2. Vehicle selection and replacement

18th September 1970, Page 254

18th September 1970

Page 254

Page 255

Page 254, 18th September 1970 — 2. Vehicle selection and replacement

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by J. Greenhalgh

The two remaining subjects, the choice of vehicle and replacement policy, are not the province of the fleet engineer alone, involving as they do most of the other departments of an organization connected with road transport, but they have considerable bearing on the engineer's responsibility—the maintenance of a reliable and roadworthy fleet at the lowest cost, Most vehicle manufacturers are helpful to customers, if such assistance is required, when it comes to choosing gear and final drive ratios, tyre sizes, wheelbases and the like, for the vehicle finally chosen, but this is almost the end of the story: a great deal of thought is required before this point is reached.

Extreme example

Everyone is familiar with the sight of the postman with his small van, making collections from pillar boxes. He can probably empty some 10 or 12 boxes before his van is full and he must return to the depot. The question may be asked why, in this era of expensive labour, the Post Office do not use a 32-ton.g.v.w. articulated vehicle. Providing approximately 40 times the capacity of the small van, it would enable one man and one vehicle to do the work presently taking 40 men and their vehicles, besides necessitating considerably less capital outlay. The proposition verges on the ridiculous, but an analysis will serve to illustrate some of the factors requiring consideration before deciding which is the best vehicle for a particular application.

The sheer size of the large articulated vehicle would reduce the chances of finding a vacant space at the kerb and threading its way through urban traffic which, in turn, would lead to longer journey times. For the greater part of its working day the vehicle would be consuming fuel merely to move its own weight. But probably of greater importance is the time factor. To provide an economic load, this vehicle would have to visit over 500 letterboxes, taking two days working non-stop. This would be unacceptable to the general public who demand several collections each day, and prompt delivery, and also to the Post Office who would prefer a continuous stream of incoming mail at sorting offices rather than infrequent large drops.

In other words, the biggest is not always the best, and the transport operation must be viewed as being only one part of the company's business as a whole. In manufacturing and supply industries it is sometimes beneficial to study the complete process from the provision of raw materials through manufacture or treatment, packaging and control of despatched traffic flows, when a different transport pattern may result.

Analysing the requirements

This concept is relevant to the public haulier also, who must first of all decide on which sector of the freight haulage business, and between what locations, he is going to concentrate, and set up an organization accordingly. The picture then begins to emerge of the type of commodity to be carried, the frequency of traffic flows in different directions, the optimum size of individual truck loads, which may be dependent upon the terminal handling facilities, and the nature of the terrain over which loads are to be carried. This will lead, in general terms, to an idea of the capacity re

quired either by weight or volume. It may be advisable, in an organization where two or more distinct transport activities exist, requiring different classes of vehicle, to consider whether the number of types may be reduced by accepting a compromise. This permits a degree of operational flexibility and will generally be welcomed in the workshop.

For payloads over about 16 tons the articulated vehicle has, until recently, reigned supreme as a result of the greater gross weights and overall lengths permitted by regulations, but a competitor has now appeared in the shape of the large rigid with drawbar trailer, and for many applications this requires careful consideration. In the lower payload range the articulated vehicle scores over the rigid in providing greater operational flexibility. A variety of body types may be utilized with the same prime mover, which is not immobilized during loading and unloading. On the other hand, the rigid is less complex mechanically, usually has an enhanced performance, takes up less kerb space and—of great importance nowadays—constitutes one, rather than two or three, separate vehicles requiring annual testing. Some flexibility has been restored to the rigid vehicle with the resurgence of containers and demountable bodies, which provide many of the advantages of articulation with the added bonus of a reduction in taxation weight. A possible drawback is that, unlike articulation, the various demountable body systems are not, as a rule, compatible.

Settling on the performance

Vehicle manufacturers' literature shows the unladen weight and maximum plated weight for each vehicle in their range. so that knowing the type of body required, and its approximate weight, enables the payload to be ascertained. It is advisable to study the individual axle plated weights to determine whether any possible combination of loads, including part loads badly distributed, cause any of these to be exceeded, even though the gross weight is below the maximum stipulated. Performance (that is, maximum speed, rate of acceleration and gradeability) is a function of the vehicle's laden weight, engine power and torque, overall transmission ratios and tyre size and at most gross vehicle weights there is ample choice between vehicle manufacturers and as available options from a single manufacturer to tailor a vehicle to give the desired performance.

There is often a wide variation in capital cost of vehicles available for a particular payload capacity and with comparable performances. In general, although there are exceptions, the higher priced vehicle is more robustly constructed, hence offers increased reliability and the potential for a longer useful life. The latter factor is not always necessary or desirable, particularly in an age of rapid technological development and only the fullest consideration of the task the vehicle will be called upon to perform, coupled with the widest experience of operating different classes of vehicle, can ensure the most viable selection.

