Does Tyre Rotation Pay ?
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FOLLOWING the article on tyre costs which appeared in The Commercial Motor on April 17, I have received an inquiry for additional information on tyre maintenance, including the recommended procedure for rotation of tyres on larger commercial vehicles. Whilst technical advice on engineering matters is not normally given in this series of articles, an exception is -being made here, because tyre maintenance is so closely bound up with systematic programming that there is little demarcation between this aspect and the application of technical knowledge and experience.
In the previous article it was pointed out that tyres may account for more than 10 per cent. of the total cost of operating a commercial vehicle. For example, even when the weekly mileage averaged only 400, estimated tyre cost per week for a 5-ton petrol-engined platform lorry amounts to £2 2s. Rd. out of a total operating cost of £26 2s. lid. A multi-wheeler covering 1,000 miles or more per week might incur a weekly tyre cost of over £20. Moreover, as these amounts are cumulative, week by week, substantial savings can be achieved if overall tyre efficiency is improved by only a comparatively small degree.
In conversation with both small _and large users of goods and passenger vehicles, striking variations in tyre mileages are elicited. It is significant that, almost without exception, the larger companies employ tyre fitters exclusively for their maintenance, or delegate responsibility to specific-members of their staff. It is equally revealing that these companies achieve the highest mileages from their tyres.
Compared with other forms of maintenance, care of tyres makes only limited demands for equipment and medium-sized and small operators should not be at a disadvantage. in analysing operating costs submitted by various operators for use in the compilation of " ' The Commercial Motor' Tables of Operating Costs" it becomes apparent that there is room for further economies in the tyre costs.
Successful Maintenance
To achieve them should not be beyond the means of the smaller man, because successful tyre maintenance results largely from systematic and conscientious attention. Where skilled advice is necessary, both tyre manufacturers and factors have qualified staff available and regular examination can be arranged by fleet users.
Some larger passenger operating companies pay for their tyres on a mileage basis, which is in effect a hire charge, whilst the tyre manufacturer, by arrangement with the bus company, arranges for one of his staff to be responsible for the main B42 tenance of all tyres for a group of vehicles—say, up to SO or 60 and preferably at one or adjacent depots. Smaller operators, especially if in haulage, are unable to take advantage of this scheme and may find it more difficult to devise a practical and comprehensive tyre maintenance system, especially if the size of fleet does not justify a tyre fitter solely for that job. Nor can a sufficiently large float of new and part-worn tyres be carried to facilitate matching, as recommended when dealing with twins or multi-axle vehicles.
In these circumstances it may prove economic to contract out tyre maintenance to the local agent. The cost of the service, over and above the price of the tyres, may he offset by the higher overall efficiency that the agent can achieve because of both a larger labour force and tyre. stock. In any event, every endeavour should be made, when building up fleets of vehicles, to standardize on both the size and ply of tyre, even if there are variations in makes of vehicle. An experienced tyre fitter can often correct irregular front-tyre cover to the driving axle when
wear by transferring the common sizes are fitted.
Before dealing with the rotation of tyres on larger vehicles, the rate of comparative wear on a four-wheeled vehicle (without twins) should be • understood: Assuming correct wheel alignment and weight distribution, wear varies according to position and might be expected to range from maximum to minimum as follows: (1) Near-side front (1/L); (2) off-side front (1/R); (3) near-side rear (2/L) and (4) off-side rear (2/R).
When to Fit A new tyre would normally be fitted to the near-side front wheel. It is also an advantage to fit new tyres in the autumn or winter, rather than in spring or summer. While the tyre is new and has maximum thickness, heat generation will likewise be at a maximum, and aggravated by higher road temperatures in a normal" summer. Higher mileages and lower costs would thus be obtained if new tyres were fitted in autumn or winter. Moreover, maximum use could be made of the new tyre tread when most needed.
Systematic rotation of tyres is recognized as an integral part of efficient maintenance, the object being to equalize the differing rates and types of wear which occur in the various wheel positions. This is comparatively simple when applied to fourwheeled vehicles not fitted with twins at the rear,. In that event, and assuming that a spare is included in the equipment, a recommended . method is to rotate in the following sequence:—I/L to 2/L; 2/L to 1/R; 1/R to spare: spare to 2/R. This form of tyre-position rotation was recommended and detailed in the April 17 article. Briefly, the axles are numbered consecutively 7rom front to rear--1, 2 and so on-whilst near-side becomes left (L) and off-side right Whilst personal examination by an experimeet] tyre fitter is the ideal practice in deciding a specific case of change in position, certain g.eneral principles are applicabre. when dealing . with larger vehicles and will helP tb it not avoid, incorrect tyre matching. For this ourpose such vehicles are grouped into three main categories: (1) Sixor eight-wheelers with tandem rear axles on which only the leading axleis driven.
