THE SIX-WHEELER AND ITS DEVELOPMENTS.
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The Use of the Flexible Type as a Passenger-carrying Vehicle. The Question of a Second Trailer. How the Law Can be Amended.
THE Tad "flexible six-wheeler" is here applied to the type of vehicle that is formed by bringing together a four-wheeled tractor and a two-wheeled trailer, the two portions being connected by a turn-table and gimbals.
Until recently the flexible six-wheeler possessed no distinct legal status. Its use at speeds in excess of five miles an hour was generally tolerated, but was nut legal, sinceeit was subject to the provision of the Heavy Motor Car Order limiting the speed of any vehicle drawing a trailer to five miles an hour.
The legal position has been. changed by the Heavy Motor Car (Amendment) Order 1922. This amendment order piovides :— (a) That if a heavy motorcar draws a trailer which is so constructed and by partial super-imposition attached to the heavy motorcar that at all times the weight upon the rear axle of the heavy motorcar shall exceed the,weight upon the axle of the trailer and which trailer has not more than two wheels in contact with the ground, such wheels being fitted with tyres. of soft or elastic material the legal speed limit shall be twelVe miles an hour.
(b) The axle weight of a trailer axle shall not exceed 6i tons end the sum of the axle 'weights of all the axles of a trailer and of the heavy motorcar drawing such trailer shall not exceed twenty-two tons.
(c) If the trailer is of the type defined in (a) and in the introductory paragraph, the total length of the heavy motorcarand trailer shall not exceed thirtythree feet when measured between the extreme projecting points.
What the Law Means.
It will thus be seen that the flexible six-wheeler may now travel at twelve miles an hour instead of being limited to five miles an hour ; that a substantial increase in legal 'axle weights has been granted and that a total length of thirty-three feet is permissible. At the same time, certain restrictions laid down by the Heavy Motor Car Order have not been removed. In particular, it must be observed that the rear portion of the flexible six-wheeler is legally regarded as a special type of two-wheeled trailer. If a power vehicle draws more than one trailer' it becomes in heavy a road locomotive and not a eavy motorcar. Consequently, if we adhere to the letter of the law, a flexible six-wheeler may not at present draw a trailer of ordinary type in addition to its own two-wheeled rear portion. Moreover, the law provides . that a heavy motorcar used for the conveyance of passengers far payment shall not'draw a trailer. Consequently, a =flexible -six-wheeler is at present illegal as an omnibus or motor coach.
it iti, of course, generally known that new regulations have been framed by the Ministry of Transport, -but these. cannot come into, force until an Act has been passed by Parliament, and there is as yet. no definite assurance as to when the attention of tile... House of Commons will be given to this matter. It is suggested that the following points should be borne in mind and pressed by those who -Wish to see mechanical transport developed without unnecessary restrictions :— (a) Subject to reasonable regulations as to suitable axle weights, brakes, etc., flexible six-wheelers should be allowed to travel at speeds up to sixteen miles an hour. c12 (b) Again, subject to reasonable regulations, the use of bus or coach bodies on flexible six-wheelers should be legalized.
(a) It should be legal for an ordinary two or fourwheeled trailer carrying additional load to be drawn behind a flexible six-wheeler.
(d) It seems reasonable to suggest that special privileges, not necessarily accorded to all flexible sixwheelers, should be accorded to those the movement of which in reverse is positively and exactly controlled. It might, for instance, .he found necessary to make such exact control a eondition if the carriage of passengers is contemplated.
What is Claimed for the Six-wheeler.
In dealing with this section of our subject it will be advisable to consider the flexible six-whacler in comparison with other types of vehicle—namely, (a)the rigid six-wheeler ; (b) the ordinary fourwheeled lorry; (e) the ordinary type of tractor and trailer.