When to replace?

Closely allied to the choice of vehicle is the question of a replacement policy, as the main factors which determine the optimum life in service are capital cost, resale values, maintenance costs and the degree of downtime experienced. Whilst it is appreciated that a vehicle can be maintained indefinitely in a roadworthy state, otherwise we should no longer have veteran car rallies, it is obvious that repair costs and downtime increase with age, and there must come a point in time when it is more economical to replace an old vehicle. Apart from mounting repair bills the amount of time a vehicle is out of service can reach the stage where, in a fleet of, say, 100 vehicles, an additional four or five vehicles are required to cope with the job in hand. Finally there is the obsolescence factor, which covers such items as a change in operating conditions necessitating a different type of vehicle, amendments to regulations permitting heavier or larger payloads, the improved efficiency of a more modern vehicle and the effect on staff morale and the public image.

Because an accurate assessment of the situatibn is dependent, inter alia, upon a complete history of costs incurred in maintaining and repairing a vehicle, it follows that its service life cannot be fixed in advance, unless similar vehicles have been operated previously. This does create a problem when a completely new type of vehicle is introduced, for until the book life is established, annual depreciation provisions cannot be calculated. It is, of course, possible to assume a book life and adjust it later as the necessary information accumulates with the vehicle in service.

The optimum service life is that which results in the lowest average annual cost and it is convenient therefore to express all the relevant factors in these terms: that is, the cumulative amounts divided by the number of years' service leading to those costs. As an illustration, the following tables and graphs relate to a hypothetical vehicle, with a capital cost of E1800, covering a constant annual mileage over a period of eight years. For simplicity, only the three most significant factors, depreciation provisions, resale values and repair costs, are included.

Table 1 shows the cumulative figure for maintenance costs. the anticipated sale proceeds in each of the eight years (based on a reduction of 25 per cent each year) and the capital cost, which remains constant.

In Table 2, these are converted into average annual amounts, with the addition of the column headed "net capital cost", which is the difference between the purchase price and the resale value.

The figures in the last two columns are shown graphically in Fig. 1. the net capital cost as the descending curve and repair costs as the ascending straight line. The sum of these two items, in this simplified example, represents the total average annual cost of the vehicle, which is illustrated in Fig. 2.

It can be seen that the minimum cost occurs when the vehicle is replaced after three years, but the flatness of the curve in this region also indicates that, for this particular example, there would be little difference in cost if the vehicle were kept for any period from two to four years.

The graphical representation enables the situation to be more readily apparent, but financial experts, using Discounted Cash Flow techniques have a more sophisticated approach which allows account to be made of other relevant factors including Corporation Tax relief, inflation, variations in discount rates and labour and material costs and percentage downtime. The effect of including Corporation Tax relief is to flatten the curve of total costs, thus extending the range of years over which it is conceivably economic to keep a vehicle, while inflation and increases in labour and material costs bring the minimum point closer to unity.

Compiling detailed and accurate records of maintenance expenditure for each vehicle in a large fleet is a costly and time-consuming exercise, and it may be considered sufficient to obtain the necessary information from a representative sample. The data thus provided is also useful in comparing the relative merits of different types of vehicle under the same operating conditions.

It will usually be found that the time for replacement occurs immediately prior to the need for a major item of repair such as an engine change. Thus, although a common book life may be chosen for all vehicles of one type in a fleet. it is a straightforward matter to cater for those vehicles which, because they do not follow the standard operating pattern, in terms of annual mileage or for other reasons, reach this stage at some time other than the average.

Phased replacement

Even when the optimum service life has been accurately determined it is still possible to affect overall costs by the manner in which fleet replacements are phased. If, for instance, the whole fleet were to be replaced at the same time, there would be a period when the demand for workshop facilities was at a minimum, whilst towards the later stages of the vehicles' lives, the demand would increase. Unless the facilities, including labour, were constantly adjusted to match the demand, this method would result in alternate periods of underand over-provision. The alternative is a balanced intake of new vehicles each year into each area covered by one workshop so that the work load remains reasonably constant over the years.

The management and organization of road haulage is becoming increasingly complex with each new item of legislation, and with the need to examine the costs of all sections of the business in order to remain competitive. Nowhere is this more apparent than in the engineering field. Vehicles are frequently more complicated and more difficult to repair than their predecessors and, let us admit it, have to be maintained constantly to a higher standard of mechanical fitness than ever before. Engineers, at any rate, feel that they now have a greater part to play in the successful running of road transport and in this paper we have tried to indicate, in a necessarily brief way, some of their problems and the manner in which they can be tackled in order to reduce costs and thus assist in providing an economically sound industry.

2. Vehicle selection and replacement

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