Akin Powel4 of . the National Tyre Distautors Association, on usage and abusage
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TWO of mankind's most important inventions are the wheel and the motor vehicle — one extremely simple and the other incredibly complex. Together, the two inventions played a pre-eminent part in the growth and development of 20th Century life, but by now both are largely taken for granted as necessary but forgetable tools for modern living.
However, there followed yet another invention without which neither of the first two could have operated efficiently and that was the tyre.
This is now a highly sophisticated and technologically advanced product. Nevertheless, its all too often abused, neglected. taken for granted, expected to cope with poor roads, no roads, snow, rain, debris, heavy loads and yet to perform faultlessly, cheaply and, above all, to last as long as possible at all times.
The tyre's reward is usually a cursory glance and maybe a swift kick to see if it reacts. If nothing happens, it must be all right! Does this perhaps strike a chord?
Tyres are the last link in a vehicle's steering, braking, transmission and suspension systems, and they have to steer, drive and stop it, while simultaneously performing adequately on all roads, in all weathers and at all speeds. They also have to cope with misalignment, under and over inflation and overloading, and yet tyre manufacturers have still managed to make tyres that, in varying degrees, meet all these demands. And this is not a song of praise for such people, just a statement of fact.
All tyres are based on four basics — the air chamber, the carcass, the tread and the beads.
The carcass contains the air under pressure, which enables the tyre to carry out its design function, while the tread is the basic link with the road surface and protects the outer contact area of the carcass. The beads hold the tyre on the wheel rim, assisting in the creation of an air-tight seal when the carcass is inflated.
This article will deal with the radial tyre, since this is now the most widely used type in commercial vehicle use; it is well known that the pioneer of this type of tyre was Michelin, particularly in its use of steel for bracing, which is now widely recognised as the most efficient method of construction.
In the simplest terms, the radial tyre is constructed so that sidewall flexing does not affect the tread area, which is separately braced, so that both can function effectively without limiting, restricting or even "influencing" the other.
The cross-ply type construction, on the other hand, means that sidewall flexing causes movement in the tread area, so that it is the complete, inflated casing that controls tread movement, and hence what can best be called "squirming" of the tread.
Tyres wear out because they are in constant frictional contact with the road surface, but the rate of wear, while dependent to a large degree on the use (and abuse) of the tyre, is greatly speeded up by the excessive build-up of heat through friction.
This, the steel-braced radial tyre dissipates at a far greater rate than its cross-ply brethren; this immediately leads to a higher mileage and improved fuel consumption, since there is also much less rolling resistance in a radial tyre and less energy is needed to move the truck.
These are the good points, or the uses, of the tyre. So what about the abuses. and the signs to look for, apart from the obvious ones of tears in the sidewalls or a semi-flat? The basic things to watch out for are as follows: Under-inflation: The stresses here are in overheating since, with insufficient air pressure, there is excessive deflection and distortion of the casing. The centre of the tread is "lifted," so that the outer edges take the increased loading and there is irregular wear, loss of grip and, from that, reduced tread life. The main sign will be wear on the outer edges of the tyre, with the centre section still showing tread.
Over-inflation: In this case the edges of the tread are "lifted," so that all the load forces are concentrated on the centre of the tyre, causing loss of adhesion and more rapid wear, The ride is much harder because of the loss of flexibility and there is more danger of shock damage, such as when hitting one of the now numerous potholes, or kerbing.
Misalignment: This is one of the commonest causes of too rapid tyre wear, and is due to the wheel trying to follow a different path from the actual line of travel. Signs are usually "featherinn" of thp tread nattern and some of the most likely causes are.
DIncorrect front wheel setting, ie toeing-in or toeing-out. The setting should follow the manufacturers recommendations but, for radial tyres, parallel is the more usual norm.
Distortion or breakage of a part of the steering linkage, usually because of wear or accidental damage.
Li Axles out of alignment, which can be caused by a number of factors, such as breakage of one or more leaves in the spring, distortion of the chassis or damage or distortion to radius arms or other suspension parts.
[1 Defective brakes, which can lead to uneven tyre wear. Signs to look for are tread wear in patches.
Li Irregular rotation, which usually comes from defects on some part of the wheel geometry, such as suspension joints, hub bearings or loose wheel nuts.
Hard driving, cornering or rough braking can be guaranteed to reduce tyre life, whatever the vehicle, even though tyre manufacturers provide heavy-duty tyres or sidewalls in their armoury where there is an undue amount of stopping and starting, off road work or where kerbing is more than usually likely.
There are a number of simple ways whereby the greatest benefit can be obtained from tyres, and obviously the knowledgeable and efficient operator is well aware of their importance in the scheme of things — like a profitable return on outlay! But it is worth reiterating them again, as it takes little time but can repay the care many times over.
Pressures should be checked only when the tyres are cold, no matter what the vehicle, and tyre caps should always be fitted, as apart from air leakages and hence underinflation, dirt can get in and cause the valve to open, usually at the most inconvenient 'time. Stones or other detritus should be removed as soon as possible, as they can work their way into the casing and cause damage, leading to eventual .failure. This is particularly true with objects caught between the twin tyres of a truck.
A quick repair of cuts can prevent water entering the casing and affecting the plies, while oil,, fuel or paint are all harmful to rubber and should be removed with a detergent (but not oil-based, such as paraffin).
Uperators who have their own trained fitters, or who use the services of tyre specialists such as members of the National Tyre Distributors Association, can obviously minimise these problems, for inexpert or careless fitting can damage a tyre before it even starts its life, while correct wheel balancing is of the utmost importance. There is some argument over the wisdom of mixing cross-ply and radial tyres. This is perhaps not surprising considering the fact that while it is clear that cars and light vans with mixed tyres must have them on the rear with absolutely NO exceptions, this rule does not apply so strictly to larger commercial vehicles.
On trucks having twin rears or wide singles, the fitments can be cross-ply even if the fronts are fitted with radials. It is now recommended by the British Manufacturer's Association, however, that where there are textile and steel-braced radials the steel braced should be fitted on the rear axle.
Matching twin assemblies needs care hut, basically, the inner twin can be more worn than the outer because of road camber but, with double-driven bogie units, there should be no great differences between tyre wear on the two driven axles.
Modern radial tyres are made to perform adequately and over great distances, provided they are properly maintained at correct pressures for the job they have to do and are not abused by excessive acceleration, braking, hard cornering or by regularly being driven over obstructions such as the aforementioned kerbs, potholes or traffic islands.
Usually if a tyre fails other than from a straight puncture, it is not the fault of that tyre — everything still rests with that famous nut holding the wheel! So it is up to everyone with economies in mind to make sure that there are no loose nuts about the place, inside or outside the cab, and to remember that you get what you pay for, in tyres as in everything else, so that the cheapest is not necessarily the best.