THE PROBLEM OF ST NG MULTI-WHEELERS.
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The Outcome of a Long Series of Exper Designed to Carry Bulky Load Conducted with Multi-wheeled Vehicles Large Number of Passengers.
THE road has once again returned to the position which it originally occupied—that of being the greatest of our transport traffic arteries. From the times of the Romans, who were our first road builders, 'down to the commencement of the railway era, the road, assisted in the later portion of the period by the canal, was the chief means of communication. With the development of the motor vehicle the road rapidly regained the position it had lost to the railways, and in the meantime traffic has grown so enormously that, already, before the mechanical road transport industry has done much more than gain its majority, congestion is rapidly becoming a serious factor. If we cast our minds, back a period of, say, 10 years only, we can
compare the motor traffic of then and now, and from it visualize the conditions 10 or 20 years hence, whilst if we are lacking ha imagination we have only to look at America, which has taken to the motor vehicle far more extensively than any other country, to see what congestion is likely to be in a few short years.
Helpful towards the solution of this problem of growing congestion, or to its diminution, is the passibility of expanding the load of each vehicle (where possible), thereby cutting down the number of vehicles in certain categories by about one-half ; for it is obvious, if, in numbers alone, certain groups and classes of vehicles are reduced by 50 per cent., that the future condition of the roads must be materially altered for the good. Consistently, over the past few years, The Commercial Motor has strongly advocated the employment of c32 vehicles with more wheels than four, having in view the two main ends of conserving the roads and reducing congestion. To distribute the load of motor vehicles over many wheels is, of course, dependent upon the nature of the roads themselves. In the city and along the main arteries which have been constructed and in use for many years, the foundation to the roads will be deep and strong. The road crust, or surface carpeting, will be ample in strength and sufficiently resilient to enable such roads to carry heavy loads. Over such roads heavy loads can be hauled and a high weight factor per unit area of contact can be permitted. Over the secondary roads of the country and most of the suburban roads a lower load factor is calledfor because the foundations are not of such a character as to warrant the passage over them of heavy loads.
When we come to roads in the Colonies and undeveloped countries, the fact arises that heavy loads again may be transported, mainly because produce has to be sent over the roads during and at the end of the harvest season when weather conditions are good and the rainy seasons have not started. The roads, being mainly cleared tracks, are not prone to be broken up by an excessive load passing over them, as would be the case with made roads of light or medium construction. The conditions applying to all these types of road obviously call for as many wheels as can be employed to advantage on a vehicle. Six is the present workable number, whilst it may not be long before the eight-wheeled vehicle will be developed.
The coming of the practical pneumatic tyre of large air capacity at low internal pressures is helping in the same direction, and for light-weight transport it is proving the most economic means of equipping a wheel, for it permits of high speed and has a substantial load capacity. For very heavy traffic—that is to say, trucks carrying five tons—the solid tyre still seems to be indicated as the most suitable type of cushion between the vehicle and the road, but, unfortunately, the heavy vehicle equipped with solid tyres causes vibration to buildings and damage to country roads where, owing to the nature of the soil, for example, the foundation of the road is not so good as it might be. It will be remembered that the President of the Municipal Council of Engineers, in an address at Bristol last year, said: " The cost of road maintenance is reaching a colossal figure, and I sometimes wonder if it would become so prohibitive that the use of roads will have to be restricted and very heavy traffic once more diverted to the railways."
As it is eminently desirable that no undue check shall be put upon the development of road transport and as the heavy loads have to be moved, it behoves manufacturers of vehicles and users to study very care
fully every idea in load-carrying vehicles that may be put forward, if only in order to conserve the roads and to reduce the cost of road transport.
We contend that the multi-wheeled vehicle goes a long way towards solving this problem. The sixwheeled vehicles so far introduced have undoubtedly constituted a step in the right direction. They may be divided into two classes ; first we have what we describe as the articulated six-wheeler, based on the pivotal centre of a fifth wheel, disposed horizontally and forming a connection between the chassis and the load platform. Secondly, we have the rigid-frame six-wheeler, which consists of an extended chassis with three axles which' are kept parallel to one another.
In the case of the articulated six-wheeler the load is distributed over the two rear axles, and as this type of vehicle has a heavier loading capacity, in practice the wheel pressures are not materially reduced, nor is road upkeep, so that the advantage gained by the use of this vehicle is the ability to carry a heavier load.
With the rigid-frame six-wheeler its load capacity, so far as one can tell at present, is limited to the capacity of the pneumatic tyres upon which it is mounted— pneumatic tyres being virtually necessary because. when a curve is taken by the vehicle, the wheels on the two driving axles tend to get out of track to a degree
depending upon the distance apart of the two axles. When brought as close together as is practicable, say 42 ins, apart, the track followed by the Wheels on the two axles differs to the extent of about in.; this can be taken up quite well by pneumatic tyres, and it has yet to be shown that this is an unimportant point when solid tyres (which are unable to swerve sideways) are employed. Theoretically, solid tyres would not be suitable for use in such circumstances.
