Front Driving, with Special Reference to Electric and Hydraulic Transmission.*
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In climbing hills there are certain theoretical advantages in front driving of a very interesting character. The problem, as far as I know-, has been first worked out by Mr.
'I'. II. Brigg (the inventor of the Plywell), who showed me his diagrams recently; and Mr. Brigg has been good enough to write for this paper a note setting forth his views. He does not in that statement take into account the difference in distribution of weight owing to the change of positions, and deals only with the case where front and back• wheels are of different diameter, but, as will be seen from the note, he has worked out these effects. It is, however, in relation to the vexed and important question of sideslip that the theoretical differences between front and back driving and braking are most striking, not to say perplexing. .I think we are justified in thinking that side-slip cannot be understood, in view of the large number of futile and often extraordinary attempts in the way of inventions to grapple with this difficulty, most of which, apart from studded tires, are in the nature of special attachments. Now there are reasons for asserting, both as the result of theory and experiment, that side-slip with pneumatics may be almost entirely prevented by applying the brakes on the front wheels, even without the use of non-skid devices. It is easy to understand that so long as the vehicle is being propelled by the front wheels the vehicle as a whole will follow the tracks of the front wheels, and would not he inclined to skid—that is, assuming the back wheels to run easily; but it is not so easy to understand what happens when the brake is applied to the front wheels. It looks at First as if the free-running back wheels and most of the weight behind would tend to over-run the front wheels and slew round the vehicle, causing what is commonly known as a side-slip. On looking, however, into the matter a little more closely we see that the difference between the cases when indiarubber and pneumatic-tired wheels ure freerunning and with the brake applied should be taken into account. It only needs a simple experiment to-demonstrate the fact that, whereas a wheel of this kind when running free tends to move most easily in the direction of its plane of rotation, when the brake is applied the exact opposite is the case, and it is much easier for a wheel on a greasy surface to slip sideways than to move forward in its own plane. We therefore have this result, that if the brake is applied on the front wheels, any tendency to slip may be checked by the driver adroitly using his steering wheel, the hind wheels which are free running following in the direr
tion of the vehicle, without any tendency for the mass of the vehicle to swerve sideways. If, however, instead of the front wheels being braked, the brake is applied to the back wheels, it becomes an extremely difficult thing to prevent side-slip taking place, unless non-skid devices are used.
The Technical Committee of the Royal Automobile. Club some time ago conducted some experiments on a vehicle with brakes on the front wheels on a specially-prepared greasy surface, the vehicle. being run at a considerable • speed, arid the brakes suddenly applied in the middle of a-. greasy patch. It was extraordinary to see how the vehicle, under these circumstances, kept its course without any appreciable side-slip, whereas a similar car, under identical conditions, but with the brakes on the back wheels, twirled round immediately the brakes were applied in a most bloodcurdling manner, and, unless it turned completely round, it slipped broadside on until it struck the other side where the greasy patch ended, owing to the failure of the gripping action of the wheels in moving sideways. It ought to be mentioned that when front brakes were used they kept the wheels moving in the direction of their plane, more or less locking them with so much greater retarding action as to pull up the vehicle, often on the patch -itself.
1 have been speaking of light vehicles with normal distributiOn of load, but there are instances of attempts to bring into use heavy vehicles in which the load has been excessive on the front wheels, where side-slip has been bad, and, in fact, apparently worse than with a rear-drivenvehicle. Very little seems to be known as to the actual effect of varying the distribution of weight and its effect upon skidding, and the views on this matter are so divergent, and so little is known about it, that I thought it worth while conducting some experiments which might throw light upon this and other points. The Krieger Company kindly afforded me every assistance, and at their own expense provided both front and back driven electric vehicles, and prepared a greasy patch for the experiments, which were conducted on Saturday, January uth, at their garage in Gillingham Street.
Comparative Experiments.
