The Front Driving of Steam and Petrol Vehicles.*
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The first steam vehicle that which was constructed in 1769 by Cugnot, was a three-wheeler, driving and steering on a single front wheel, while the first vehicle Hancock built was on the same lines exactly. Again, in 1862, we find Pattison in evidence with his combined front-driving and steering four-wheeler, whilst in our own clay, in 1895 and 7896, Messrs. Thornycroft made their first attempt at road locomotion with a vehicle that drove the front wheels, but steered by means of the rear wheels. Messrs. Thornycroft, after trying this vehicle, abandoned it in favour of the usual type of to-day (driving the rear and steering on the front wheels). In the earlier vehicles such as Cugnot's and Hancock's the. reason why front driving and steering was attempted was undoubtedly clue to the fact that, using, as they did, a single front wheel, this eliminated the then difficult problem of compensating a pair of driven wheels.
The more recent attempts have been made solely as the result of the one, and really only great, difficulty met with in the rear-driving, front steering type of vehicle. The difficulty referred to is the indisputable fact that, although universally adopted because of its innate adaptability to motor traction conditions generally, yet this type of vehicle is mechanically incorrect because, under certain road surface conditions due to the manner in which the force is applied relative to the whole vehicle, these vehicles are quite uncon
trollable. The force propelling the vehicle is, in a rear driver, parallel in its direction to the centre line of the car, whereas the front wheels are set at an angle to this direction varying to suit the exigencies of the moment.
Analysing the forces which are brought into action at the moment when an attempt to turn is made, it is plain that the car itself will only follow the curve as set out by the position of the rear wheels, because these lay out a curve of least resistance, resulting from the fact that the wheels are vastly easier to revolve on their axis than they are to slide bodily sideways. Although this is generally true, it is not always so; and as the resolved force along the direction of the turn becomes less, the greater the angle of deflection IKcornes; there comes a time when, if the roads are greasy, the resolved force is too small to act upon the vehicle and alter its direction, because the reaction of the road on the wheel caused by the propelling force is itself too small to prevent the wheel sliding bodily forwards in the direction of the propelling force. This, then, is obviously one serious defect in the rear-driving car.
Considering for a moment a long narrow rectangular block of metal, it is evident that if this is moved by haposing a single force acting right at the back, that such a block (if the surface over which it is being pushed varies in its nature) is quite uncontrollable as regards the direction it moves in. The surface it slides upon, reacting upon the imposed force frictionally, will have more to do with its direction than the applied force itself. In other werds, applying a force at a position behind the centre of gravity puts the block into a state of unstable equilibrium. Although this is an extreme case, it is still true that even in the case of a vehicle propelled from behind this same objection applies, from which it follows that in such a vehicle, if once any external forces set it moving laterally, the propelling force then acts in a manner which increases such deflection, because the whole system of forces acting and reacting on one another when such a vehicle is in motion is an unstable one. This second defect, then, in conjunction with the one already pointed out, makes the matter one which requires consideration, It is now necessary to consider the question of vehicles using their front wheels to obtain their tractive effort to see how they stand in relation to the afore-mentioned evils. Front-driving vehicles are broadly divisible under two heads, which will embrace all types, e.g. :—(1) Front driving, but steering by the rear wheels ; (2) combined front driving and steering. Considering, first, the front-driving, rear-steering vehicle, a moment's thought will make it clear that in this case the conditions resemble the block of racial, referred to before, pulled from the front instead of pushed from behind, and that in consequence, at least while pur suing a straight course, this force will tend to stability; but further reflection will show that in turning the pull will act across or against the tendency to alter the direction of motion, and that in consequence when required to turn this vehicle is in no better case than the rear-driving frontsteering type. Further, when traffic conditions are brought into the case another glaring defect becomes apparent, as the direction of motion can only be changed by the rear of the vehicle running outwards in the opposite direction to that in which the front is required to move, a state of affairs quite out of the question in crowded streets or narrow entries. This does not exhaust its bad points, however, because conditions are such that, in providing accommodation for the driver, almost all the load is thrown on the rear wheels, when, as these are steerers, a new difficulty is created by heavy steering.
