Is Tyre Rotation Worth While?
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A Tyre Change Form is Detailed Here With Recommended Procedure for Matching Covers to Increase Mileage Life IN this series last week it was emphasized that considerable economy in tyre costs could be obtained by comparatively simple, but persistent, routine maintenance. This has long been recognized by the larger operators, but such tyre maintenance schemes are by no means universal with smaller users. For such schemes to be effective it is essential to inaugurate, and subsequently maintain, a corresponding tyre record system.
It is probably largely for this reason that many small operators fail to obtain the maximum mileage from their tyre equipment because of a reluctance to undertake the additional clerical work which they consider is involved in keeping such records.
A recommended Tyre Record Form was described last week, and it was emphasized that it was the whole basis of the system that one form should be made out for each cover— either as soon as a new vehicle is put on the road or when replacement covers are issued at a later date. The several movements of the cover are then recorded on this form and at the same time the mileages run in each of the various positions. The total cost of the cover and subsequent repairs or retreading is proportioned out relative to these several mileages.
By this procedure a fairer proportion of costs as between the various vehicles to which a cover may have been fitted is obtained than would have otherwise been the case if the total cost was allocated in full to the first vehicle to which it happened to be fitted.
TN conjunction with this Tyre Record Form it is neces
sary to also have a Tyre Change Form, an example of which is shown alongside. It is unfortunate that, when inaugurating a tyre record system, a substantial amount of work is involved in taking the initial inventory. In many instances. especially when mediumor long-distance journeys arc made, such an inventory can only be satisfactorily taken at week-ends when the whole, or at least the majority, of the fleet is back in depot.
Once such an inventory is taken, however, it will be the Tyre Change Form which will be most regularly employed. Moreover, it will normally be completed by the fitter detailed to undertake tyre maintenance or, alternatively, by a driver when it is necessary to make a tyre change on the road. For this reason it might be advisable for the Tyre Change Form to be printed on a card rather than a sheet, to give greater permanence. As, however, the Tyre Record Form would normally be completed in the office by clerical staff assigned to that work, a sheet should suffice.
Also, because the Tyre Change Form would not normally be completed by clerical workers, it should be drawn up as simply as possible. In this instance it consists of three sections. In the first section are detailed the fleet number of the vehicle and wheel position, followed by the date on which the change was made and the mileometer reading at the time.
In the second section are recorded details of the tyre which has been removed. These include the serial number, size and ply rating, together with the make. Corresponding details then follow relative to the replacement tyre, whilst in the last section the fitter or driver making the change adds brief remarks of the reason.
At this stage in describing a tyre record system it is convenient to refer to the method of abbreviating the various wheel positions. It is advisable at the outset to adopt a standard method so as to facilitate completion of the various Tyre Record Forms, and additionally to avoid misunderstanding.
For many years the use of the terms " nearside " and "offside" was almost universal in this country, but mare recently there has been an increasing tendency to use the more simple description of either " left " or "right," possibly as a result of the influence of increasing export of 1 commercial vehicles. For the purpose of Tyre Record Forms this shorter description can be still further reduced to simply " L " or " R" and correspondingly the description "Inner" or " Outer " when applied to twin tyres is indicated by " or "0."
A complication arises when multi-axle vehicles of any description are operated. By modern standards former methods of describing the several wheel positions which could occur on a multi-wheeled vehicle are considered too cumbersome. Moreover it is not easy to abbreviate such terms as "first steer," "second steer," "first driving axle" or "rear trailing axle." Once staff have become familiar with the alternative method of numbering axles from front to rear it will be found much simpler to indicate wheel position by this means. It has been assumed, however, that the operator will also be making use of a fleet numbering system which indicates the type of vehicle, and not merely the chronological sequence in which the vehicles were purchased. It would then follow that the fleet number would have already indicated whether the vehicle concerned was fitted with either two, three or four axles.
Consequently, although the figure " 2 used in this context could indicate any one of three axle positions, namely the rear axle of a standard four-wheeler, the second front axle of a six-wheeler "steer" or alternatively the corresponding position on an eight-wheeler, it could also indicate the first of two rear axles of A six-wheeler. In each specific instance, however, it would be known from the fleet number to which axle it actually referred. By the adoption of these abbreviations any position of a cover on a standard vehicle can be clearly shown by a combination of a maximum of three letters and numbers tn each case.
THUS on all vehicles 1/L would denote that the cover was fitted to the left (i.e., the nearside) of the first axle. Similarly 4/L/0 would indicate the outer cover on the left of the fourth axle, which would normally be either the second driving axle tar the trailing axle of an eight-wheeler. Assuming twin tyres were fitted at the rear, the six tyre positions on a standard four-wheeled rigid vehicle when adopting these abbreviations could be indicated as follows from front to rear: I/L-1/R; 2/L/0-2/L/I; and 2/R/0-2/R /I.