It is at this point that the Jonkhoff system of carrying heavy loads on multi-wheeled vehicles, or drawing • Fig. 10.—The effects on the relation of the pivotal centre of a bogie supporting the rear of a long body, of the various methods of controlling the bogie, compared also with the cutting-in of the rear of the usual semi-trailer shown at D. At A the front wheels of the bogie are turned, but the pivot of the bogie frame cuts in from the curve taken by the centre of the support over the tractor rear axle. At B, steering all four bogie wheels ,effects a slight improvement, and at C, steering only the rear wheels brings about correct tracking.
trains of two or more vehicles, comes up for consideration. Our attention was drawn to the system in 1921, and we dealt in that year with the vehicles employed in Java, where Mr. Jonkhoff had to transport, as cheaply as possible, a large number of native labourers from the town to the plantation and back again every day, using the vehicles as well for the transport of loads, which, because of their nature, were bulky. With awkward, narrow and winding roads to deal with, Mr. Jonkhoff was compelled to develop a type of vehicle the wheels of which followed as nearly as possible in the same track.
Fig. 1 shows an articulated six-wheeler being taken round a sharp curve, and it will be seen that the rear wheels of the trailer portion cut in to a considerable extent. Fig. 2 represents the first attempt of Mr. Jonkhoff to solve the problem. He mounted the rear of the platform on a bogie, the two turntables being connected by a drawbar which steered the front wheels of the bogie. This form of construction was not entirely satisfactory, because at high speed a certain amount of swaying was set up, and various experiments were made in order to overcome the difficulty. It became obvious that the steering action of the trailer must be controlled by the tractor, and yet the instability of the connection reacts on the steering device, this difficulty obviously growing with each additional trailer, so that it was not possible to run road trains of several trailers except at slow speed.
In the course of the experiments consideration was given to the type of rigid-frame six-wheeler which has steerable rear wheels. With this system the rear steering wheels are connected with the front steering wheels, both pairs being simultaneously operated but in opposite directions. An example of this is to be found in the six-wheeled bus now in use in Paris. The system is somewhat dangerous in congested traffic, because when the front of the vehicle is turned to the left the rear end swings out to the right; when a vehicle is turned away from the kerb of the road the rear wheels swing in towards it. Whilst the swinging of the vehicle itself cannot be avoided, a fouling of the kerb can be minimized by reducing the track of the two rear wheels. This type of vehicle is represented in Fig. 3.
The result of a long series of experiments has brought the inventor to an extremely simple system, which is illustrated in Fig. 11. We are illustrating it in its proper sequence, but anticipate its position in the sequence because we think that the steps by which it was reached can be understood better if the ultimate aim be kept in mind. It will be seen from Fig. 11 that the four rear wheels are mounted on an actual bogie (not the semi-bogie of the rigid-frame six-wheeler, which can only oscillate in a single plane) ; the bogie of the Jonkhoff system actually pivots, as well as oscillates about the spring centre. There is a rack fixed on the bog-le and a segment of a wheel rigidly mounted on the frame of the vehicle. When the steering wheels are turned so that the vehicle is made to describe a curve, the bogie swings out in tile opposite direction, and, owing to the influence of the rack and pinion, the two wheels on the rear axle, being mounted on stub axles and connected with the steering device, are turned outwardsto a slightly greater extent. The net result is that all six wheels follow one track and the centres of the axles follow the same curve.
It is interesting to see some of the methods that were tried before this end was reached, because they show that the subject was very thoroughly explored, and by these experiments the defects that were discovered in connection with each method are brought to light. Thus, in Fig. 4 it will he seen that an elongated coupling bar acted as the steering leverJfor the front wheels of the trailer. With this form of construction the transverse movement of the tail of the tractor portion served to steer the front wheels of the trailer, but the trailer wheels cut in too much when making curves.
The next experiment entailed dividing a coupling bar into two with a hinge at B, as shown in Fig. 5, two cables passing round the rollers (C) and being connected to the body at A on the central line of the body. The body, by the way, will be seen to extend over the rear portion of the tractor and over the whole of the trailer. The drawbar on the tractor could turn horizontally about the pivot (D) of the tractor turntable. The dotted lines and circles should be carefully studied, as they show the defect of the system, namely, that the distance between the two turntable centres is variable, so that the rear turntable had to be connected to the trailer chassis by means of some sliding device.
