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OPINIONS and QUERIES
Page 133
Page 135
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OPERATING COSTS OF MOTORBUS, • TROLLEYBUS AND TRAMCAR.
The Editor, TEE COMMERLAL MOTOR.
[4268.1 Sir,—Your contributor Mr. H. Scott Hall, in his article "Operating Costs of Motorbus, Trolleybus and Tramcar," in your special passenger-vehicle issue, states that he has not seen any method of comparing these factors. I may say that I have dealt with this subject at some length in the columns of several journals, and in my text book, "Street Traffic Flow" (Chapman and Hall).
His method of approach to this subject is quite
but one or two points invite comment. His interest rate of 2A• per cent, is too low. I suggest 4 per cent. He calculates depreciation without allowing for the interest that can be earned by such allocations. The cumulative sinking fund much reduces the annual amount with longlife assets, e.g., tramways. His tramway depreciation is thus too • high. His life of 10 years for trams and 15 years for track is much too short. when many cars 30 years old are in use, as well as tracks nearly 30 years old.
am aware of the obsolescence argument, but it cuts no ice with tramway managers. His estimates do not allow for a varying life of track and varying repair costs, according to traffic density. These greatly complicate comparisons.. A life of 15 years for cars and track at a three-minute interval is reasonable.
Your contributor plots his cost on a basis of the number of vehicles in service on a 10-mile route. It is much more convenient to use as a basis average-minutes interval. Straight lines then result and theit significance is more apparent.
Tramcar costs must fall below trolleybus costs at some • frequency, though his curves hardly suggest this. In my calculations they do so for intervals of three to four minutes or less, with new construction and no consideration of road costs.
Except for the above, his values seem reasonable to me, and we must all agree that your contributor's method of approach, i.e,; by laying down specific "conditions of service, building up an estimate and by plotting a graph, is the only intelligent way of discussing the subject.
Wallington. . HENRY WATSON', [I am interested in your criticism of my article on opera t ing costs of motorbus, trolley-bus and tramcar. In the first place I should state that the title was a unsnarl-ter, and the words "operating costs" are not strictly accurate in their application to the subject matter, which was meant to be comparative and not a statement of absolute costs. It is my misfortune that I have nor seen your articles, and I was unaware that the subject had previously been treated in the manner extinplified in my article. I have, however, seen your text book "Street Traffic Flow." I should like to take this opportunity of complimenting you upon it, I was simply amazed at the mass of detail and the evidence it affords of prolonged, careful and accurate study of a most difficult subject. You criticize the rate of interest, and, at the same time, point out that I calculate depreciation without allowing for interest earned by allocations for.. depreciation. In actual fact, for the sake of simplicity. I combined the two, and the 2i per cent, is a rough average to allow for the effect of the one upon the other. As regards depreciation. I am well aware that some tramways and tramway tracks are kept in use much
• longer than the periods used as a basis for my calculations. This, however, also applies to the trolleybus and to the bus, and, if it be kept in mind that my figures are
comparative, your criticism there cancels out. In an article published on February 23 I have drawn a diagram -of the kind you suggest, in which straight-line graphs represent the relative values of the different methods of traction. I must conclude by thanking you for the interest you have taken and express my appreciation of your kind criticism—H. SCOTT HALL, M.I.A.F..1
OIL FILTERS IN COMPRESSION IGNITION ENGINES.
The Editor, THE COMMERCIAL MOTOR.
[42691 Sir,—May we suggest, regarding your recent editorial on this matter, that the over-riding cause of excessive filter choking is the use of oil of inferior type?
Some time ago, we carried out a microscopical examination of used lubricant received from one of our customers operating over 40 A.E.C. oilers and were astonished to find it absolutely free of mechanical impurities (carbon, metal, silica, etc.)—a fact which proves the inestimable value of the filter device fitted to the vehicles. "
This operator fixes the routine cleaning of filters at 8,000 miles and has no trouble,whatsoever.
To rectify choking troubles, therefore, we suggest the removal of the cause, viz., oil which is not stable enough, rather tt■gn" eliminate or modify this very necessary
fitting: T. R. PARuv, Director
London, E.14. (For T. R. Parry and Co., Ltd.) The Editor, THE COMMERCIAL MOTOR.
[42701 Sir,—I strongly disagree with your editorial in xegard to the non-elimination of the felt type cif 'filter element in the lubricating system of C.I. engines. From 3a 11 my own experience of this type of filter in both petrol and oil engines, I find it necessary to clean them at least every 1,000 miles, and even then most of the oil is going via the bypass owing to the filter being fairly badly choked in this relatively short time.
I come in contact with quite a large number of operators, both large and small, and it appears that at least 50 per cent, do not appreciate that this type of filter needs regular cleaning. Those who do make some attempt to carry out this operation at regular intervals usually run at least 5,000 miles between cleaning, for 75 per cent, of which distance the filter is probably hopelessly choked.
I consider that the most suitable arrangements to keep the lubricating oil in good condition are, first, a generous sump capacity, I suggest a gallon for every 20 b.h.p. ; secondly, a sludge trap which can be cleaned without draining away oil ; thirdly, a gauze box surrounding the suction pipe.
Incidentally, I would advise operators who are using lorries with o.h. camshaft engines to use an oil with a good viscosity at high temperatures, as this type of unit
tends to increase the temperature of the oil considerably more than is usual.
We have just taken delivery of two lorries built by one of the largest makers, which are not even fitted with a drain plug for the gearbox. This manufacturer's instruction book states that the oil should occasionally be drained from the gearbox, but it omits to say how. I have written the maker on the subject, but so far have not had a reply.
As an operator I come across dozens of oversights in design, and I think it may help if you were to throw open your columns to users so that they could criticize any bad design in the machines they are using. I think the manufacturers would take more notice of these than they do of private letters. J. L. G. BREWSTER, Newcastle-upon-Tyne, 6. For Orrell and Brewster.
[With regard to the suggestion made by Mr. Brewster in the last paragraph of his letter, we are quite prepared to open our columns to unbiassed criticisms of commercial-vehicle design, provided that these deal with the latest models. It would obviously be unfair to impeach makers in respect of model faults which have since been rec lified.—Eo.1