I will only give a brief summary of the results, as it was obvious that neither the building nor the patch were large enough for a complete investigation: which it is proposed to carry on elsewhere at some future time. Nevertheless, the experiments were of very great interest and demonstrated conclusively the superiority of the front driven vehicle both in driving and steering upon a slippery surface, and also the difference in behaviour caused by varying the distribution of the moving 'mass upon the wheels. Runs were made at speeds varying from 2 to 15 miles per hour with both. vehicles, the brakes being suddenly applied when they were upon the patch. In every case the front-driven vehicle held on its course in a perfectly straight line, as tested with a cord afterwards stretched over the mark left by the wheels. The rear-driven vehicle went nearly straight on several occasions, but upon extending the patch, whilst no difference was observed with the front-driver, the rear-driver had some severe side-slips, which were, of course, checked when it reached the untreated concrete. When the attempt was made to divert suddenly the course of either vehicle at fairly high speeds on the patch they both (as was to be expected) refused to obey the steering wheels. At slow speeds on the patch the difference, however, was very marked, and while the rear-driver kept on its course, the front-driver was much more under control and could be diverted from it in any required direction.
In ordinary conditions the weights of the Krieger frontdriven cab is distributed thus
As a result of the trials under these conditions it was found that not only was the vehicle much less under control and evinced a slight tendency to side-slip, but tractive. power on the slippery patch was reduced in a very marked manner,
Electrical Front Driving.
Coming now to the mechanical details, it may be said at once that electrical and hydraulic methods of driving may be applied directly at the wheel in connection with front driving, when the front wheel is steered, in a way that does not seem feasible in any other kind of self-propelled vehicles. I have confined myself to a description of the Krieger electric car, and 1 am indebted for my descriptive information and diagrams to Mr. C. J. Potter, the secretary, and Mr. F. S. Coles, the engineer. Each front wheel is driven by a separate compound-wound motor (M M), which is supported by two bushed motor-arms {A A), the bearing being taken on the inner extension of the front wheel axle (B 13). The motor (M M) is thus kept always at the proper radius with respect to its gearing. The motor is suspended by a laminated plate spring (S S), linked on to the motor at one end, and rigidly clipped to a bracket, which is securely fixed to the middle part (B B) of the inner extension of the front axle. The motor-arm bearings work on either side of the spring clip (C C). This mode Of suspension absorbs the shock automatically when either starting or stopping.
The gearing of each motor to its front road wheel is arranged as follows : The raw hide or fibre pinion (P) is keyed on to the tapered end of the motor armature spindle, which extends outside of the motor case bearing, and meshes with the steel gear wheel, which is secured concentrically with and to the road wheel bearing hub.
The foregoing system gives a direct drive combined with high efficiency, the ratio of the gear being as g to t, the front road wheels being 34 inches in diameter.
The latest type (1907) of the Krieger front-drive vehicle includes many alterations and improvements in its construction. One of the chief of these alterations consists in discarding the spring-supported motors and substituting for them a motor directly fixed to one part of the front wheelhub, having the steering pivot inside the hub, vertically above the centre point of contact of the road-wheel to the ground. This system, a sectional view of which is shownin Fig. 2 and an elevation in Fig. 3, not only ensures very light steering, but prevents the tendency in the wheels to pull the car to one side or other when one of the wheels meets any obstruction. Further, the tractive effort of one of the motors when the other even momentarily is for any cause not operative does not, with this new arrangement, tend to throw the car out of its course as is the case when the pivoting takes place outside the vertical axis of the wheel.
