DOES ACKERMANN STEERING SPELL FINALITY?
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Some Notes on Steering and Its Problems.
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f HE STEERING of a vehicle is one of those things which are too often taken for granted. f HE STEERING of a vehicle is one of those things which are too often taken for granted. The average man, if asked to describe the steering lay-out of a particular vehicle, will probably reply that it is fitted. with Ackermann steering, but he usually has a somewhat vague idea of what that term actually means and as to whether it is an inventiOn or a principle. As to its advantages or disadvantages, usually he can say little and is apt to be-. lieve that Ackermann steering is bound to be correct.
We propose in this article to deal with some of the main points which should be known with regard to this vitally important question, for it is a question not only affecting the safety of those persons actually in the veh:cle and the general public, but also affecting the pocket of the owner of the vehicle, insomuch that the tyre wear can be materially decreased or increased by the correctness or incorrectness of the steering.
-Some time ago, we received details of a specific case of tyre wear in which the average life of the solid front tyres of a two-ton van was reduced from well over 10,000 miles to something like 2,000. During the whole of the period that these tyres were in use the steering had not been touched, and it was found on examination that there was an inaccuracy of in. between the rims of the wheels ; the steering pivots were also loose and the wheels wobbled. An example like this shows how much can be gained by paying more attention to the steering, not only as regards keeping it in good order, but also as regards its actual design and lay-out.
We were prompted to reintroduce this subject by seeing the patent specification • of what is called "Improvements in Mountings for the Front Wheels of Motor Truoks." In this invention, which is mainly applicable to vehicles fitted with front wheel drive, the inventor in his diagrams shows the front wheels mounted in a rectangular frame which can be converted into a rhomboid by the action of a steering arm. Whatever the angle of lock, the wheels are constantly parallel, and the error in steering must therefore be such as to place the design quite beyond practical application.
Strictly speaking, the Ackermann steering gear, as first brought out, was not designed on the principle • which we to-day call the "Ackerman." It was merely a convenient method of connecting wheels B40 which were run on divided axles. No attention was paid to inclining the steering arms towards each other when the cross-rod was at the rear of the axle, or away from each other when it was at the front. This refinement, upon which practically all our steering designs are based, was introduced by a young French carriage builder, named Charles Jeantaud, and lie was the first to take into consideration the fact that when rounding a curve the steering wheels of a vehicle must be turned at different angles, SO that the intersection of the radii drawn from the points of contact of the wheels with the two circles, round which they are travelling, is at the point round which the whole car is turning. This point, for all practical purposes, lies on a line drawn as a continuation of the back axle and to either left or right of this according to the direction in which the vehicle is being turned. Since then designers have concentrated on this principle and few efforts seem to have been made to discover a principle which is more correct than that employed by Jeantaud. Few people realize that what is known as the Ackermann steering is not correct and cannot be made so except for three definite wheel positions. These positions are : when the vehicle is driving in a straight line, and at one steering angle to the left and a similar angle to the right ; over the rest of the range on either look there is more or less inaccuracy, which inaccuracy can be transferred either to the small angles or to the greater, so that a vehicle can be arranged to steer fairly accurately up to angles of a few degrees only, or it can be made to steer accurately at angles approaching the full lock whilst being inaccurate for lesser angles.
From this it would appear that, when using Ackermann steering, vehicles for particular work should have their steering lay-out designed for that class of work only, that is to say, a vehicle intended to run in countries where the roads are, on the average, straight and where there is little traffic, should have its steering designed to give accuracy over the smaller range of steering angles, whereas a vehicle for traffic work should have its steering as accurate as possible for the larger angles at the expense of the lesser. Naturally, this is not feasible, as it would over-complicate matters, and vehicles must also be prepared to run both in town and country. A mean, therefore, has to be taken between the two.
The problem of making. the wheels track accurately for the major part of their work is not by any means the only one which has to be.faced. In some steering designs the wheels have a marked terideney to wobble ; in other words, when locked well over, the wheels tend to lock over still more, and we know of
particular instances in which this action is so accentuated that the steering wheel has been wrenched out of the driver's control.
In order to prevent this, in some cases the front axle and steering pivots are inclined in a forward direction ; this gives the wheels a castoling action owing to the fact that the point of contact of each wheel with the ground Iles behind the point at which a continuation of the centre line of the pivot would meet the ground.
The steering pivots are also often inclined in the vertical plane through the centre of the front axle, so that the virtual lever formed by the distances between the point of contact of the tyre and the ground, and the point at which a continuation of the centre line of the pivot would touch the ground is reduced. In some vehicles this virtual lever is reduced to nothing, that is to say, the continuation of the centre line of the pivot meets the point of contact of the tyre. By this method the road resistance on the wheels does not tend to turn the steering wheel. Raking the steering pivots and axle in a forward direction also prevents, to some extent, the shocks due to road inequalities acting upon the wheels and turning them.
In some vehicles the front wheels are inclined instead of the pivots, but this method is not to be recommended, for if the wheels are canted there is continual slip between the tyres and the road.
The direction of rotation should always be at right angles to the direction of motion and in the same plane with it. On the other hand, it does not do to accentuate the outward inclination of the steering pivots, as, if this is carried to excess, it means that the whole front of the vehicle will have to be lifted excessively by the Wheels when these are locked over in either direction. A rake of anything up to three degrees is permissible, and the wheels will then automatically swing back into straight-ahead position if freed.
Another consideration is the effect of bumps on steering. In passing over a bump, the axle rises and i carries wfth t the steering rod, the forward end of the latter thus passing through an are of a circle the radius of which is the length of the rod. The result is that the rod, if normally below the horizontal, pushes forward and twists the wheels ; as the axle drops, the reverse action takes place, giving a -twist in the other direction, so that the wheels swing first one way and then the other. This action is particularly noticeable where the steering rod is very short.
It will -sometimes he noticed that a vehicle which has no play whatever in the steering connections will wobble when rounding a curve. This is a direct proof of the inaccuracy of the Ackermann steering, and is caused by the fact that the :steering of the car is done first by one wheel and then by the other, according to which wheel has a greater grip on the ground. On sharp corners this wobbling is not particularly noticeable because the weight of the body is thrown
by centrifugal force on to the outer wheel, which, consequently, does all the steering.
There is a widespread belief amongst designers that the point at which the continuations of the steering arms should meet lies at the centre of the back axle. This is quite incorrect, the point lying on the centre line of the vehicle somewhat in front of the axle. Definite figures cannot be given because the enact location to give the best mean depends upon the wheelbase, the track and other dimensions.
The question of having the steering tie rod at the front of the axle or behind is of importance. The internal quadrilateral is not so accurate as the external from the theoretical point of view, but the proper application of the external is limited by the proximity of the ends of the steering arms to the wheels, so that the front angle cannot be large ; also the front tie rod is in a position which render § it more liable to damage.
We trust that these general remarks will, cause designers and inventors to turn their attention more fully to the -question of accuracy in steering. We have purposely dealt with the Ackermann type only and have refrained from criticising traction-engine and similar steerings, in which crudity is counter-. balanced by simplicity and strength.