P, that means the tyre is a light-truck tyre. Light-truck tyres are designed to have higher-load carrying capacities and are usually found on pickups and SUVs. These vehicles are not required to have LT tyres, and in many cases, the original-equipment specification calls for passenger-car tyres.
A tyre’s speed rating is most important if you do a lot of highway driving. Tyres are speed rated from 140 Km/pH to 300 Km/pH. The most common speed ratings are T (190 Km/pH) and H (210 Km/pH). Both of those ratings clearly exceed the nationally posted speed limits and would make excellent long-distance highway tyres. If a consumer were to drive only in urban situations at low speeds, a tyre with an S (180 Km/pH) speed rating might be completely acceptable.
You see, the speed rating translates into the the tyre’s ability to dissipate heat, or better yet, to prevent heat build-up. Heat is a tyre’s enemy. The more heat, the faster the tyre wears, and the faster a tyre might break down and fail. So a tyre with a higher speed rating can dissipate more heat on long highway trips. If a consumer were to spend little time on the highway, the speed rating might not be an important factor in choosing a replacement tyre.
Another important factor in choosing a replacement tyre is the load rating. The load capacity number on the tyre-size code indicates the load-carrying capacity of that single tyre. When selecting replacement tyres, consumers have to be careful not to select a tyre with a lower load-carrying capacity.
Tyres need to rotate in more than one direction
Rotating a vehicle’s tyre is essential to prevent uneven tyre wear. If left unchecked, unrotated tyres will cause increased road noise, lower fuel economy, and decreased wet-weather traction. Additionally, badly neglected tyres will have to be replaced sooner.
It is generally accepted that on front-drive vehicles, where all tyres are the same size, you rotate the front tyres to the rear in a straight line and cross the back tyres to the front. In a rear-drive vehicle, you rotate the backs in a straight line to the front and cross the front tyres to the back.
On all- or four-wheel-drive vehicles, the rotation pattern most often suggested is a simple “X.” The left front and right rear swap and the right front and left rear swap.
Many sports cars and some luxury and sport-utility vehicles have unidirectional tyres. Unidirectional tyres have tread patterns that are designed to perform in the direction denoted on the tyre sidewall only. They should always be rotated front to rear (assuming they are the same size). This ensures that the direction of revolution does not change.