Conning now to the second division of front drivers, that in which the front wheels both drive and steer, a very slight consideration of the conditions show that, whether running straight or making a turn, since the steering wheels are also the tractors or drivers, it is plain that in this type at least since the propelling forces are always in the proposed line of travel, that here is a design which is mechanically correct, as the propulsive force always tends to stability, being never pre-disposed to skidding.
Dr. Brightinore, late head of the engineering department of the Royal Indian College, is of opinion that it is wrong to attempt to build heavy vehicles on this principle while yet retaining the usual methods of steering. He proposed to develop a vehicle which was scientifically correct, which would be adaptable to all loads, and would also meet actual road conditions thoroughly.
In the first place, it must be capable of carrying six to eight tons on its own platform, and also, if required, able to trail another load similar to the usual type of vehicle. To carry out this latter condition it meant that the vehicle must carry at least two-thirds of its weight (eight tons when loaded) upon its front axle, and yet it must be as easily steered as an ordinary rear driver, which only carried something like a gross weight at the tire of from two to three tons. Further, it must be readily Inanceuvred (this means a short wheel base), must have a joint in its construction to allow for every possible movement on the road, be capable of giving a very large platform capacity for bulky goods, and also so designed that it would withstand high speeds. So far as the author's knowledge extends, the Brightmore vehicles, for general purposes, are the only ones that have seriously tackled the question. The first Brightmore vehicle was designed as a shooting phaeton, to carry five to six passengers, but it was only intended as an experiment. In this vehicle, as in the two later types, all the mechanism was carried on a fore carriage which was pivoted by a large ball-and-socket bearing, and supported at the back by a runner which travelled in a quadrant-made channel section, and bolted firmly to the body of the vehicle. The whole of the fore carriage revolved, and at the same time had a universal movement relative to the body, which was built with the rear wheels carried by laminated springs, so as to form practically a trailer of the vehicle body itself. On the fore carriage itself the engine and gear were arranged across the vehicle before the front axle at a distance to practically balance the'boiler, which was behind the axle. The final transmission to the axle was by Hans Renolds' silent chain ; the axle itself was of the plain live type. The experiment was in the steering. The road wheels (front) ran loose on the axles on roller bearings, and were only indirectly connected to it as follows Each road wheel carried a pawl, whilst this pawl geared into a ratchet wheel of special type, which ratchet wheels were keyed on the driven axle. By a special arrangement of a friction pulley revolving only with the axle, and with a cam to keep the pawl in gear or lift it out, it was possible to engage or disengage either pawl, and, therefore, its road wheel, with the ratchet wheel and axle. The friction pulleys in an ordinary course kept the pawls engaged, but by apply.. ing a hand-brake to either of these pulleys the corresponding pawl was forced up out of gear and (so long as the brake was held on) kept out of gear, and the now free road
wheel was further slowed up by this brake. This resulted in the fore carriage revolving on its pivot one way or the other as required. The defect was that the finest steering obtainable was the length of tread on the tire corresponding to one tooth in the ratchet into the distance from centre to centre of the front wheels. This was modified, however, by putting different numbers of teeth in the two ratchet wheels.
In Igo° this gear was replaced by using an ordinary differential gear and steering by applying a brake to one side or other of the gear, whilst retaining the ball-socket connection with a rear runner between the fore carriage and the main platform, and to give easier control by placing the driver himself on the fore carriage with the driving mechani sm.