In the next experiment, shown in Fig. 6, the tractor portion of the divided drawbar was eliminated, so that The sliding connection of the trailer turntable could be avoided ; the guiding pulleys were fastened to the body
instead of to the tractor chassis and the cables were passed over the rollers (C) and connected to the drawbar at B. D is the pivotal centre, whilst A are the points on the chassis to which the cables are connected, these points being midway between the two axles. The farther forward the connecting points (A) were taken the greater the sideway movement of the hinged point B, and, with careful adjustment of the haulage point, cutting-in could be virtually corrected.
This led to the employment of a much longer body, the trailer turntable being taken much farther to the rear, as is shown in Fig. 7. Here the two cables were taken from the haulage point A on the tractor, over rollers fastened to the body, to the hinged point at B, the rear end of the body pivoting on the rear of the end bogie at the point d. But in order to bring the two pivot points D and d (1) on the tractor portion and d on the rear bogie) in the same circle, all four wheels of the rear •bogie had to be steered. This prevented the trailer rear wheels from cutting-in on curves, but increased the angle of the rear bogie to the body to an extent that was not favoured. It was at this point that an endeavour was first made to eliminate the cable connections, and a good deal of research was directed towards discovering the relation between the angles made by the tractor chassis and the front pivoted body when the vehicle was taking a curve.
Thereafter followed a series of experiments aiming at the construction of a central bogie-steering device which should work automatically by means of the difference in direction between the axis of the body and the axis of the truck chassis. One such experiment used a hydraulic form of transmission, into details of which there is no need for us to go.
In Fig. 8 a remarkable achievement is illustrated, a long length of timber being supported on a tractor and a trailer, the wheels of the latter being steered by the rack-and-pinion device already referred to, so that the wheels approximately correctly track on making the sharp curve shown. A bus is shown in Fig. 9 supported in exactly the same way and, owing to the absence of any connecting bars between the tractor and the rear bogie, a low central entrance to the bus can be provided. With a body length of 40 ft., this vehicle can turn at a street junction where the streets are only 16 ft. wide. A further series of experiments from this point showed that it is not desirable to cause all four wheels of the rear truck or bogie to steer. In the illustrations contained in Fig. 10 we show, four types of vehicle describing a curve with a radius of 30 ft. At A is shown a vehicle with the front wheels of the bogie steering, and if the curve described by the vehicle as a whole be compared in relation to the straight line passing through the two turntables, it will be seen that the lines do not exactly meet and the bogie is turned to an angle of 17 degrees, whilst there must be an attempt on the part of the bogie to slip sideways towards the inside of the curve. If all four wheels of the bogie are steered, as at B, the angle of the bogie to the body is reduced to 13 degrees, but now the bogie desires to slip towards the outside of the curve. If, however, only the rear wheels are steered, the angle of the bogie is reduced to 9 degrees and there is no tendency on the part of the bogie to slip either way. It will thus be seen that the system shown at C (Fig. 11) is the only device which keeps the bogie centre on the circle described by the centre of the front axle of the tractor and which ensures that there shall be no transverse movement in the bulk weight of the body. , The objection which could be raised to this system • is to be found in the fact that to give ample movement around the pivoting point, the wheels must be of small diameter so as to pass beneath the body.
In Fig. 11, as we have said before, we see the-developed system, claimed by Mr. Jonkhoff to be the only correct one for overcoming all steering troubles and for avoiding tailwag on the part of a long-bodied vehicle.
As a matter of fact, the steering device for the bogie rear wheels is a very simple mechanical construction. It consists of a vertical shaft (A) secured at one end to the body, the other end being fixed to the segment (B) in the balancing bogie frame, the body, being pivoted on the bogie turntable, can turn horizontally over the bogie, and by such transverse movements the axis and the body always correspond in direction with the axis and the segment B. This horizontal movement occurs as soon as the front end of the body enters on a curve. The teeth on segment B engage with the teeth on the rack C; this being part of the steering bar which is connected to the steering arms of the rear wheels, these wheels automatically take a correct angle, which keeps the centre of the bogie on the line followed by the centre of the front axle. Probably the only difficulty that this form of construction entails is the need for an independent steering device to be used when the vehicle is driven backwards. This consists of the lever shown near to the driving seat. The vertical shaft (A) has at its upper end the segment indicated by the latter E, the teeth of which engage with the pinion F, which is connected by a shaft to the steering lever. When the vehicle is being driven backward, the front wheels of the bogie become the front wheels of the vehicle, being controlled by the lever, whilst the wheels of what was the front axle are left to track automatically.
We think that Mr. jonkhoff has now brought his invention to such a stage that it should be considered by manufacturers of different types of vehicle and by users thereof, as it should materially help to solve many of the transport problems that still confront us. We should imagine, for instance, that the bogie system which embraces one steerable pair of wheels would be of interest to managers of tramway systems, where the wear and tear of the permanent way is largely increased by the use of two four-wheeled bogies on the trams, none of the wheels now being 'capable of deflection.