IS THE TEN-WHEELED VEHICLE PRACTICABLE?
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A Description of the Invention by a Notable Engineer for Distributing the Load Over Five Axles. Easy Riding Obtained, but Reliability in Doubt.
IN THE Patents page of our issue of June 29th, 1920, we described and illustrated an invention by Mr. L. E. de Mole for the distribution of the weight of &vehicle and its load over five axles instead of the normal two, the objects being the reduction of road wear and of shock to the vehicle whenever it was passing over bad roads or uneven ground. There are prospects of this invention being commercially developed in the near future, and for that reason we refer to the matter again.
The actual invention, for which letters patent have been granted, applies to vehicles having at least four pairs of road wheels (so that six-wheeled vehicles do not fall within its scope), and in which the wheels are associated with supporting springs, and it takes the form of a device by which the springs are subjected to a stress of the same order as that due to the normal load, either with or without a. damping device.
The Originator of War Tanks.
It is of interest to note that Mr. L. E. de Mole is the Australian engineer who, in 1912, sent to the War Office the plans, drawings, and details of an invention which, in the course of the inquiry into the origin of the Tank, was stated to have been the first suggestion for such an instrument to have been received. It was pigeon-holed and forgotten, and came to light only by accident. Mr. de Mole received no part of the awards made for the invention of the Tank, but he received a substantial sum for his elk penses, and was honoured with a Commandership of the Order of the British Empire.
The need for a better distribution of the load upon the road is acknowledged on all hands. It has been urged by road engineers and accepted by vehicle designers. There are two instances of the increase of the number of separated wheels per axle—apart from the practice, now fairly general, of mounting twin wheels in close juxtaposition on an axle shaft end. We have described the Erhardt tractor and the Detroit semi-trailer, which fall into this category.
The six-wheeled vehicle has appeared in a number of forms which need not be here enumerated. But this is the, first attempt to apply load distribution to four or five axles. Five axles seem to be favoured by Mr. de Mole, the first, second, third, and fourth steering, and the third, fourth, and fifth driving. The steering is on the Ackermann principle, the lock being greatest in the case of the wheels on the front axle, and being gradually less until the fourth axle is reached. The amount of lock on the wheels on the second axle woulch be such that differential troubles would be insistent if it were attempted to drive as well as to steer with them. Hence driving is limited to the wheels on the three rearmost axles. It is claimed that, with a 12 ft. wheelbase, the vehicle can turn in a 20 f t. road.
With ten wheels on five axles to carry the load and six wheels to-transmit the driving power, it is obvious that an axle weight of, say, seven tons would be distributed nearly equally over all wheels, or, say, 14 cwt. per wheel, consequently the weight of the wheels and of the transmission gear should reasonably be capable of being made lighter. With brakes acting upon a greater number of wheels, brake actuation should be easier and more effective. Again, the wear and tear of the roads, surfaces, and the crushing of road foundations should be lessened. The total weight of the ten wheels would, of course, be greater, but Mr. de Mole thinks that no more than 10 cwt. would represent the difference in the case of a seven ton vehicle. A further 10 cwt. should be allowed for the additional driving and steering gear. The existing 6 ton (axle . weight) type of vehicle would, therefore, weigh 7 tons, but, instead of the maximum load being 40 cwt. on each back wheel and 20 cwt. on each front wheel, as in the ease of a two-axle vehicle, the maximum load would, in the case of a five-axle 7 ton vehicle be the 14 cwt. mentioned.
In the running of a ten-wheeIed vehicle on the road, the surface defects to be expected are pot-holes and hollow places.. With a four-wheeled vehicle one of the
wheels passing over a pot-hole would inevitably drop into the hole, giving a shock to the vehicle, followed by a second shock as the wheel struck the edge of the hole and essayed to climb out of it. The following spring reactions would produce further wheel slip and vibration. With a ten-wheeled vehicle, the effect would be the same multiplied five times, as the wheels following in the same track successively dropped into the hole. But, with the ten-wheeled vehicle, with nine wheels available at the moment to carry the load, it is feasible to suspend the unsupported wheel in its passage across the pot-hole, and this is the basis of the invention.
The springs are first subjected to a stress equal to that due to the normal load ; in other words, they are tied back. This could be .done by means of a strong chain, but preferably should be effected by means of tie rods carried by the frame of the vehicle and compressing the springs, a dashpot being connected therewith to reduce shocks. If each spring is thus initially stressed equal to that due to its normal load, when
the load is placed upon it, it will obviously not be further stressed. Consequently, when passing across a pot-hole, the axle and wheel cannot drop, the tie rods holding the axle in its normal loaded attitude. Hence the pot-hole would be passed over by the successive wheels without any axle dropping and without any shock to the frame of the vehicle. When, instead of a pot-hole, a bump is met on the road, the wheel striking it communicates the blow to the spring and imposes extra stress upon it. The load on the wheel, however, in the case of a ten-wheeled vehicle, is less, and the wheel flows over the bump with a diminished shock to the frame.
The Obvious Difficulties.
There are, of course, mechanical difficulties to be overcome in the design of such a vehicle. The driving gear, steering gear and brake gear are more complicated, and difficulties of adjustment are not lessened with such a multiplicity of joints and connections.
We also imagine difficulties arising in the passage of a ten-wheeled vehicle over uneven roads or aough ground. With a badly worn road it is not impossible that, at times, four or even six wheels would be "floating"—not taking their share of the load, and there would suddenly be imposed upon the remaining few wheels the whole load of the vehicle, and it must be remembered that lightness of wheels is claimed as a merit of the scheme. Would they be able to stand up to such sudden strains ? Many a bridge approach is concave in longitudinal section, and at such a place the weight of the vehicle must, for the moment, be carried on the most forward and rearward axles. It is doubtful if they could be relied upon in such circumstances.
There is another point which cannot be ignored. In the case' of the goods vehicle, there is a considerable difference between the stresses imposed upon springs, axles and wheels when the vehiele is loaded and. when it has, as is inevitable in general practice, to run empty. The stressing of the springs under load slows down their periodicity to the designed rate, so that when the vehicle is unloaded the vibration set up in the chassis and body is much greater than when the vehicle is fully loa,ded. If the springs are tied down, as is suggested by de Mole, and thus subjected to a stress equal to a normal load, and the vehicle be run without that load, the initial periodicity must necessarily be considered increased and the vibration from what are virtually very stiff springs must be great indeed, as what might be called the initial soft part of the deflection has already been taken up.
The fact of the matter is that, once a departure is made from the conventional two-axle design, difficulties and complications arise.
We firmly believe that, in the development of the motor vehicle, more than four wheels will be employed. What is the ideal number of wheels or contact points—that is to say, the smallest number effectively to carry the load with the minimum amount of road wear—it is not at present possible to say, but our own feeling, at the moment, is that it would not be inadvisable to secure the fullest knowledge from the employment of six Wheels, taking eight or even ten wheels in the next and following stride.