Even with a comparatively small fleet of vehicles there will be a comparatively large number of Tyre Record Forms, so that the manner of filing justifies some consideration, and should bear relation to the extent to which vehicle costs arc segregated. It might be advisable to adopt some grouping relative to size as, otherwise, filing on the basis only of serial numbers could prove confusing.
Normally the technical aspects of maintenance are considered to be outside the sphere of this series of articles. An exception is made in respect of tyre maintenance, however, since this is so dependent upon the system adopted and the extent to which records are kept. Whilst the work involved is comparatively routine, many large operators consider that the possible economics which can be effected by careful maintenance justify the employment of one or more fitters solely on this work.
Where such a policy is not justified, opportunities are now available for smaller operators to enter into contracts with tyre manufacturers or factors to check tyre equipment periodically on the operator's own premises. In cases where the local agent's premises were sited comparatively near to the operator it might prove worth while to hire out this part of vehicle maintenance and so avoid outlay upon tyre maintenance equipment which would in any case be only occasionally used by a small operator.
ALTHOUGH, as already stated, a.good part of tyre maintenance consists of routine work it will be recognized that, when a fitter is employed solely on such work, his special knowledge of both the cause and effect of exceptional tyre wear could decide at an early stage possible remedies to mechanical defects, for example in the steering of a vehicle. But obviously a fitter employed only part-time on tyre maintenance could not be expected to associate cause and effect so readily.
In devising a formula to give the most economic rotation of tyres, it is first necessary to determine the relative wear on a standard vehicle. Assuming the vehicle concerned was a small four-wheeler with single tyres on the rear, and that the wheel alignment and weight distribution were correct, the rate of wear would normally increase from minimum to maximum in the following order:-2/R, 2/L, 1/R and 1/1.
On this basis a new cover would be fitted to the left front wheel, and incidentally when it is possible to arrange this it is preferable to fit new tyres in the autumn or winter rather than the spring or summer. This is because, whilst the tread has its maximum thickness, the heat generated will correspondingly be high which would unfortunately coincide with the higher road temperatures normally experienced in the summer. Maximum mileages would therefore be achieved by fitting the covers in the autumn or winter, which, additionally is when the benefit of the new tread is most required.
Where the need for proper tyre maintenance is accepted, it is also recognized that rotation of tyres is desirable. But the underlying reason and the manner in which this should be done are not always fully understood.
THE main purpose of tyre rotation is to endeavour to equalize as much as possible the rate and type of wear relative to the various wheel positions. Thus on the four-wheeler, with single tyres on the rear, and assuming a spare is available, the recommended rotation would be I/L to 2/L; 2/1 to 1/R; hR. to spare and spare to 2/R.
Over and above the requirements of any predetermined rotation system, the actual condition of individual covers must still remain the prime determining factor. Here, again, the advice of an experienced tyre fitter would, of course, be invaluable.
The importance of correct matching so as to avoid excessive wear is especially important in multi-wheelers which would, in any event, invariably be fitted with twins on at least one or more axles. For the purpose of considering tyre matching, multi-wheelers can be grouped as follows: (1) Sixor eightwheelers, with tandem rear axles, on which the leading axle of the bogie is driven.
(2) Sixor eight-wheelers, with tandem rear axles, both being driven, but with a third differential.
(3) Sixor eight-wheelers, with tandem rear axles, both being driven, but with no third differential.
Providing adequate maintenance is given to the overall diameter of twin tyres, vehicles in categories 1 and 2 should not provide any exceptional difficulties relative to tyre match ing. Even so, improvements might still be obtained by applying the suggestions recommended, where appropriate, for vehicles in category 3. It should be pointed out that whilst differentials are admittedly included in the transmission of a vehicle to balance out variations in the relative revolutions of each wheel, it is nevertheless undesirable to overwork the differential unnecessarily on account of the possibility of unequal tyre diameters.
TN the case of vehicles in category 3 it is particularly import"ant to pay particular attention to the diameter of tyres fitted to each axle, even though it is appreciated that there are practical limitations to the extent to which this can be achieved.
Where the ideal is not possible, say when fitting four new and four worn covers, then two new covers should be fitted to the left side of the first driving axle and the other two new covers to the right side of the -second driving axle. With the part worn covers in the remaining positions, a diagonal fitment is achieved.
Another arrangement would be to fit all four new covers on the left side and the worn ones on the right side. In both examples the objective has been to try to reduce the amount of work that the differential has to do in balancing out overall revolutions, whether caused by variations in tyre diameters or any other reasons.
Whilst the fitting of all four new covers to one axle would undoubtedly achieve correct matching relative to the twins on that. particular axle, the resulting fitting of the four worn covers to the second driving axle would cause the two axles
to revolve at different rates. There would then be either substantial mechanical damage to the vehicle or rapid tyre wear caused through the exceptional amount of slip necessary
for the two axles to marry up their revolutions. S.B.