Doubts over downplating
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I/ Fresh attempts are being made by manufacturers of artic tractors and coupling suppliers to get the Department of Transport to revise its downplating requirements. At present the DTp bases maximum allowable gross combination weights on a little-publicised law which regulates the number and size of the bolts used to mount the fifth-wheel coupling baseframe to the chassis.
For example, a tractor with a design weight of 38 tonnes which the operator wants to have plated at only 32.5 tonnes must have a fifth-wheel installation held down by ten bolts of 16mm diameter — no more, no less. To qualify for a 28 tonne plate, there must be precisely eight 16mm holddown bolts. Between six and sixteen bolts are called for, of either 14 or 16mm diameter, to "qualify" for GCWs from 16.5 to 38 tonnes.
All parties involved concede privately that the relation of bolt strength to GCW is arbitrary, though the DTp claims the system is necessary for enforcement purposes, following the 1981 vehicle excise duty changes which made gross rather than unladen weight the basis for VED rates.
A meeting of tractor and coupling makers was held recently at the Society of Motor Manufacturers and Traders' London base to discuss practical problems which have arisen through downplating. Not least is the problem of how to make use of existing (pre drilled) holes in tractor chassis coupling-mount flanges when they are bigger in diameter and/or more numerous than those stipulated by the DTp.
One manufacturer, who does not wish to be named, said last week that it was hoping to arrange a meeting with the DTp in the next month or so to persuade the legislators of the engineering naivety of the system.
It will argue that the approved and VED-licensed gross weight shown on the DTp plate provides more than adequate evidence at roadside spot-checks.
Few vehicle inspectors, it is claimed, are fully-conversant with the coupling bolt code, and on many tractors some or all of the bolts are hidden by other chassis equipment.