Front or Rear Driving ?
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Home Developments of the Front-wheel Drive are Imminent.
Immunity from, or practical elimination of, side-slip is being advanced by the advocates of combined front-driving and front-steering motor vehicles as merits peculiar to their systems, and these claims demand closer examination. The need for such enquiry arises, if for no other reasons, from the success of the avanl-frain Lath in securing the gold medal, at the last French commercial trials (vide this journal, Vol. III, Nos. 66, 67, and 68, June 4th, 21St, and 28th, 1906) against well-established constructors of rear drivers, and also from the success of the " Pullcar " motorcab, as manufactured in this country (vide "TIE MOTOR," Vol. III, No. 72, page 4381. It is interesting to consider the features incorporated in each system, And to endeavour to ascertain if there are any underlying principles in fore-driving to warrant its more extended introduction_ Let us first see what are the theories involved in the front driver which should, if practically applied, conduce to that immunity from side-slip which is claimed by its supporters, since, if these can be substantiated in practice, this bete noir and ever-existing danger of motoring, especially in winter time, will be removed. Several of the factors which may set up side-slip in the rear driver, or be contributory thereto, are greatly minimised in the front driver, with the result that the tendency to side movement should be reduced to quite a negligible quantity. It is permissible to neglect, in the present consideration, the centrifugal force created in the rear of a vehicle when turning corners at a high velocity.
If we consider the rear driver first, and examine the causes wholly or partially accountable for side movement, the principal one must be found in the fact that the power of the engine is constantly being applied to the rear wheels in a straight line; and these wheels, perforce, cannot accommodate themselves to the varying deflections, or lines of direction, of a vehicle on common roads, for they are constantly deviating from a straight line, owing to the exigencies of their running on unconfined tracks_ Thus, when the steering wheels are deflected to take the vehicle from its straight path, there are at once
SF.T UP TWO ANTAGONISTIC FORCES, each acting to get the better of the other : the tangential setting of the front wheels operates against the straight line of the drive from behind. The former is, usually, the master, for, through the medium of the differential gear, the rear driving wheels will generally track to the front ones. There must, nevertheless, always exist a tendency for the driven rear wheels to resume their straight line of drive; and they will endeavour to bring this about by aligning themselves to the deflection given to the steering wheels. The action of setting the steerers at an angle to the straight, is, to arrest the front portion of the car, and, the momentum of a moving body being greater behind, a tangential force is bound to be created, having, approximately, the centre of the front axle as a pivot. The adhesion of the back tyres decides the mastery of the one force over the other. This adhesion will, on a dry road, prevent the rear drivers from getting back to their straight line of driving; but, if the conditions for adhesion are changed to a greasy consistency, or a pronounced camber, then the rear wheels will assert their " right by design "to a straight line of movement, the rear part of the car will slew round for this to take place, and the result is a side-slip. How far the action of friction in the differential gear may be auxiliary to this effect is a debatable question ; but it is, doubtless, a point to be weighed as one in esse.
If we now consider a combined front driver and front steerer, carried into effect by means of universal joints or other compensating method, the power of the motor is always being
APPLIED IN THE EXACT DIRECTION OF MOTION, because the centre of the steering pivots and the universal or other flexible joints must be co-axial, and, therefore, must synchronise one with the other_ The equilibrium of the driving direction is not, thus, abruptly set at naught, as in a rear driver, but is thoroughly harmonic at any angle of deflection, and, thus, the principal element conducive to side movement when the tires cannot adhere is, for all practical elects, overcome. So long as power is being transmitted to the front driving axle, the back wheels, which are mounted upon a fixed axle and arc free from whatever friction is to be found in any differential gear, are bound to follow the direction of motion imparted to the fore wheels. Should the rear portion be disproportionately heavier than the front, centrifugal force, on quick turning, or at great speed, would be intensified and could cause a slew, whilst it is difficult to see how any design would successfully overcome a high centrifugal force. Some American cars, wherein the engine and major part of the weight are centrally disposed, are said to be less subject to side-slipping, than the accepted European design, with the engine in front. This may or may not be attributable to the more correct design of the former, but the writer is of opinion that it is more largely brought about by the facts that the engine is placed transversely to the frame, and that the centrifugal force created by the revolving fly-wheel and other moving parts is practically in line with the direction of motion; whereas, with a longitudinal disposition of the engine, such force is at rightangles to the frame and accessory to the straight-line driving of the rear wheels in their endeavour to re-align themselves to the varying angles assumed by the steering wheels. In rear drivers, the back wheels must, necessarily, be small in diameter, but, in a front driver, the rear wheels are merely trailers and may be of any desired height. We have become familiarised to seeing practically even-sized wheels on motor vehicles, and rear wheels of a greatly increased diameter may appear to us, now, to be somewhat incongruous : but the fact remains that they are, mechanically, more correct, inasmuch as they afford a better rolling action and absorption of the vibration set up by road impact, so that solid tires may be employed, in place of pneumatics, to give equal or better results, in point of comfort, to the passengers, whilst, to the same end, much finer and longer springs may be introduced_
There is no reason to surmise that the universal joints used on front drivers, if properly designed, With sufficient wearing surfaces and due provision made for lubrication, should not prove perfectly satisfactory and last as long as any other part of the car. It is evident that the compensating movement only takes place at times when the car is changing track, and that the drive is, usually, being transmitted over, or through, two such joints, which can be well supported in bearings. In a cardan-shaft drive, the conditions are not so good, as the whole drive is being transmitted THROUGH THE STRENGTH OF ONE JOINT, whilst the central portion of the shaft, connecting the two universal joints, is without a bearing, being supported only by and between the joints.
There would appear to be another advantage gained by the front driver, in its ability to turn in a lesser circle; and, for town carriages or omnibuses this is especially advantageous, in order to obviate the necessity for using the reverse to effect a turning in an ordinary street : the " Pullcar " cab, with a wheel base of 7 feet, makes a complete turn in a circle of 24i feet in diameter. Care will, of course, have to be exercised to ensure that sufficient tractive weight is carried upon the fore wheels to haul the loads up hills without their slipping round; but there is no difficulty in design to secure this. From a manufacturing cost paint of view, all things considered, the advantage would probably rest with the fore driver, and, from the foregoing few notes, it will be apparent that the front driver presents several commendable features. It calls for more serious study and attention than has been given to it, for commercial work, perhaps, more than for private uses; at any rate, the firms which are already in the market, and whose managements may be expected to realise its best possibilities, are making the commercial applications their leading line.