TRAILERS, FOLLOWERS, AND ATTACHMENTS.
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Some Remarks on the Subject of Potentially Useful Traffic Units, -the Legal Status of which is at Present Doubtful.
SOME OF our readers may remember that, several years ago, experiments were made with the .... Renard road train in this coulhtry. It is to be presumed that fine success of these experiments was not sufficient to justify the big expenditure which would doubtless•bave been necessary—to secure any substantial development of the principle. At the time, it was,recognized that the legal status of the train was very doubtful, and this was due to the lack of any workable definition as to what constitutes a trailer.
We all know that the law, as it stands, provides that no heavy motorcar, whether its tyres are of metal or of rubber, may draw a trailer at a greater speed than five miles an hour. One of the consequences of this is, that all tractors used for haulage have metal tyres. If a rubber-tyred tractor were permitted to travel at a higher speed limit with a trailer, doubtless the use of rubber tyres in this connection would presently become general, because the extra cost of tyre maintenance would be more than balanced by the extra efficiency of the tractor and trailer as a traffic unit, and by its ability to do far more work in a given time.
Thus, we have not to go far to find one potentially useful type of machine which may fairly be included within the scope of this article. If the rubber-tyred tractor were encouraged by a reasonable speed being4 permitted to it and its trailer, it is at least open to argument whether the roads and the public would not . benefit thereby. Other things being eq_ual, the rigid metal tyre is certainly more harmful to the roads than is a resilient tyre.
There is, of course, the question of whether the added speed would introduce any material degree of danger owing to the ordinary type of trailer not being perfectly controlled by the power vehicle. This brings us fa the question of what are really the underlying reasons that the speed limit of a trailer combination is put so low, and what is the relative importance to be accorded to each of these reasons.
• One of these reasons is certainly that the length of al, power ve'biele and the trailer coupled to it is altogether considerable, and a long vehicle is, by its nature, more inclined to be obstructive and dangerous in tra-ffic than a short one, any danger of this kind being, no doubt, more or less increased as a result of an increase in speed.
There are, however, many other points which must have influenced the framing of the law. One is2 that a trailer merely connected to the power vehicle in the
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ordinary way by a central drawbar does not necessarily follow in the track of the power vehicle. When going down a hill on a greasy surface, if the brakes of the tractor have to be applied, there is some risk of the trailer pushing it forward and causing it to swerve round ID the road. This is particularly the
ease if, at the moment of application of the brakes, the tractor is rounding a curve, so that its wheels and those of the trailer are not in line. In this case, the pushing force of the trailer is oblique and the tractor may be forced broadside on and possibly over-' turned. This, of course, would not occur if, when the brakes are applied to the tractor, equally effective brakes were simultaneously applied to the trailer so that there would be no danger of over-running by the latter.
It does not follow, however, that the existence of strong trailer brakes will achieve the desired result. The trailer brakes may conceivably skid the wheels and so partially defeat their own end. Moreover, the load on the trailer may be such as to render it very difficult to prevent a tendency to over-run under these conditions. Another point against the ordinary form of trailer is, of course, that it is difficult to back accurately because its movements are not exactly controlled by the tractor driver.
Yet another and a very important point is that the load on the trailer does not assist the adhesion of the driving wheels upon which the traction of the entire combination is solely dependent. Thus, the weight upon the driving wheels of the tractor must be considerable if a substantial load is to be hauled in the trailer. Moreover, under bad conditions and on heavy gradients, there is some risk of loss of adhesion of the driving wheels, resulting in spinning round and consequent damage to the road.
If the weight of the load were instrumental in assisting adhesion the, would evidently be less objection to a trailer combination. Consequently, if this could be arranged, it would apparently be'quite reasonable to allow either higher speed limits or alternatively to permit more than one trailer to be drawn and a greater total load to be hauled. A motor tractor to which more than one trailer is • attached becomes immediately an illegal traffic unit. unless it is registered also as a road. locomotive and obeys the laws and local regulations governing that type of engine.
One would imagine that there are circumstances under which a tractor of reasonable limited weight might well be permitted to travel at upwards of five miles an hour with more than one trailer. It depends really on whether the trailer wheels can be used to assist adhesion and also whether the connections between the tractor and trailer can be made so that the movements of the latter are adequately controlled. These considerations bring us back to the question of the doubtful legal position of a road train of the Renard or other type.
For the moment, however, we are discussing the question of what constitutes a trailer and why, any trailer combination should be so limited in speed. So long as the word trailer is taken as a fair description of any type of following vehicle which carries a load and is connected to another preceding vehicle, no encouragement is offered to the design of trailers which , do not possess the objectionable features of the ordinary types. Fig. 1 shows a form of trailer which might very conceivably be employed. It is, in fact, nothing more than an extension of the idea contained in the Auto-trailer. In this case the following vehicle may be connected to the power vehicle at two points on the rear of the frame. Hinges permitting relative turning movement in a vertical plane may be introduced at H, so that when travelling over varied 'gradients the weight is still borne on all the wheels as indicated in Fig. 2. It will be observed that the wheels of the trailing vehicle are arranged
on the castor principle, so that they can swing round independently. The result is, in effect, a six-wheeled vehicle of considerable length hinged or articulated at H, so as to .provide a simple means of keeping the load properly distributed over the various wheels.