This vehicle ran from the day of its completion without a hitch. As, however, the general details are very much the usual practice, it will be wise to pass on to the performance of this vehicle. The method of steering by brake application was a very easy method indeed, and it was very singular to what nicety and with what small manual effort this wagon could be controlled, whilst the design of the vehicle made is perfectly impossible to throw any cross stresses on to the gear. The action of this vehicle on a rough road was the easiest imaginable, being only a pure roll from side to side, however rough or uneven the surface was; and, moreover, such was the advantage of the design for durability that, although fitted with all copper tubes between both the main and auxiliary pumps and the boiler, as also with a plain straight copper steam pipe without any of the usual expansion coils to take up vibration, throughout the three years this vehicle was on the road no trouble was experienced with broken pipes. There were, however, difficulties; the first of these was that going astern the wagon was in a state of instability, and it took a lot of care when going astern to follow any desired curve, as the rear of the vehicle tended to increase the lock, and these had to be counteracted. To go astern and steer in a given direction needed, first, to gently bring the lock over to approximate curve, keeping the speed low, and then to put the steering over to the opposite hand, and keep the rear of vehicle from increasing the angle by gently checking the inside road wheel by means of the steering brake. This was, however, far from impossible. The only difficulty that was discernible when going ahead was if one wheel got into soft ground, soft enough to let it sink in, then it would be difficult to pull out, although it was quite possible to keep the wagon straight. The vehicle was driven in Lancashire on cobbles, and generally over all sorts of roads in all sorts of weather, and finally in and round about the City of London itself, and such was its freedom from skidding there was not a single instance of the kind; whilst with its short wheel base and the rapidity with which the lock could be altered, it could be worked at seven or eight miles an hour through traffic that an ordinary rear-driving wagon would be quite unable to get through without following whatever was in front of it. The vehicle was so well under control for anyone to drive going in a forward direction that it was considered only necessary to make the design such (by some addition) that it would be equally as easily handled in going astern. This involved a careful consideration of the conditions to eliminate the factors which did not bear upon this particular question, after which it was found that the proposition stood as follows :— (a) That the problem would be solved by positively locking the fore carriage and vehicle together by means of some mechanism acting on the slide or quadrant. This, however, had against it the real difficulty (as was then thought) of the fact that the movements of the wheels rising and falling to the road inequalities also imposed a movement between the slide or roller and the quadrant or supporting sweep for the back of the fore carriage. Hence it was argued to positively lock here would set up serious vibration in a horizontal plane. (b) The only other solution that appeared feasible was to introduce a clutch on the differential gear locking the two driving wheels positively together. This would, of course, under some conditions be a detriment, for if one wheel on very greasy surfaces slipped whilst its fellow gripped, an involuntary turn would result. This was clearly foreseen, hut that such conditions would occur seemed with the actual weights, three tons per wheel, quite out of the question. However, after carefully considering tne evils of each system, the latter seemed most likely to cause least trouble and definitely promise success.
To give this system a thorough test, Dr. Brightmore built a completely new vehicle. It was then discovered that the conditions inimical to the second solution, which seemed likely to be so rare, were most common—in fact, almost insuperable—on routes where tram lines were laid, as these in places were above the road surface, polished quite bright, and when one wheel rested on such a raised length, the coefficient of friction on that wheel was 90 greatly reduced that the wheel would not bite, and the result foreseen of turning involuntarily at once occurred. This difficulty resulted in the clutch which locked the wheels together being abandoned.
