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The Operator Is Not • Always to Blame

10th February 1939
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Page 51, 10th February 1939 — The Operator Is Not • Always to Blame
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Which of the following most accurately describes the problem?

pERHAPS it may seem, from my previous articles on tyres, that I have laid most of the blame for tyre failures upon the operator himself. This is by no means unfair, because practically all present-day failures are due to circumstances for which the operator, directly or indirectly, is responsible.

These circumstances include, mainly, misuse and neglect, as well as accidental damage, for, although nobody is morally responsible for damage sustained by sheer mischance, it is, nevertheless, obvious that the ultimate financial responsibility rests with the operator.

Faulty Tyres are Rare.

Faulty tyres are a great rarity, although it would be well-nigh impossible to convince some fleet owners that this is true.

Let us put it in this way. Out of every 50 tyres which are the subject of complaints and optimistic claims, not more than one is genuinely faulty. , The other 49 are perfect tyres which have failed through adverse conditions of usage. To the operator they are all the same—just burst tyres which must be faulty— and the respective makers have the unenviable task of trying to convince 49 out of every 50 operators that their conclusions are wrong.

It would be unfair to deny that there are a few forms of trouble for which the operator cannot be held primarily responsible, the blame resting either with the maker of the tyre or with the designer Of the vehicle. Sometimes; as we shall see, it is due to a lack of co-operation between the two.

In the first place, we must admit the possibility of the genuinely faulty cover. We all make mistakes occasionally, and tyre manufacturers are no exception. It must be recorded to their credit, however, that the percentage of failures as the result of a tyre being faulty is microscopically small.

Apart from the " dud " tyre the main troubles for which the operator is not responsible are due to faults in vehicle design, or, alternatively, to the design of the tyre in relation to the vehicle.

A typical example of the latter is provided in the practice, prevalent among lorry manufacturers, of guaranteeing their vehicles to carry an overload. Let us say, for instance, that the lorry is nominally a 3-tonner, fitted with 32 by 6 truck-type tyres. This equipment is only just capable . of carrying the vehicle and a normal load, but some manufacturers guarantee their lorries to carry overloads of as much as 50 per cent.

The Load for the Tyre.

The operator can hardly be blamed for this. He naturally believes that every component of the vehicle is included in this overloading guaran tee. Only in rare cases does an operator appreciate that the tyre equipment is limited to the original nominal load.

I, personally, believe that there is a great share of popularity awaiting a vehicle manufacturer who will fit really adequate tyre equipment.

It is known that a vehicle maker, when placing an order for a supply of tyres, will make stipulations as to the maximum weight. He, more

' or less, forces the tyre manufacturer to depart from the accepted principles of tyre design, in order that the total weight of the complete vehicle shall come within a certain taxation class. In making this stipulation, the chassis designer does not necessarily allow' the tyre maker to choose the size of tyre which shall be produced at the given weight; the size and weight are specified, and the manufacturer must conform.

The Alfieight Factor.

Thus, on some makes of vehicle, we find that the original tyres are slightly deficient in the number of plies of cord which comprise the carcase, or have slightly less tread

rubber. These reductions are not made with a view to economy, but solely to reduce the total unladen weight of the vehicle. In the great majority of cases, such tyres are quite equal to the task they have to perform, but it is only natural that they are more susceptible to failure, through unfavourable conditions., than are covers of normal construction.

It is rather unfortunate that the vehicle designer, having made exact specifications as to how his tyres are to be made, should then repudiate all responsibility for their failure when in use. Tyres, being a component made by another manufacturer, are not generally included, in the vehicle guarantee.

Sometimes, on the other hand, the design of the vehicle is entirely to blame for unsatisfactory mileages. Most operators have noticed that certain makes of machine are heavier on tyres than the corresponding models of other makers. The various causes of this are innumerable, but are mainly connected with special features such as braking systems, suspension, load distribution, etc.

Faults in Chassis Design.

Such features seldom cause carcase failures, but are frequently responsible for rapid tread wear and low average mileage. They might be described as minor faults. Every now and again, however, a major fault in chassis design is encountered which is responsible for many serious tyre failures.

Who would think, for instance, that a large brake drum could be the cause of many bursts? One such case is shown in an accompanying illustration. The tyre was fitted on to the usual type of disc wheel which, when fitted to the hub, left only a small clearance between the brake drum and the rim—a feature for which the vehicle designer was responsible. Naturally, every time the brake was applied, the heat generated was virtually trapped in the narrow air-space between the brake drum and the rim.

Another great problem at the moment concerns low-pressure lowloading tyres as fitted to semi-trailer types of vehicle. Whether the fault lies in the design of the chassis or the tyre it is rather difficult to say, but it is a significant fact that the trouble is prevalent irrespective of the make of tyre fitted.

By some, it is considered to be due to the excessive sway of the vehicle, which throws enormous stresses on the side walls of the tyre, particularly on bends. Others believe that the unnatural proportions of the tyre section, in relation to the small rim diameter, result in, severe distortion ofthe tyre casing during rnanufacture, causing a weak spot which manifests' itself when the cover is put into use. Another form of tyre trouble connected with some types of trailer is irregular wear of the tread. As shown in one of the illustrations, the pattern is deep in some places and practically non-existent its others, with buttons of tread standing out at intervals, and., over all, that "rasped " appearance which is normally associated with misalign ment

Tylvs.Sometimes Faulty.

There are other cases, of course, where the blame for tyre failure does not rest with the operator but, generally speaking, they are of a similar nature to those already mentioned. The tyre manufacturer is responsible for an occasional fault, and the vehicle designer is liable, at times, to make mistakes which react on the tyre equipment. These examples are quoted in all fairness to the operator, but it must not be forgotten that they do not constitute 2 per cent. of present-day tyre

failures. L.V.B.

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