Tyres: For a safe and smooth ride

THE WORLD of automobiles has witnessed many changes over the years. The earlier tyres were just a rubber tread clamped to the wooden rim. Then came the basic tyre that resembled what is now used in animal driven vehicles (ADV), and with the advancement of technology came by, came the process of refinement.

There are four main types of tyres - cross-ply, radial-ply, bias belted and steel belted radial tyres. Besides improving ride comfort by carrying the vehicle on a cushion of air, tyres have to cope with the considerable forces generated while accelerating, braking and cornering. Many technical problems are involved in tyre design, but great advances have been made in this field in recent years. A major challenge to tyre designers is that one single type is not suitable for all driving conditions. The tyre must be strong enough to resist damage and provide superior service life, yet be flexible enough to cushion impacts. It must respond accurately to steering inputs, without deflection due to road irregularities. It must provide optimum traction during acceleration, cornering, braking and wet weather use. It must perform optimally in all weathers and on all surfaces, wet or dry, without overheating, and at all times. From the motorists' point of view, it must give a comfortable ride, run quietly, have long life and above all be cheap to buy and maintain.

Usage-governed design

In the recent past, suspension systems of vehicles have been upgraded or rather tuned to support radial tyres. Mac-Pherson Struts, one of the most popular and economical suspension systems is normally found in most automobiles. A recent trend is for car suspensions to be designed with an individual make and type of tyre in mind. Tread patterns have to vary to cope with the rigorous requirements of different road surfaces. For example, the chunky-treaded all-weather tyre is ideal for snow and mud, but unsuitable for high-speed driving on tarmac because the thick tread generates excessive heat, which could result in tyre failure. Hence tyre design governs usage and vice-versa and misuse can lead to premature failure.

Tubed and tubeless tyres are made in similar ways. The difference between them is that the tubed tyre has a separate rubber inner tube to hold the air, whereas the tubeless tyre holds the air itself. The tubeless tyre has a number of advantages over the tubed: it is easier to install on the rims; when punctured it deflates slowly, because the soft lining has a self-sealing effect which reduces the chance of loss of control in the unfortunate event of a puncture during driving; and a temporary puncture repair can be made without removing the wheel, by plugging the hole with a special rubber plug or anything hard, even a nail or even a wooden stick, will do and since there is no tube in the tyre there is no question of the tube getting damaged. Worldwide preference is given to tubeless tyres; unfortunately, in India we still continue with the relatively primitive technology of tubed tyres, on the presumption of ease of repair which it is not.

The trend now is to make tyres wider and shallower, reducing the height from tread to rim while increasing the width across the tyre's section. The relation between width and height is known as the aspect ratio. It is usually expressed as a percentage. A tyre, which is 4 in. from tread to rim and 5 in. wide, has an aspect ratio of 80 per cent. Wider tyres put more tread footprint on the road and give better high-speed performance, better cornering, better load carrying capacity and longer life than the old symmetrical type. Today car tyres are being made still wider and shallower. The latest radial-ply tyres for road cars have an aspect ratio of 65 per cent, sometimes even lower. On racing cars, with their extra-wide wheels, the ratio can be much lower, down to 45 per cent or less, but the critical thing to remember is that the lower the aspect ratio the harder the ride quality. This is because the smaller side-walls do not provide adequate cushioning effect.

The cross-ply is the oldest type of tyre and has a casing made up of two or more plies, or layers, of fabric. A tyre's strength and load carrying ability were at one time indicated by its number of plies: a four-ply tyre indicated that the casing was built up from four layers of inner lining material and could carry a specific load safely. Modern materials and man-made fibres, such as polyester and glass fibre, embedded in the rubber, are much stronger than the cotton fibres originally used; and the ply rating can no longer be related to the number of plies. Ply- ratings are still used, but only to indicate the strength and load-carrying capacity: a four-ply rating may well be assigned to a tyre which has only two plies in the casing but has the ability to carry loads equivalent to that of a four-ply rating tyre.

Radials

Radial tyres also give the impression that they have low inflation even though the air pressure is as recommended by the manufacturer. This is primarily because of the soft sidewalls. The physical difference between the radial and cross-ply tyres and in their behaviour on the road is governed by the difference in their individual carcass construction. The radial-ply has a stiffness and resistance in its tread area, so that in motion the tread in the contact patch (the section that touches the road surface) retains virtually all of its pattern and grip. On the cross-ply, the contact patch becomes pinched and compressed. The radial-ply tyre corners more tightly than the cross-ply because it has a lower slip angle. This is the difference between the path in which the wheel is pointed on a corner and the path it actually follows. The cross-ply steers better at low speeds and is easier for parking. The radials also have a lower rolling resistance and hence result in better fuel consumption figures.

Tread patterns

A wide variety of patterns are moulded into the tread to drain away water and cope with various other requirements dictated by the road surface and the type of vehicle. On a dry road the best grip is afforded by fairly smooth tyres (in the racing world they are known as Slicks), as they provide the greatest possible contact patch, that part of the tyre in direct touch with the road at any moment. But on a wet road, smooth tyres have hardly any grip, and the least trace of water acts as a lubricant. If there is a lot of water on the road, it will form a wedge in front of and underneath a smooth tyre, causing a form of skidding known as aqua-planing, in which the tyre virtually floats on a thin layer of water as it is unable to displace the same in real time driving conditions.

Tread patterns differ to suit different purposes. A tyre that is to be used for long periods in cold weather conditions needs a tread with a deep bite to get rid of snow, slush or mud. Other tyres may be needed for use in generally wet weather, when the most important requirement is a tread pattern design that ensures rapid water drainage. A tyre with a heavy rugged tread intended for rough roads would quickly wear out if used on dry roads at high speeds.

Cutting costs on tyres is false economy. Not only should all four tyres and the spare be of the same type, whether radial or cross- ply, but they should also, for maximum safety, be of the same make, size and tread pattern.

Code letters are imprinted on tyres to show the maximum speed upto which they can be used. The addition of the letter R to this coding signifies that the tyre is of the radial type. Radial tyres marked SR are suitable for speeds up to 113 kmph, those marked HR can be used at speeds upto 130 kmph, and those marked VR for speeds over 130 kmph.

Some final observations on the subject of tyres are:

You will always find new tyres give a smooth and soft ride. This is mainly for two reasons; one, that little extra rubber on the tread makes that big difference and the other, the freshness of the rubber that gives the elasticity making the side walls supple resulting in the cushioning effect.

Radial tyres normally give twice as much mileage as cross-ply tyres and the difference in cost is not more than 30 per cent sometimes even less.

Radials give a marginally harder ride but are safer at high speed.

Radials make the steering a little harder than cross ply tyres, but provide much better braking performance.

It is always recommended that the last bit of rubber, approximately 1 mm or less be ignored and a new set of tyres put on. Don't try to run the tyres till they become slicks as they become extremely unsafe for use.

If you have a flat while driving in the fast lane don't try fixing the same in the same place. Roll off the car into a side lane andthen do the needful after putting the hazard/warning triangle behind the vehicle. There have been many fatal accidents as people try changing tyres in the fast lane.

Tyres coming from fresh stocks must be used for high speed driving as aged and worn out tyres have a tendency to blow out at high speed.

Tutu Dhawan

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