It was now decided to try the first system referred to— that of locking on the quadrant between the quadrant and the slider, in order to see if the supposed difficulty was real. In the result this acted perfectly in its own capacity, but the vibration in a horizontal plane set up by the bad road surface was even worse than it was expected to be. Once more pure thought was resorted to, this time to try and find if a neutral plane existed where the same point (whether considered as a part of the fore carriage or of the vehicle body) would have the same amount and direction of motion for all inclinations of the two axles. It was discovered that such a plane did exist. When this new position was actually tested by altering the sweep and the locking slide it was found that a great part of the vibration had been eliminated. In dealing with petrol vehicles, as a typical instance of good practice, the Pullcar vehicles arc taken as examples. The system of transmission is one in which all the cornponent parts—engines, gears, tanks, etc.—are carried on the vehicle proper, the only great or radical departure being that an axle is used which steers on the ordinary Ackermann principle; but the pivots upon which the stubb axles turn are divided, each consisting of two pins which are concentric as regards their axis, but not extending across, the intersection of their common axis with the axis of the driving member of the axle; at the point of intersection referred to, some type of an universal joint is arranged to join up a short driving axle (extending through the hollow stubb axle and to which the wheel is connected by some usual driving joint), with the driving shaft extending from the differential gear through the fixed section of the axle. The 'axle proper, consisting of all between the two steering pivots, is much the same, and, if necessary, could be exactly the same, as any usual type of back axle which is used both to carry the load and act as a casing to enclose and carry respectively the driving shafts from the differential gear to the road wheels. The difference in design is that the ends of this axle, instead of carrying the road wheels, have arms cast upon them vertically, and then extended laterally to carry two short steering pivots; these pivots form the hinges for a hollow stubb axle to be carried upon. The road wheels run, in the later design, on ball bearings carried on the outside of these stubb axles, while the power is transmitted to the wheels by short driving spindles, which are keyed into the bosses of the road wheel hubs, and on the pivot side of the stubb axles are formed as a plain jaw. Similarly, the driving spindle extending from the differential gear at its outer end is formed into an exactly similar jaw, but arranged, when put together, at go degrees to the jaws on the stubb axle spindle, whilst between these two pairs of jaws a loose floating block—rectangular in section—is arranged to act as an universal joint. This loose block has curved ends to rock in the respective jaws; this simple job is a notable feature in the Pullcar system, as it leaves nothing to be desired as regards simplicity or accessibility. In actual practice the whole of this joint is enclosed in an ingenious set of covers, which allow the full lock to be obtained without any difficulty.
The most distinctive feature in front driving and steering by this method is, of course, the universal joint in the driving shaft system. This has often been credited with a great loss of power. This notion is based upon the misconception that it is often working on a big lock, whereas, as a matter of fact, almost 95 per cent, of the driving is such that the joint is only a loose coupling in a practically straight length of shaft, and as such, of course, has no movement in its individual parts; hence it cannot, while so acting, be a source of loss. There is, of course, a fairly large loss of power when the vehicle is turning on full lock (so degrees),. but even then the front driver would compare. favourably with a rear driver on the same lock, because in the front -driver the loss of power through the joint is more than compensated for in the fact that the front driver exerts its energy in the direction of required motion, while a rear driver exerts its forces in a direction inclined at so degrees to the desired direction. The loss on turning in a frontdriver can be greatly reduced, with a little added expense, by having two universal joints working across the steering axis, and so halving the angle of the joint, in which case, instead of 50 degrees in a correct design each joint would only work at 25 degrees; and as the loss of power is not directly proportionate to the angle itself, but approximately to the cosine of the angle, the loss would thus be reduced to about one-half of its original value. This scheme of using two universal joints working across the steering pivot would admit of larger steering angles, and hence of the use of longer wheel-bases, whilst maintaining easy manceuvring ability as regards quick turning in a narrow compass. Before leaving the question of the axle, it is desirable to consider the question of the scope of such a type of front
driving and steering vehicle, which is obviously limited by two considerations, the ability to steer such a vehicle (as regards the gross weight on the steerers at the tires), and, having reached a point which is the actual limit of load upon the steering wheels to allow of easy manipulation, then the further consideration as to what load this will admit of being carried on the rear wheels as trailers.
In experiments it has been shown that to steer easily at the speeds attained, say, in motorbus traffic, it is not advisable to exceed 35 cwt. on the steering wheels, which, allowing for extra greasy surfaces, would probably admit of a gross load on the rear axle of about three tons. This would only admit of, say, a vehicle, With its load complete, weighing 4 tons 15 cwt., or, say s tons. Considering, finally, the benefits of the system, first and foremost stands the fact that front driving and steering vehicles under ordinary fair usage are non-skidding. The whole position is summed up in the remark that the direction of operation of the propelling forces can be modified in a moment to counteract any suddenly arising disturbing forces.