Merityre Article Archive

Your car tyres are all that keep you on the road and in dealing with heat, cold, wet and dry conditions and various surfaces, they have a hard life. Knowing what tyres are made of and how they’re built will help you understand how they cope.

Car tyres are built round an inner liner of halobutyl rubber sheet with additives that help keep air inside the tyre. The next part out is the body ply, which consists of three layers, one of rubber, one of reinforcing fabric and a second layer of rubber. Rayon, nylon, polyester or Kevlar have replaced cotton as the reinforcing fabric and it is the tyre’s one or two body plies that give it structural strength while making it flexible.

Externally, the car tyre’s sidewalls make it resistant to abrasion and chemical attack. Sidewall material includes antioxidants and antiozonants to help maintain strength and flexibility. This goes on in and behind the sidewalls on which you can see the tyre maker’s name and the various code digits that the confirm tyre’s size and rating.

The tyre beads lie at the edges of the sidewalls. They are bands of high tensile steel wire, coated with an alloy of copper or brass to prevent corrosion. The beads, inside their rubber casings, are strong points in tyres. They are what holds it firmly to the wheel and help seal in the air. The apex is the section that joins the bead and sidewall. Triangular in section, it is the cushion between the two.

Now for the belt package, the part that lies beneath the tread. This consists of a two rubber layers sandwiching a layer of steel cords. Here’s where ‘radial’ tyres gets their name; the cords run radially in the package. This steelwork gives the tyre the strength to resist dents from road impacts.

Like the sidewalls, the tread is a visible part of a car tyre. The tread compound is as important as the tread pattern. Tyres made of hard compounds wear well, while softer ones grip the road better. ‘Compromise’ is the watchword.

How are car tyres built? This process starts by wrapping the inner liner, body plies, sidewalls and beads around a special drum. The belt package and tread are then added. During this process, the components are ‘spliced’ together, before the ‘green’ (uncured) tyre is blown up and shaped. These tasks are generally automated.

Now comes the curing process. Though spliced, the various parts of the tyre have yet to be bonded together. This involves applying heat in a mould, to speed the chemical reactions between the different materials. During this stage, a rubber bladder, the equivalent of an inner tube, is used to force the tyre into the mould. Unlike an inner tube, the bladder is filled with steam, hot water or an inert gas. Tyre curing is done at very high pressure – about 350 psi – and at around 350 degrees farenheit. A passenger car tyre can be cured in around 15 minutes. Finally, the tyre is run on a mock road surface, to test for imbalance and local inflexibility. All tyres are visually inspected, some are even X-rayed.

So as this shows, there is more to a car tyre than meets the eye. It’s no wonder car tyres are tough enough for the tough job they have.

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Most garages selling car tyres offer a wheel alignment service but not all customers know what this is or what it’s for when they come to buy new tyres. As its name suggests, wheel alignment is to ensure that your car’s wheels are parallel to each other and perpendicular to the ground. Aren’t the wheels like this anyway? They should be but wear and tear, especially that aggravated by speed bumps, can put the wheels out of alignment, as can damage caused by touching a kerb or hitting carriageway debris. Misalignment can cause a braking car to pull to one side or the other, give inaccurate steering and drastically shorten tyre life.

What is involved?

There are usually three factors to be checked and if necessary, adjusted.

The first test is to ensure the wheels are vertical as viewed from the front of the car – this is checking the camber angle.

The second check is slightly more complicated. A car’s front wheels are turned from side to side when you steer. Imagine looking at a front wheel from the side of the car. If you had X-ray vision, you’d see that the steering pivot isn’t vertical but leans back at the top, towards the rear of the car. This is the castor angle, which, if out of adjustment, can cause all manner of strange steering behaviour when you drive.

The third and last check is the tracking. Again looking from the front, the car tyres are usually ‘cross-eyed’, they ‘toe-in’. This is so slight you can’t see it but the setting is important. The tyres are generally set to toe-in so that the when the car is driven, the forces acting on the front wheels and suspension make the front wheels run parallel to one another. If the tyres ‘toe-out’ (think Marty Feldman) through misalignment, or toe-in too much, steering accuracy will suffer and tyre wear will increase. On many cars it’s also possible for the rear wheels to be misaligned.

Should car tyres be worn through misalignment and/or have worn or damaged steering or suspension parts, or even the wrong tyre pressures, correct alignment could be difficult or impossible. The technician will ensure you know about what, if anything, needs adjusting, repairing or replacing.

Man at Work

Wheel alignment used to be achieved with mirrors, weights and string. But such paraphernalia is old hat now; technicians usually use computerised, laser-equipped measuring installations. With these, they can measure and adjust wheel alignment quickly and extremely accurately. So while you enjoy a refreshing cup of tea or visit the shops, your car’s wheels will be aligned with precision, which will extend the life of your car tyres.

What is involved?

How can you keep them this way? Simple – all that’s needed is a little care when you drive. Avoid any debris on the road and watch out for potholes. Take your time over speed pads and straddling speed bumps damages tyres and suspension; drive so that only the wheels on one side of the car go over the bump. Most importantly, when manoeuvring, do not hit kerbs.

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