(a) Up to the present a really practical type of rigid six-wheeled vehicle has not been evolved in this country, although the Goodyear .designs have proved successful in the United States. The problem of its design is very complex, and serious difficulties are introduced in respect of springing and steering when solid tyres are used. Theoretically, it is on an equality with the flexible six-wheeler from the standpoint of distribution of load over three axles. In practice its rigidity makes it, in the opinion of somee undesirable for. great length to be permitted, amItherefore handicaps it as a load carrier. The flexible six-wheeler has been proved to be practical, is sinp pier and cheaper to construct, and is less objectionable in traffic, assuming the overall length in bot4 cases to be considerable. (b) The outstanding advantages of the flexible sixwheeler over the ordinary four-wheeled lorry is that it enables larger loads to be carried without the need for exceeding a givenlimit of axle weight: There is no need to labour the advantages_ of employing the largest possible -unit for many classes of haulage. We know, for example, that the total operating cqsts of a 5-ton lorry are nothing approaching donble those of a 4-ton lorry. It follows that, if full loads can be found, the costs per ton-mile are reduced when the larger unit is employed.
The same argument applies in the case under Consideration. The saving results largely from the fact that the cost of labour does not go up in proportion to the capacity of the unit. Moreover, as a rule, the costs under the heading of fuel and other items are not doubled when the carrying capacity is. doubled.
More Engine Power Required.
It is often argued that the six-wheeler has a greaL advantage over the four-wheeler, inasmuch as the paying load is pulled instead of being pushed, and less power is, therefore, required for the operation. In any attempt to analyse the case fairly, it must be admitted that it is qUeStionable-whether this argu. inent can be substantiated. It is, of course, easier . to pull than to push, let us say, a wheel-barrow, but in that case the pull is exerted at an angle away from the ground while the push is at an angle towards the ground. tending to force the wheel down in soft soil. In time ease of a motor vehicle, the power is transmitted exactly horizontally from the tyre to the
road, and it is difficult to see why appreciably less power should be needed if it is applied to the front or centre wheels than if it is applied to the back wheels. There may be some advantage in this respect, but.the point is not one on which a strong claim should be based.
, In the case of an average four-wheeler' with a 14-ft. wheelbase and a total length of 21 ft., only about 14 ft. at the outside is available for body length. In the case of a flexible six-wheeler 30 ft. in length, approximately 21 ft. is available for body length. In other words, the proportion of useful length to total length is much greater in the case of the six-wheeler, and, by increasing the total length by less than 50 per cent., we increase the available body length by 75 per cent. The limit of length. of a six-wheeler is 33 ft., so that we can already double the space available for load, whilst only increasing the length of the vehicle by about 50 per cent. The advantage of the six-wheeler from the standpoint of load-carrying capacity, assuming adequate strength and power, is thus illustrated. So also is its advantage when the problem is the carriage of large quantities of bulky but comparatively light goods. The argument would apply again in favour of the six-wheeler for passenger-carrying purposes, provided that this use of the type were made legal.
Another advantage is that, if a vehicle is required from time to time for various and differing classes of work, the six-wheeler may be made absolutely suitable by the provision of a spare after-part, at an added cost amounting to no more than about 15 per cent. Suppose, for instance, the vehicle is hired out to a manufacturer of chairs or boxes, an afterpart can he provided with a body of the maximum legal dimensions; if, on another occasion, the load is to consist of compact but very heavy machinery, a second carrier with a totally different kind of body, sufficiently strong but comparatively small, can be employed. The work of changing the one rear portion for the other is a quick and simple matter.
The six-wheeler may, in fact, be used in the same way as ordinary tractors and trailers are sometimes employed. Given three rear carrying portions, the one may be in process of unloading at one terminal, the second may be on the road behind the tractor portion, and the third may be in process of loading at the other terminal, so that the work of loading and unloading can be carried on continuously and the mileage of the tractor portion, which costs most of themoney, is kept up to the maximum.
The Sphere of the Four-wheeler.
