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More can be squeezed out of drawbars

21st July 1984, Page 38
21st July 1984
Page 38
Page 39
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Page 38, 21st July 1984 — More can be squeezed out of drawbars
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Which of the following most accurately describes the problem?

As the law stands drawbar potential cannot be fully exploited John Dalzell looks at this aspect of operation

WITH the trend towards payload being limited by bulk rather than weight, drawbar outfits are capturing more and more attention. With a long drawbar a total body length of 15 metres is possible — a rather useful 25 per cent gain compared with an artic. Exploiting recent techniques in close-coupling, the gain can be 30 per cent, with up to 15.7 metres (5 11/2ft) total body length.

Just how much extra space can be gained, by any system, is exactly predictable on the drawing board. What is not so evident is the effect in operating conditions on stability, manoeuvrability and ease of reversing.

On an articulated outfit, the permissible spacing between the tractor and the front of the body is determined by the trailer's corner-swing clearance — the arc swept by the front corner of the body, as it pivots around the king pin. That applies to the front end of drawbar trailers too, but with a conventional drawbar outfit a second factor has to be taken into account — the corner swing of the tail of the drawing vehicle about the coupling. This has to be added to the trailer's front corner-swing.

If the pivot point were directly in line with the end of the towing vehicle's body, the required swing clearance would be half the body width. Moving the coupling under the tail by a distance equal to half the body width reduces the swing clearance drastically — to just over a fifth of the body width. The swing arc is similarly flattened on the trailer if the turntable be tucked further under the front end.

Keeping these arcs as flat as possible enables the gap between motive vehicle and trailer to be reduced. It means that the drawbar coupling should be positioned as far forward under the drawing vehicle as possible. The turntable should be set back under the trailer as far as is practical, taking account of weight distribution and stability.

As an example, if the drawbar coupling can be positioned 1.25m (49in) forward of the tail of the vehicle, and the turntable set back by a similar amount under the trailer, a gap of 1235mm (481/2in) between the bodies is sufficient to ensure that no foul can occur. This still leaves 200mm (8in) to spare for vertical angularity (enough to cope with a 1 in 12 dip in the ground). This is with maximum-width bodies and conventional A-frame trailer.

Achieving such geometry calls for a low-mounted A-frame and an underslung hitch on the chassis. Although this is becoming the most popular layout both in Britain and on the Continent, chassis manufacturers still persist in fitting their standard drawbar couplings in the rear cross-member. That is about the worst possible position for keeping a small gap between the units.

The disadvantage of a long Aframe is that it brings more cutin during tight turns. As it happens, in normal road conditions this is less noticeable than the theory might suggest. In most manoeuvres with a long outfit, the drawing vehicle is starting to unwind from a bend before the trailer has got itself screwed into the turn.

Any further reduction in the gap between the bodies can be achieved only by modifying the corner-swing arcs of one or other of the units. Continental designers have been concentrating on this aspect with varying degrees of ingenuity and complexity.

Two broad concepts have now emerged. In one, the effective length of the trailer drawbar is increased as the dolly turns relative to the rest of the trailer. This means that the trailer corner keeps its distance from the drawing vehicle in a turn, and the corner-swing arc is actually flattened into a virtual straight line. This means the gap between truck and trailer can be as little as 2ft (600mm) when the towing hitch is tucked far under the tail of the truck. An extending drawbar is quite legal, because official length limits are measured in a straight line.

The other approach is to modify the steering geometry of the dolly so that any lateral displacement of the A-frame, relative to the drawing vehicle, causes the trailer front axle to steer through an even greater angle. This means that the front of the trailer can track behind the drawing vehicle with a much-reduced cut-in of the trailer's front wheels. Although the turntable centre still moves in a true arc about the coupling, its reduced lateral displacement means that it does not close the gap so much between the units. The main snag with such steering systems is that they call for special equipment on the drawing vehicle — usually at least an extra coupling — as a means of modifying the steered path. They do not usually afford quite such a close gap, at the limit, as a telescopic drawbar.

A variation on the theme is to force the turntable to keep more or less in line behind the vehicle by using a reversed A-frame with its apex just ahead of the turntable centre. As the turntable subframe then swings on its short arc, the movement is used to induce extra steering on the axle.

I have visited Holland to have a closer look at two of the systems — one of each type. They were the extending drawbar Contar Trombone, and the Bulthuis, which uses a hydraulically-controlled Ackerman steered front axle on the trailer.

On the road, both systems behaved impeccably, and no amount of manoevring when travelling forwards produced any threat of contact between th?. bodies.

Reversing was a different matter, however. The extending drawbar behaved in exactly the same way as a conventional drawbar outfit.

When shunting, a steering drawbar, the usual technique did not work. The reversing process had to be re-learned. The Dutch operator said that his drivers were allowed a day to familiarise themselves with the outfits, in the yard, before venturing out, and needed at least a week before they became proficient at reversing.

Once they had mastered the technique no further difficulty was experienced. Nevertheless, contact between the units could still occur if the driver's concentration lapsed while reversing.

By contrast, the Contarequipped outfit with its extending drawbar, was relatively foolproof and introducing it into an existing fleet of drawbar outfits brings no risks of confusion. This was a big factor that swayed Ray Smith Demountables Ltd to market the Contar arrangement in the United Kingdom, I understand.

