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H.Kishie Singh
Tyres are probably the most discussed item in this column. The reason is simple. The tyre is the most critical fitment on your car. It holds you to Mother Earth’s bosom. A quick look at what is required of a tyre to give you the ultimate performance: correct tyre pressure, condition of the tyre, age of the tyre, nicks in the side wall to mention a few.
In the last 15 years, tyres have been through a sea change in the evolution of technology. Every make of tyre is available in India, every tread for any terrain. Any terrain?
By any terrain, we take for granted what Nature and Indian road conditions provide. No one paid any attention to man-made conditions. The Yamuna Expressway is an eye opener. And a problem! It is a Cement-Concrete, CC highway, and not the black-top bitumen we are used to. Numerous tyre burst cases have been reported on the Yamuna Expressway since it opened in 2012.
The first thing to understand is that the same tyre will not behave the same on a CC road as on a bitumen road. Bitumen provides a smoother and softer surface. This gives the tyre a better 'rollability' i.e. less road resistance. This improves your fuel average and gives you a smoother ride and less road noise. CC surface is rougher and more expensive but lasts longer. It offers greater resistance, fuel average is affected, road noise increases and the heat build up is also greater. This may be the reason for the blow outs.
Where the CC roads score over bitumen roads is under wet conditions. Even under dry conditions, CC roads provide better adhesion for braking and cornering. So, common sense tells me that CC roads are suitable for the Kalka-Shimla Highway. The air temperature is cooler. Speeds will be slower, no danger of tyres heating up. Good tyre traction on corners under normal conditions. This will be a great advantage when there is snow, mud, slush and monsoon rain on the road. Obviously, this CC surface was not ideal for the Yamuna Expressway. The unusual tyre blow-outs do make a statement. While buying tyres, you should concern yourself with the speed rating. This is imprinted on the side wall of your tyre. It will read something like 145/ 70. R. 12. 69. S.
145 is the width of the tyre in millimetres. Next is 70, also in millimetres. This is the aspect ratio. It is the distance of the rim to the road surface. R is simple. It denotes radial construction.
12 is the googly! Read on to get confused. It is the rim size in inches! Diabolical! The formula starts off in millimetres and suddenly lapses into inches! 69 is the load index. More on this later.
S is the speed symbol. In this case, S means your car tyre is good for speed up to 180 kmph. Beyond that, it is not safe.
Refer to the accompanying chart. These are manufacturer's specifications for brand new tyres. Other factors that control the speed limit of the tyre are tyre balancing, alignment and tyre pressure. A tyre that is five years old, having been driven 60,000 km with cuts in the side walls, mounted on an SUV and overloaded, will not conform to the manufacturer's specifications. Obviously, as the tyre ages, the safety decreases.
Happy Motoring!
Speed Ratings
F 80
G 90
J 100
K 110
L 120
M 130
N 140
P 150
Q 160
R 170
S 180
T 190
V 200
5
8
9