Closer attention for double bottoms
Page 48
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I could not help feeling that the Reading study of double bottoms had missed some of the alternative propositions which might have found a place in their report, and to which the wider economic issues are relevant.
Within overall national economics, the total ton-mileage requirement is closely related to the gross domestic product. In recent times almost all of the increased requirement for tonmileage has been taken up by road transport. Had vehicle sizes, the road network, legislation, etc, remained static, then the increased ton-mileage would require an increased number of vehicles. In practice, none of these has been static, the major change being in vehicle size. The migration to larger vehicles has allowed an increased ton-mileage to be accomplished by an actual decline in the total vehicle park (from 591,000 in 1968 to 566,000 in 1973).
So any increase in vehicle size, either weight or cube will improve the efficiency of road transport and help to prevent an increase in the "vehicle .park", once the possibilities within current legislation have been exhausted.
What the Reading Study does not do is explain why only one of the ways of increasing size was studied in depth, when there are a number of possibilities which deserve closer attention.
The "short-double" with 2><27ft load space and overall length of 65ft operating at 24 tons for the artic and either 16 or 24 tons on the dollytrailer (40 or 48 tons gcw) compares with a three-axle rigid and drawbar under demountables offering the
same cube and weight within a slightly shorter length. These weights stay within the spirit of UK axle-load restrictions. The additional weight of the demountable gear on the rigid-drawbar brings it into direct comparison with the heavier artic and dolly rig. The steering of the two outfits is similar with the rigid drawbar having less cut-in on bends and better manoeuvrability. The splitting down time for the artic rig is directly comparable with the swopping time for demountable bodies from trailer to rigid (for local delivery) and the demountable has the advantage of relating closely to current thinking on urban distribution.
The outstanding difference is the stability of the rigid drawbar compared to the artic/dolly, and the much better loading on the drive axles (18 tons). I shall be surprised if the DoE can get the Volvo shown in your picture to behave itself without the addition of anti-lock brakes.
Within present legislation the 16+ 16-ton outfit using 24ft demountables already offers 20 per cent improvement in cube over the maximum size artic, and every foot of improvement beyond this must be a foot on overall length regardless of the type of rig. MAT Transport are operating with great success an outfit which demonstrates the principles involved.
It appears therefore that the "strong economic case for short doubles" claimed by Reading does not exist, because they have simply failed to compare "apples with apples". What they have done is to underline the improved economics of larger vehicles. Perhaps I am being too hard on the authors, as most of the work was done before the legislative relaxations for drawbars, though that itself does not excuse the single alternative approach.
There is a case which argues that short doubles could use existing tractive units for the urban delivery and would therefore be a cheap shortterm route to the improved ton-mile economics. This falls down when one considers that light artics (up to 24 tons) constitute only 41/2 per cent
of the vehicle park compared with 8 per cent to three-axle rig ids and 14 per cent to the heavy two-axle machines. I think this indicates where we should look if we wish to utilize existing equipment. More important, rigids fit better into the long-term legislation for urban use, minor road use, daily mileage restriction, European practice, etc.
It appears that the short double has been overtaken by events. Better by far that the DoE issue special dispensation for overlength drawbar outfits.
J. ANDERSON, Brentwood, Essex.