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In rubber we trust

Your car can weigh anything from 700-odd kilos to 2.5 tons, depending on what you own.

autos Updated: Aug 27, 2009 10:11 IST
Grease Monkey

Your car can weigh anything from 700-odd kilos to 2.5 tons, depending on what you own. That much mass can easily hit speeds well beyond 100 kph — all of it connected to the road by four little patches of rubber at each corner.

Your tyres are made of rubber, yes, but it’s not just plain old rubber. They’re vulcanised, which means the rubber that goes into them has been treated with sulphur and lead oxide so it doesn’t become brittle in cold weather and sticky when it’s hot. It also helps the rubber increase its strength, elasticity and resistance to solvents. This process was discovered by Charles Goodyear in 1839.

Automobile tyres used to be of the ‘crossply’ kind. Crossply tyres are made of rubber outside and a textile weave underneath. As far as modern cars are concerned, crossply tyres are history, because they can’t meet the demands today’s cars make on tyre performance.

So you have radial tyres, which have belts running along the circumference of the tyre, and parallel to the surface of the tyre that makes contact with the road. These belts are usually made of steel, but you’ll also find belts of nylon or polyester in addition to the steel belts in the tyre.

Of course, they aren’t the same fibres as the ones used in clothing, they’re a whole lot stronger. Radial tyres cushion shocks better and deform less under extreme conditions, so they’re more comfortable on imperfect surfaces. And when you’re driving on your favourite twisty road, you’ll get better feedback and much better grip from them.

A major advantage that radial tyres have over crossplys is that the ‘height’ of the tyre — the distance from the wheel rim to the tread that comes in contact with the road — can differ while the width of the tread itself can remain the same. Crossply tyres, on the other hand, will have equal width and height, and no exceptions.

The ratio of width to height is called ‘aspect ratio’ and is expressed in percentage in the puzzling jumble of letters and numbers that denote a tyre’s dimensions and capabilities. For example, a Suzuki SX4 ZXi has ‘205/60 R16’ tyres. No, it isn’t a code that NASA dreamed up to keep customers confused.

Let’s break that down: ‘R16’ is the diameter of the wheel rim in inches — meaning the SX4 possesses wheels that are 16 inches in diameter. ‘205/60’ isn’t a fraction, it is two numbers separated by an oblique. ‘205’ is the tread width in millimetres and ‘60’ is the aspect ratio, which means the height of the tyre’s sidewall is 60 per cent of its width, which in the SX4’s case is 123 mm.

There is no clear reason why the imperial and metric systems have been thrown together, or why the height of the tyre cannot be expressed in mm, but there have been far more complex and strange systems of conveying tyre size — this is one of the simplest ways of doing it.

Another thing you should pay attention to is the recommended pressure for your tyres — there is usually a sticker on the driver’s side door sill that is visible when the door is open. Use these recommended pressures while topping up your air; if you inflate them too much or too little, you’ll lose out on grip, fuel efficiency and tyre life. Do not leave it to the attendant at the pump.

Car and motorcycle racing concern themselves with decisions like hard or soft compounds, slicks or grooved tyres, temperature, graining and flat-spotting, which is a little more complex compared to non-racers like you and I looking for a replacement set.

Here’s something for you to think about till we revisit automobile rubber: if racers use slick tyres, which have no grooves on the tread because they have the maximum grip, how come our cars have patterns on the tread?

Doesn’t more grip mean increased safety?

If you have questions or comments for Grease Monkey, email him at carsnbikes@ with Auto Tech 101 in the subject line