Now a combination of this kind might be regarded either as a six-wheeled vehicle or as a lorry drawing a trailer. In the latter case its speed limit is reduced to live miles an hour. In the former its speed limit is that of the power vehicle alone, provided that there is
no restriction in respect of the overall length. The objection to such a combination lies only in its length and in the fact that, so far as other traffic is concerned, it is in the nature of a vehicle, with a very long overhang behind the back axle. All the objections of an excessive overhang cannet be urged' against it. -The overhang does not result in any particular diffieulties as regards body design ;
neither does it tend to place an excessive load linen the driving axle. *hen rounding a sharp corner, however, the following vehicle sweeps round in exactly the same way as would the tail of a vehicle with a long overhang, and this may conceivably intro duce an element of clanger in dense traffic (Fig. 5a). Consequently, one imagines that it would be necessary to put some sort of limit upon the length of a trailer of this type or alternatively upon the length of the complete combination. Thus, the law as regards this particular form of trailer might be satisfactory if a trailer of this kind
were merely engaged as a continuation of the leading vehicle introducing no special obligation as to speed limit, provided the maximum, overall length were prescribed. For convenience in operatian two-wheeled trailer of this sort would carry legs at its forward end which could be lowered before detaching the trailer from the power vehicle.
Even if a maximum overall length were prescribed, the combination would have the advantage that a part at least of the loading and unloading could be going on inpon trailers while the power unit with another trailer was out upon the road doing useful work. At present,' as already pointed out, there is no incentive towards any developments at all along the lines indicated because of the doubt existing as to the legal status of the combination.
Another somewhat doubtful case is illustrated in Fig. 3. This drawing shows a newer vehicle or trac tor of short wheelbase, over the rear, axle of which is mounted a turntable upon which the forward end of a load-carrying vehicle can be suitably fixed. 535
The load-carrying vehicle has only two wheels placed near the .rear. When detached from the tractor it can be supported on legs let down at the forward end. 1ts attachment must be such as to introduce some form of universal joint •or trunnion, so that, when travelling on a varying gradient, the combination may adapt itself to circumstances as illustrated in Fig. 4.
It is evident that this type of trailer, if it must be so called, has much the same freedom of lateral movement as the ordinary type .connected through a drawbar,. though, being brought closer up and further forward, it is less liable to push the tractor round in the road when the brakes are applied on a down grade. As regards length, the whole combination would not be very much longer than a large fourwheeled lorry. In fact, it need not necessarily be any longer at all, unless to adapt itself to the requirements of -special types of load, such as timber, •iron bars or loag packing cases.
The weight carried would be distkibuted over three pairs of wheels, so that one would imagine that the total weight might. reasonably be higher than that permitted to an ordinary four-wheeled vehicle. This would be particularly the case if the tractor were fitted with a four-wheel drive, in which ease a very large proportion of the total weight .would help in securing adhesion. It would evidently be possible to arrange connections which would make the movements of a .trailer, of the type under discussion, capable of being exactly controlled from the driver's seat. Such connections would, in effect, convert the whole combination into a six-wheeled vehicle, drive jag on the centre axle and possibly also on the frout axle, and steering upon the front and back axles.
In order to take comparisons a little further, we may refer to Fig. 5 (a and b), in which the two types of trailer discussed are compared from the point of view of the obstruction of other traffic.
Fig. 5 (a) shows a vehicle of the type illustrated in Figs. 1 and 2 rounding a curve. It will be observed
that the tail of this vehicle tends to swing right across the road.
Fig. 5 (b) is a similar view of a tractor of the type shown in Figs. 3 and 4. Here it will be observed that there is not the same tendency to block up the road while turning, and this is an evident advantage of the
second method. • Putting the point in another way, this latter method does not share the disadvaata.ges of an ordinary fourwheeled vehicle with a very long overhang.
Both types approximate very nearly to a sixwheeled vehicle. In Flueh a vehicle some special' device must necessarily be introduced to enable all, the six wheels to bear properly on the road surface under all conditions. It, would not be reasonable for the law to consider that, if these devices happen also to be such that the tail of the vehicle can be removed from the forWardapart, the result is to bring into existence a tractor and trailer with.a ridiculously low legal speed limit. In fact, the speed limits of the combinations discussed shadd be those of ordinary four-wheeled vehicles with the same axle loads.
The only limit that should be considered at all should be overall length. The combination, shown in Fig. 3 might well be permitted a greater overall length than the ordinary four-wheeled vehicle, for the reason illustrated in Fig. 5 (b). On the other hand, if the overall length is very great, then it is reasonable that a lower speed limit should be im
posed. , Thus, the result of the whole analysis appears to . be that attachments of these kinds should not be classified legally as trailers but that a, provision ' might be made under which, if the overall length of any vehicle exceeds a certain maximum, the legal speed of that vehicle is automatically reduced. This would be a very different thing from imposing. a, definite limit of overall length, which would be, undesirable as altogether preventing motor vehicles from dealing with certain special loads, such as long. balks balks of timber.