REMEDIES for FRONT WHEEL OSCILLATION.
By “THE CHARGE HAND.” [This article is from the pen of a new contributor with wide experience of workshop methods, and is submitted to our readers as representing the views of a practical expert, who can rarely be persuaded to commit them to paper.—EluxoR.]
ONE of the troubles in tackling the job of correcting front wheel oscillation, or “wobble,” as we usually term it, is the difficulty in deciding whether it is due to wear on the steering gear, to some kind of impact, or a defect in the design of the car ; for all these causes are sometimes blamed for the irritating and dangerous action set up when the front wheels begin to oscillate, irrespective of anything the driver may do in trying to correct matters. The subject is one that interests the practical mechanic as much as it does the sports driver, and, generally speaking, both have to work very much in the dark
when dealing with bad cases ; though, perhaps in a way, a really bad case is easier to remedy than one where the wobble is only slight and altogether intermittent.
Wheel wobble is sometimes put down to too much backlash in the various joints of the steering system, but I am not sure that this kind of looseness is an actual cause. Of course, excessive backlash has the effect of exaggerating the complaint, but it generally begins somewhere else. On sports cars, the wobble is seldom allowed to reach a very bad state, but I recently had a case of a touring car in which the wobble was so bad that, on a rough road, it was actually necessary to come to a complete stop, before the control of the front wheels could be regained. Tilting the Front Axle._ It will be noticed that with most of the easy steering cars the front axle has a decided tilt, which brings the two steering pivots out of the vertical plane, to help the wheels to “castor,” and when this tilt is produced by packing pieces between the axle palms and the springs, it is easy to put them in the wrong way round, for owing to their wedge shape, they can cause the steering pivot to tilt backwards at the bottom, instead of forward in the proper way. This misplacing of the wedge packing pieces is quite sufficient to account
for a tremendous amount of wheel wobble ; but, of course, the mistake can be easily cured.
When fitting new packing pieces, it is advisable to try tilting the axle little by little with temporary strips until trial on the road shows that the right amount of tilt is secured. Then permanent wedges should be fitted with the taper indicated by the road tests.
Incorrect Steering Lay-out.
With some designs of cars that have come into my hands for repair, there is no doubt that the drawing office has been largely to blame for wheel wobble and the accompanying sketch shows what happens when a mistake has been made in design.
The diagram shows the lay-out of an ordinary steering gear, together with the theoretical movements of the front axle and steering arms, which control wheel wobble. When ordinary springs are used, the rise and fall of the front axle is not truly vertical, owing to the flexing of the front springs. Thus the actual path of the hub spindle may be taken as roughly following in the line A–B, omitting factors which may influence it to some slight degree. The end of the steering arm to which the steering side rod is attached, naturally follows the same path, being formed on the same swivel as the wheel spindle, but the forward end of the steering side rod has a tendency to follow a slightly different course, shown by the curve C—D. This is explained by the fact that it moves in an arc struck from the bottom of the steering drop arm on the column and consequently as the axle rises and falls, when the car is passing over rough ground, the difference sets up a series of jerks, with continually reversing directions, on the lever of the swivel axle.
Now if the length of the drop arm is such that the side rod inclines downward towards the front, the difference between the two paths A—B and C—D is increased, and the wobble will become very pronounced, but as the side rod in most designs has a certain amount of upward tilt, wheel wobble from this cause is rather rare. On more than one occasion, however, I have been unable to get over the trouble except by scrapping the
existing drop arm and forging a new one, long enough to give a correct lay for the side rod, which has also entailed certain alterations to regulate the amount of lock for the front wheels.
Besides taking up slight road shocks, the springs on the ball and socket connections help to absorb any slight irregularities in the two paths shown in the diagram, so it is important that these parts should be attended to carefully when making the adjustments.
Personally, I believe that we shall see modifications of the accepted form of steering gear for high speed work, and it is even possible that we may go back to the old system of mounting the drop arm on the front axle, as in the case of the early Wolseleys, or at least to some idea embodying the principles of that lay-out. Then, of course, the practice of arranging the pull and push rod across the chassis instead of along the side, helps to do away with wheel wobble as the movement at the end of the rod corresponds exactly with the rise and fall of the axle.
Correct setting of the front wheels, freedom of backlash, alignment of the two axles and wheel balance are all important things in accurate and sensitive steering, but even if one takes every conceivable precaution, it is sometimes necessary to try the car at high speeds on the road or track, during the course of the adjustments, to secure the accurate steering so essential on a fast car.
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