To obviate shocks in the new gear, an ingeniouslyarranged Rheostat is introduced in the main circuit between the batteries and the motors, and is actuated by means of the foot-pedal (A). This Rheostat has a quick break action, only allowing the resistance to come on for a very short distance of its travel before cutting out entirely, and throws the arm carrying the carbon brush out of action, so that the pedal is able to return to its on-position without moving the resistance arm at all. The resistance arm remains stationary, with the circuit open, until the pedal is -4ressed back again to o further point than was necessary
for release, and it then re-engages with the contact arm, allowing that arm to come into action, and cutting out the resistance without shock and as slowly as necessary. The current increases gradually until full contact is again made with the copper brush at the bottom of the resistance frame. The outer pedal (B) is used only for actuating the mechanical internal-expanding band brakes on the back wheels, and not, as in the earlier type, for actuating a cut-out before the mechanical brakes came into action. The controller is of the same type and connection as in the earlier cars, giving the various connections between batteries and motors—with half or full voltage of batteries, as required.
With regard to heavy vehicles, I do not know of any attempts at the electrical driving of the fronf wheels alone, but there certainly have been numerous examples of construction of vehicles in America in which the electrical driving of all four wheels has been adopted. Examples of these are the Imperial Electric Four Motor Drive and the Michigan Four-Wheel Drive vehicles. The subject of fourwheel driving is outside the scope of any present paper, and therefore I will not enter into any description in connection with it.
Hydraulic Front Driving.
With regard to hydraulic transmission and front-driving, I do not think such a system is very practical for light and fast-moving vehicles. The case may be quite different, however, in the case of slowly-moving vehicles, especially where rubber tires can be used. Although I do not know whether any experiments have been made in the direction of such a vehicle, it is quite mechanically feasible to place within the wheels a strong and compactly-built hydraulic motor, which, if using oil as the working fluid, would be very little affected by shocks and vibrations. Experience with the Hall hydraulic gear has shown how little affected such a motor is by, and how well it continued to work in spite of, severe and continued concussions. The experience gained in this connection has shown that the durability of such a motor is practically unlimited, and there is every reason to believe that it would wear for an almost indefinite period under a working pressure of as much as a ton on the square inch. As for the transmission of liquid for such a motor, it is easy to see how the supply and exhaust passages can be made through the swivel pin of the steering wheel, a petrol motor driving an oil pump on the framework of the vehicle itself. I will not pursue the matter further into details, but have introduced the matter of hydraulic driving rather to show that it must not he overlooked as a practical possibility for commercial purposes, and that we are quite likely to hear of the introduction of a heavy vehicle of this type at some future time.
Conclusion.
I have limited my paper to conditions in which the actual motors are carried on the driving wheels, and, indeed, this might really have been taken as the distinguishing feature between my paper and that of Mr. Bailey's. Even with These limitations I have said enough to show that not only are there points of considerable advantage in driving and braking on front wheels, but also shown that it is mechanically feasible to do so.
I will conclude by expressing my own belief that we cannot consider we have arrived at a final stage of the problem of the steel-tired four-wheeled vehicle, at any rate for heavy traffic, until much more extended trials have been given to driving, steering and braking not merely upon the front but upon all four wheels.
I have already alluded to the four-drive electric vehicles of American design, and although the design itself, of all the vehicles I have seen, is evidently in a state of evolution, I cannot help thinking that the system itself has a great future in its application to heavy vehicles. Apart from prevention of side-slip, there are special advantages in the case of such vehicles running in great cities with paved streets, where not only is manoeuvring of great importance,
but where rubber tires are highly desirable, both to save the vehicle from shocks and vibrations which occur with steel wheels on paved surfaces at even very moderate speeds, and to diminish the noise which is found not merely objectionable by day, but almost unbearable by night. Further, the saving not only en tires, but also on the vehicles, which may have to be accelerated or retarded, possibly hundreds of times in a day, would be found to be so great as to far more than outweigh the additional cost of four-wheeled propulsion.
Back driving, in spite of one or two courageous attempts. to introduce front driving, is practically universal for heavy vehicles at the present time and if the papers to-night, and their subsequent discussion, lead to attention being turned to driving and steering on the front wheels and also upon all four wheels of a vehicle, I think the Institution will have reason to congratulate itself on the result.