Problems of the
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HAULIER and CARRIER IEXPECT that quite a number of readers of the previous article has considerably discounted its value, because in it I considered only legal speeds, crediting a solid-tyred steam wagon with an average sliced of 14 m.p.h. and .ft wagon and trailer, also solidtyred, with no more than 7 m.p.h. Indeed, there has Probably been many a smile over my calculations.
I wrote that article, however, with fullest knowledge of the facts and of everyday conditions of trailer usage. I stipulated for legal limits because I believe that those legal limits are going to be strictly enforced. I am quite sure that the conditions I had in mind will shortly be those under which wagons and trailers are operated.
Calculations on "Current Practice."
For those hauliers who prefer to base their calculations on current practice rather than future limitations, the formula is still as useful, needing to be corrected only . for the alteration in average speed. If, for example, it is the user's expectation that the steam wagon alone will average 16 m.p.h. and that the addition of a .trailer will diminish that average speed to only 12 m.p.h., then the relative values of the wagon alone and the wagon and trailer can be set out as follow:,
The formula is : H (S + R) ± CM, in which S is the total standing time, It the running time, M the mileage, H the cost per hour and C the cost per Mile. It may be eliminated when the average speed is known, because it is equal to M divided by the average speed. In the previous calculations I assumed S to bean. hour Mr the steam wagon and 1ihour for the steamer and trailer. The other values will be apparent as the formula develops. . For the lorry alone the formula—I should have added that it gives the cost' of the journey, total mileage M, provided that journey be completed within a normal working day—becomes 42 (8 + —) + 6.8 M, -which equals 42 S + 9.43 M. As S is 1 it disappears and the cost Of a journey with tile steam wagon alone
is simply 42 + 9.43 M. . .
For the wagon and trailer the formula becomes
54 (S + —) 8.3 M, which is 54 S + 12.8 M. Substi12
tilting II hour for S this simplifies to Si + 12.8 NI,
Directly Comprehensive Data.
The cost per journey is, of course, not sufficient to enable a fair comparison to be made, because it does not take into account the greater load capacity of the trailer combination. To obtain directly comparative data I assume that each vehicle is loaded one way and on that basis I obtain the figures for cost per ton-mile by dividing the steam-wagon formula by three and that of the lorry and trailer by five. The cost per ton-mile then becomes 14 + 3.14 M for the -steam wagon and 16.2 + 2.56 M. for the wagon and trailer.
Now, in the previous calculation where the average speed of the wagon was assumed to -be twice that of the combination outfit, the two formulaa gave results which differed so slightly one from the other as to make it fair to deduce that the cost per ton-mile was the same In 'either case, leaving the solo steam wagon alone with the advantage that it could, in -working day, cover nearly double the mileage. That made it fair to 'say that the steam wagon should be employed for trips in excess of 25 miles, but that either would be useful for journeys within the radius mentioned.
That is not true of the present instance, as is shown in Fig. 1, which indicates the comparative cost per tonmile of the two types. It is clear that, except for extremely small mileages, the wagon and trailer is much the cheaper outfit to operate.
There is, of course, still the limitation of distance imposed by the lower speed of the trailer and the figures now are : 45-mile radius as the limit of the trailer combination and 65 miles for the wagon alone. In the diagram named this factor is indicated by stopping the lines short at the distances given. The pneumatic-tyred steam wagon presents a different case altogether. Incidentally, it may be said that the owners of pneumatic-tyred steam wagons whom I have met are loud in its praises, which is something to be borne well in mind by steam-wagon users who are considering the purchase of new vehicles. The speed limits are 20 m.p.h. and 16 m.p.h. respectively, so that, legally, the pneumatic-tyred vehicle can attain the speeds just considered in reference to the solid-tyred wagon and trailer, The figures there given are sufficiently near to the truth to be taken as applicable, especially if the tyre makers extend the practice of guaranteeing their products for 20,000 miles, when used on steam wagons, as is now being done by certain manufacturers.
If the legal limits of speed be exceeded in anything like the same proportion as is the case with solid-tyred vehicles, it is likely that averages of 20 m.p.h. and 16 m.p.h. will be regularly experienced with these vehicles. In that case, again, modifying our formula?, that for the steam wagon alone becomes 42 (S + + 6.8 M, which is 42 + 8.9 M for the cost per journey, that is to say, 14 + 3 M for the cost per ton-mile.
For the steamer and trailer the formula is 54 ( S + —) 16 + 8.3 M for the cost per journey, and 16.2 + 2.34 M for the cost per ton-mile. The cost per ton-mile for different lengths of journey, plotted in the same way as in the foregoing example, gives the diagram shown in Fig. 2. The advantage is still in favour of the wagon and trailer and, if anything, over a greater range of mileage.
• The limits of the length of journey now become 60 miles for the wagon and trailer and 80 miles for the lorry alone. (This in all cases is the length of the outward run and is half the distance exemplified in M2 in the formula.) Consideration of this subject is not complete without some reference to the effect of the proportion of standing time.
Standing Time Against the Wagon and Trailer.
It is quite apparent from the foregoing that increased standing time tells against the wagon and trailer combination and reduced standing time leans in its favour. It is the extra 50 per cent. of standing time debited against the wagon and trailer which makes the latter unit less profitable for short runs, that unfavourable effect diminishing as the length of the journey increases and the proportion of standing time to running time therefore decreases.
It will be useful now to take a case where the loading is effected overhead from a chute and unloading is by means of tipping. The standing time for a wagon alone may in favourable circumstances then be reduced to aquarter of an hour. It is only to be expected however, that the idle time for the wagon and trailer will be approximately double, that is, half an hour.
The formula for the steam-wagon performance then becomes 3.5 + 3 M for the cost per ton-mile, and 5.4 + 2.34 M for the wagon and trailer. The relative costs per ton-mile for various mileages under these conditions are shown in diagram 3.
Fig. 4 indicates a comparative cost per ton-mile of a solid-tyred steam wagon and trailer, and of one shod with pneumatic tyres. The comparison is of interest particularly in the way in which it shows bow the use of pneumatic tyres diminishes the cost per ton-mile, notwithstanding the fact that the actual cost per mile may be approximately the same. The 'reason for the difference is, of course, the greater speed of the pneumatic-tyred vehicle, which decreases the cost and allaws a 'greater radius of action.