On the road, it was noticeable how little the Bulthuis steering trailer dolly cut in during cornering. The wheels hardly strayed at all from behind the drawing vehicle. Even in the tightest manoeuvres there was surprisingly little cut-in. So this might be a consideration in its favour.

The only close-coupled system of the steering type to be offered for sale in Britain is the Ackermann-Fruehauf GLZ, which Crane Fruehauf is selling just on its own trailers.

In this case, the trailer is pulled by a single bar, rather than an Aframe. The tail of the bar is pinned to the front of the dolly subframe, ahead of the turntable. The drawing bar is restrained from moving sideways relative to the vehicle by a chunky bracket like a small reversed A-frame. It is hinged at its wide-spaced roots on hangers, either side of the coupling, and with a guide slot at its apex for the drawing beam. The towbar therefore always points straight backward in line with the drawing vehicle; in effect it extends the motive vehicle's tail.

The dolly subframe is thus pulled, but not positively steered. It simply follows behind the drawbar, relying on castor action to do the steering by sideslew action. Hydraulic dampers between the drawbar and the dolly help in preventing oscillations.

The effect of all this is to make the dolly turn about a point just A popular Dutch way of getting maximum body volume is to have a centre ahead of its turntable. There is bogie drawbar, caravan style, and buy a sleeper-cab DAF with a top sleeper. therefore no corner swing arc

from the drawing vehicle to be taken into account. The movement of the trailer corner, however, is affected by two arcs — the corner swing of the trailer about the turntable plus the arc of the axle centre about the tail of the drawbar.

The net effect is that the units can be coupled closer together than with a conventional Aframe, but not as close as with some of the other systems. Crane Fruehauf says the gap can be 900mm (2ft 111/2in).

As the dolly moves laterally when it steers, there is hardly any cut-in of the front of the trailer when cornering.

There are two possible approaches to reversing a Crane Fruehauf GLZ-fitted outfit. The pin, at the tail of the drawbeam, can be lifted out and the chassis driven forward to extend the beam out of the dolly subframe until it reaches a stop. The pin is then reinserted and this locks the beam relative to the dolly.

The reversed bracket on the tail of the drawing vehicle is hinged down out of the way (the drawbar has to be uncoupled first to do this) and then the dolly behaves as a conventional long A-frame when the drawbar is reconnected. So, a driver accus

tomed to drawbars can reverse the outfit using his normal technique.

Alternatively the outfit can be reversed in its normal running state, but at the expense of modified driving technique.

Interest in drawbar outfits in Britain seems to be reviving, and the relaxation of the speed limits might further enhance their popularity. An increase in train weights in line with those of artics would do even more, and we can but hope that common sense will eventually prevail on that issue.

Even so for a lot of operations bulk is more important than weight, and it is in this field that the extra load-space offered by a drawbar outfit, compared with an artic, can pay off. The introduction of closer-coupling systems could make the exercise even more worthwhile. It seems probable that more will soon be heard in Britain of such devices, especially if overhanging containers are banned.

The fact is that the 18 metres overall length is now out of step with modern operational demands. For one thing, sleeper cabs have become commonplace. The need to accommodate sleeper cabs was the prime reasoning behind the addition of half a metre to the overall length limits of artics. The argument is just as valid for drawbar outfits. At present, when a sleeping compartment is needed on a drawbar outfit the extra room has usually to be found on top of the cab.

Apart from that, there is the frequent need to match body lengths to standard pallet sizes. A drawbar outfit with about a metre gap between units can be arranged to give a 30-pallet total floor area — 25 per cent more than in an artic. However, even Ray Smith Contar-style close coupling still cannot give enough extra space to give a 32-pallet area; it would leave only a metre for a cab. Half a metre more on overall length would (with co-operation from chassis manufactureres to get rid of ancilliary equipment behind the cab) just about make a 32-pallet area possible — affording an attractive 33 per cent boost in transport productivity.

Some operators with a volume problem have overcome the normal restrictions by putting the loads in containers or demountable bodies. Under the present law they can project beyond the basic vehicle length. Now there are proposals to outlaw such practice, ostensibly to reword the regulations in tune with case law established by the infamous Premier Transport case.

The Premier Transport issue concerned overhang, the limits on which have to be observed by containers and demountables anyhow under present law. The judge's reference to the definition of overall length was to point out, that an exemption from the length limit applied only where the "receptacle" increased the carrying capacity of the vehicle. In Premier's case it did not. That is the vital point.

It needs asking why the drafters of the present law made the length exemption for containers in the first place. The envisaged circumstances are unlikely to have changed.

If "receptacles which increase the carrying capacity of the vehicle" are to be brought within the lengths that apply to the vehicle themselves there will be frequent problems. The law says "receptacles". It does not say "containers" or "demountable bodies". Long crates could be illegal, not to mention the occasional carriage of 30ft containers on rigids and the need for the same length of demountable body on a drawbar trailer as on the truck towing it.

The truck-trailer length problem would virtually disappear if the present limit were raised by the same half-metre, as has been granted to articulated vehicles. In the absence of such an amendment the practical transport problems will persist. In any case, what is to be done about those vehicles already in use that go over-length when they carry containers and demountable bodies?

As for the proposal to exempt fittings for railway-vehicle attachment, that is a concession that should apply to any vehicle.

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