Wednesday, December 12, 2012
Car Care: Selecting and maintaining tyres -- Part 1
[Article copyright Ewe Paik Leong]
A tyre is not just a part of the car that is black, round and made of rubber. It plays an important role in its performance and safety. Also, regarding appearance, the car may look like an amateurish "boy-racer" if the sports tyres do not complement it.
Now for some tyre basics: The illustrations below feature many basic tyre terms that you should be familiar with:
Section Height/Section Width x 100.
Nominal Rim Diametre
Diameter of rim seat supporting the tyre head. Examples: 13", 15", 16".
The diametre of the inflated tyre without any load.
Maximum width in cross section of tyre without any load, including protruding side ribs and decorations.
Distance from rim seat to outer tread surface of a tyre without any load.
Linear distance between outside of side-walls if inflated and unloaded tyre (exclusive of protruding side ribs and decorations).
The portion of the tread design that comes in contact with the road. The second important aspect of tyres is their speed rating. A speed rating of a tyre is a letter indicating its maximum speed capability. The five major categories of speed ratings, from low to high, are: S, T, H, V and Z. As the speed of a tyre increases, its components start to deform, leading to heat generation. This is a normal effect of the deformation of the tyre due to centrifugal force. The faster the tyre spins, the greater it deforms, and as a result, heat is generated. For tyres driven at high speeds, engineers must try to control the deformation so that heat levels are confined to acceptable levels.
There are three basic types of speed-rated belting systems. For S- and T: rated tyres, two steel belts provide reinforcement for the tread to ensure a full, stable contact area. For V-rated tyres, extra reinforcement is necessary. One manufacturer achieves this with two nylon cap piles at zero degress; another manufacturer, by having one nylon ply folded over the edges of the steel belts. For Z-rated tyres, meant for today's high-performance cars, an extra measure of integrity is required. A few brands incorporate a steel belt wrapped by folded belt of Kelvar. This wrapping provides superior strength in the shoulder and allows a reduction of overall weight.
Speed ratings are determined using measurements obtained from an indoor test machine. The tyre is mounted under a specified load and pressure, and spun against a large roller to simulate harsh driving conditions. The speed of the roller is increased periodically until the tyre is damaged. Based on the results, and giving a margin of safety, the tyre is assigned a speed rating.
In USA, the Uniform Tyre Quality Grading System is a tyre information system that provides buyer with standardized information on treadwear. traction and temperature. Treadwear ranges from 60 to over 500. Traction grades, ranging from A to C (with A being the best), are an evaluation of the tyre's ability to stop a car in straight, forward motion on a wet test surface pavement. Temperature grades, a measure of the tyre's ability to dissipate heat, also ranges from A to C, with the former being the highest. Low profile tyres are getting popular. The term "low profile" generally refers to the 60 or 50 series tyres even though they go as low as the 30 series. As the term "low profile" implies, the tyre is lower lower than the normal tyres. The 60 series tyres have been around for a while and nowadays, more motorists are opting for the lower profile, 50 series tyres. Many imported cars, especially the more expensive and powerful ones, come with 50 series tyres. If you are not sure what type of tyre you are riding on, take a look at the markings on the sidewalls of your tyres.
For example, if you find the markings "185165 R14", the figures "65" denotes that you are using low profiles 65 series tyres while the figures "14" indicate that your rim should be l4-inches in diameter. The letter R stands for "$adial" which means that you are using a radial ply tyre and not the cross ply tyre. The numbers 185 is the section width of the tyre mea-ured in millimetres. You may find markings such as HR, VR or ZR. As explained, they indicate the maximum speed that particular tyre is good for.
If you see an arrow sign it means that it is a "directional" tyre and the tyre must be fitted to rotate in the direction to which the arrow is pointing. You may find "tubeless" indicating that the tyre need not have inner tubes but you can put them on if you wish to and there are occasions when you will have to use them depending on the kind of rim you are using.
Low profile tyres, in addition to being better in handling, also look better on the cars. Therefore many motorists change to such tyres not so much with safety in mind but because they make the car look sleeker and faster. However, it must be remembered that sometimes when changing to low profile tyres, the rims have to be changed too. so it becomes an expensive affair.