12) 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.
Categories 1 and 2 do not normally present undue problems , of tyre matching, because the routine attention to oycrall
diameter which should be given to thick or tins 'tyres twin formation on any vehicle would be iiifficientfor satisfactory operation. Even so, if the additiOhal• attention which it is subsequently recommended should: be-applied-tocategory 3 is given also to categories 1 and 2', a.furtherimprovernent should' be achieved. This is because, although differentials may have been incorporated to absorb any difference in. the comparative revolutions of each wheel, it is undesirable to canse.them.to do unnecessary work as the result of uneqnal tyre.diameterS.
When dealing with the maintenance of tyres fitted to vehicles in category 3, strict attention must be paid to the overall diameter of tyres fitted to each axle, the ideal being, of course. to fit a full set of tyres of identical size and type. This is, however, not always practicable., Where a compromise is necessary-for example, the fitting of four new tyres along with four part-worn--two of the new covers can be fitted to the near side of the .first driving axle and a further two new covers to'. the off side of the second axle, with the part-worn tyres in the other two positions. This is termed the' diagonal fitment.
AlternatiVely, all four ne* covers can be fitted tei the near-side position and the part-worn to the off side. -Both theS'e pro
cedures have the effect of giving ,each, differential the minimum arnOtint of Work to do, tO correct overall revolutions.
If all four used tyCes were fitted to, ode axle, as might at first appear reasonable, and the :four'part-Worn to the second axle, the two axles would be rev'o,lving at a -different rate per mile. There would then be the possibility of severe mechanical ", damage to the Vehicle or:of rapid tYre Wear through the. slip necessary for theaxles: to operate in unison despitethe
difference in tyre diameters. .• • "
Where difficulty arises In preserving a good match of tyres on multi-axle vehicles, as just: outlined. •any odd-sized tyre should be run out in the front position, where differing reyolutiOns would not have much influence. Where a different front fitinent.waS specified. this would not he practicable, of course. and for satisfactory operation a small float 'of part-worn tyres might be netessary to ProVide a' chOice of sizes sufficientfor This is. one' of' the 'difficulties Which ha'Ve to. be faced by the small operator whose limited. finanCial..resotirCes restrict his . '• stock..of. tyres: .1f,-.in an enaergency,-one or' two tyres of a. . larger. diameter, than those, already, on the vehicle, have. to be • fitted, they should, if 'possible, be .placed on the near side.. . A diagonal change'. of front. tyres _would . normally.. be sufficient to rectify any positional wear., and tyre wear should , be carefully watched for arty. sign of m,isalignment which could .
reduce life and increase cost. '
Much Less Common
Operators may well consider. whether, a reasonable balance is being kept in the allocation of funds for garage equipment. Whilst many such items on the mechanical side are generallv used in most garages, equipment for the setting-up and testing of tyres and wheel alignment is much less common. Yet the cost of misalignment in terms of excessive tyre wear can be substantial and well justifies the expense of purchasing special tools for the purpose. Even if only one tyre is saved from premature failure, the initial outlay would be recouped.
When considering the amount of time and money that can be apportioned to tyre maintenance it should not be overlooked that, as with -other items, efficiency or inefficiency in this department can have repercussions on one or more of the remaining nine items of operating costs. Thus, careful selection of the correct tyre, and subsequently its pressure, will reduce both chassis and body wear.
This particularly applies with vans or where there is a high proportion of empty running. Conversely, whilst the effect of excessive speed on tyre wear is, within reason, limited to reducing the mileage life, overloading (or the parallel fault, of under-inflation) can completely wreck a cover. Incidentally, for this reason, and because of a similar adverse effect on maintenance costs, estimates' or recommended charges cannot be given when readers' requests obviously imply a measure of overloading. For the benefit of operators who may wish to calculate their operating costs on the basis of ,their experience I have prepared a ready reckoner, . From this the cost per mile can be obtained
for .sets of tyres ranging in Price from £.1ff £600 and with an estimated tyre life of 20.000i. 30.000 or 40,000 per set. S.B.