Understanding Your Tyres

Must all four tyres on a car have the same specifications?

Tyres are the single most important safety feature on your car. They are the only thing that connects the car to the road, and life-saving technologies like antilock brakes and electronic stability control cannot do their job if the tyres do not have a good grip on the ground. But tyres are the least-understood components of our cars mostly because there are so many different kinds and little information is provided about them.

The law says that the two wheels on the same axle (that is two fronts or two rears) must be of the same construction and size. The tyres on one axle do not have to be the same as the tyres on another axle.

However, this does not mean you can just change from say 16560R14 to 17560R14. Because the aspect ratio is 60% on both tyres, but the tread width (165 and 175) are different, the rolling radius is also different.

This affects the effective gearing of your car and the accuracy of your speedometer. The speedometer reads the speed at the gearbox, and is calibrated for the tyres fitted by the factory.

To increase from 16560R14 to a 175 tyre without significantly affecting the rolling radius, you will have to use a 55 profile tyre – 17555R14. You also need to ensure that your car has enough clearance in the wheelhouses for the wider tyres when on full steering lock, it may be necessary to have your steering system altered to reduce the amount of steering lock you can apply.

There is also an issue with the change on your car handling ability. You will have increased the grip at the front of the car, without a similar increase at the rear. This may make the car less stable in hard cornering, so that the rear end is more likely to lose grip, allowing the car to spin. In general, it is safer to have more grips at the rear than at the front, even on a front-wheel-drive car.

You must not mix cross-ply tyres with radial ply tyres. Tyres have to be the correct speed and load rating for the vehicle they are fitted to.

Do I really need to replace all four tyres at the same time?

This question really depends on your car. Most of the cars on the road today are front-wheel-drive, and a few are rear-wheel drive. These do not necessarily need to have all four tyres replaced at once. Usually, two at a time is sufficient. But all-wheel-drive systems are becoming more popular, and they do require all four tires to be replaced in one go. So for AWD vehicles, the short answer is yes.

In vehicles with all-wheel-drive systems, the differential and the computer work together to send the right amount of torque to each wheel to minimize slippage and maximize control. If one of the tyres is a different size than the others because three tyres are worn and one is brand new, the computer will take an incorrect reading and the differential will work too hard. If you drive this way long enough, you will burn out the drive train. Also, if you need to use the small spare to limp to the nearest repair shop, which is probably not far enough to do any serious damage.

Remember to rotate your tyres because it extends their life on any vehicle but for AWD vehicles, rotating the tyres also makes sure that the tread wears evenly and does not place a strain on the drive train.

How to know when tyres need replacing

Tyres become unsafe when they are worn, and once the tread is down to 116th of an inch (1.6mm), the tyre is no longer safe.

Look at the tread pattern.

These are small bridges that form between your treads. Look at the tread pattern and you will see the beginnings of these bars start to form between the treads, or running across the tyres. As the tyres wear, these bars will become flush with the tyre tread. At this point, it is time to replace the tyres.

Know the legal requirements.

Worn tyres should be replaced as a matter of common sense to assure safety, but in some jurisdictions, there are also legal requirements to replace worn tyres.

Make note of any irregular tread wear.

This could indicate a wheel misalignment, the need for a tyre rotation, or both. Uneven tread wear is a sign that you need to take your car for servicing.

Check for any abnormal bulges or “bubbles” in the sidewall.

A sidewall bulge indicates that the rigid internal frame of the tyre has been damaged and cracked, allowing air pressure to reach the flexible outer layers of the tyre. Such damage could be caused by driving through a large pothole or over a curb, or by driving with low tyre pressure.

Replace the tyres at least every six years.

If you are not sure, the minimum replacement time that is recommended by the law is six years regardless of use, with ten years being the maximum service life for tyres.

Source : The Independent

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