In favour of the four-wheeler certain points should be mentioned_ The six-wheeler is only to be recommended if loads are fairly considerable. It is not suggested, for example, that the system should be applied to the 1-ton or. 30-cwt. van. Its great merit being that it enables bigger loads to be carried than are otherwise legal, its appeal must necessarily be mainly to traders who are already carrying loads somewhere near the maximum legal limit for a heavy motorcar. Another point is that the four-wheeler is absolutely easy to steer in reverse in narrow alleys or in traffic. This is not the ease with all flexible six-wheelers. In some designsthere is an element of uncertainty as to the direction of move ment of -the trailing portion when in reverse gear. In practice it is found that an experienced driver can, to a large extent, overcome this apparent difficulty, hut the only complete solution is by the provision of eome mechanism which will exactly regulate the direction of motion of the trailing portion in reverse. Further reference will be made to this point.
(c) In comparing a flexible six-wheeler with au ordinary four-wheeled tractor coupled in the usual manner to a four-wheeled trailer, the former has many advantages, and, where it has no advantage, it is, to all intents and purposes, on an equality. Its
movement in reversesisemore easily controlled with exactitude. If properly designed, it is free from the risk of an accident ,being caused by the trailer overrunning the tractor when descending a hill. The main point is, however, that the useful load helps to seems adhesion of the ;driving wheels. The greater the load the more • firmly are the driving wheels held• to the around. There is, therefore, no necessity to pile on otherwise unnecessary weight when designing the tractor portion for the purpose of obtaining ,sufficient adhesion; to deal with the maximum load under bad conditions.
The six-wheeler is far mare compact than the ordinary tractor and trailer, and can handle larger loads without exceeding the legal limits of axle weight, either on the driving axle or on the trailer
a, Perhaps the one advantage of the ordinary tractor aial trailer combination is that under exceptionally bad conditions, if the tractor becomes stalled, the two portions. can be separated, and, by the use of a drum and wire rope, the whole of the engine power can be applied to the movement of either portion at a time.
How to Encourage the Type.
In order that the flexible six-wheeler may become popular as rapidly as it deserves, there are certain points which call for consideration.
In the first place, changes in the law must be care
i fully watched and n some respects must be pressed. It should be made legal for the six-wheeler to draw an ordinary four-wheeled trailer, and, in certain circumstances, it should also be legally allowed to be used for passenger-carrying, not, however, with a
i four-wheeled trailer behind t.
As regards power, it is a mistake to suppose that an engine only just adequate For a 4-ton to 5-ton fourwheeled vehicle will give complete satisfaction when fitted to a six-wheeler required to convey eight or nine tons. Such an engine can, of course, be made to deal with the load, but the average speed maintained will certainly be reduced, and, under exceptionally bad conditions, serious difficulties may be experienced. If a35 h.p. engine is suitable to propel a certain total weight, then we want a 70 h.p. engine to propel double that weight at an equal speed. It would be unfortunate if the development of the sixwheeler were hampered by a tendency of designers to cut down power and to over-accentuate the theory that, because the driving wheels are not at the back, a smaller power will be found quite adequate.
A feature in design that requires very great attention is the springing. The springs over the driving axle and those carrying the load sliould be as far apart as possible. (In this connection it will be remembered that the chain drive has been applied to certain makes of steam tractor solely with a view to rendering possible a more perfect form of springing.) i A. further point in connection with the springing s that it must be such that, when the trailing portion is running light, the springs are not so stiff as to tend to lift one or other of the trailing wheels off the road when rounding corners.
The brakes on the trailing portion must be ex tremely efficient, so that there can be no possible tendency towards overrunning down hills, and it seems desirable that, to ward against the unlikely accident of a breakage of the connections between the forward and after portions, the trailing axle brakes should apply themselves automatically when the connection is broken.
Reference has been made to the question of steering when in reversal. Various means have already been devised by which the movement of the trailer portion can be exactly regulated, but, to reach per in this respect, we must have something not only exact, but also cheap and simple.