From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article from 'THE AUTOCAR' magazine discussing the phenomenon of steering wheel wobble and related experiments.
| Identifier | ExFiles\Box 28\4\ Scan103 | |
| Date | 3rd January 1914 | |
| THE AUTOCAR, January 3rd, 1914. Steering Wheel Wobble. it off the ground, and its up and down movement whilst driving on the road is quite noticeable when the mudguard is removed. One effect of the horizontal component of the centrifugal force, since it reverses six times per second, is to make the car go faster and slower six times per second with about 1/4th of the gravity acceleration, since the car weighs 3,000 lbs., that is to say, the acceleration is over six feet per second per second. It is, however, swallowed up to some extent by the springs, and is resisted by the adhesion of the tyres to the road, and I am bound to say that I could not perceive this effect at any speed. An additional reason may be that the other effects were so much more remarkable. The next effect to consider is that this force of 600 lbs. is not applied to the centre of gravity of the car, and therefore acts with a leverage to turn the car round its centre of gravity with a moment of 600 lbs. multiplied by half the track, viz. 2ft. 2in. equals 1,300 ft. lbs. The mass of the car is 3,000 lbs., and probably has a radius of gyration of 4 1/3 feet, so that its moment of inertia is 13,000 ft. lbs., which is ten times the figure of the turning moment. This effect is also swallowed up by the yield of the springs and resisted by the tyre adhesion, and reverses its direction six times per second. The most important effect, however, is the one to which I have referred before, i.e., the turning moment round the steering swivel, or rather round the pair of steering swivels, because the two wheels are connected by the steering link. The force is the same, viz., 600 lbs., but the mass of the wheels and other steering parts is only about 150 lbs., and the radius of gyration is about 8in., so that the moment of inertia is in the neighbourhood of 100 ft. lbs. FIG. 7. In some of my experiments the weight was placed on the outside of the wheel as in fig. 5, in which it will be seen that the plane in which the weight revolves is 4in. from the centre of the steering swivel, so that the turning moment is 600 multiplied by 1/3rd = 200 ft. lbs., which number is twice that of the moment of inertia, and in this case there is nothing whatever to resist the resulting wobble, except one's grip of the wheel. The car under these conditions was fairly comfortable to drive up to twelve miles an hour, when the effect upon the steering was not only noticeable, but decidedly unpleasant. I succeeded in driving it on dry roads up to thirty-four miles an hour measured by an isochronous speedometer which I have carefully checked on other occasions, and know to be correct, but it was most difficult work and seemed to be exceedingly dangerous. On greasy roads I was not able to get to anything like this speed, because I could not control the skidding brought about by the severe wobbling of the wheels, and perhaps contributed to by the total acceleration of the car. I do not know how to describe the disconcerting and even terrifying effect of this exaggerated wobble, but I am sure neither I nor any of my passengers will ever forget it. Fortunately we were all good sailors. My next experiment was to put the weight on the other side as shown in fig. 6. My apparatus was so crude that even then it was almost exactly an inch from the centre of the plane in which the weight ran to the centre of the steering swivel. In this case the backward and forward acceleration on the car and the turning moment to spin the car right round would not be very much influenced by the movement of the weight 3in. nearer the swivel, but the turning moment round the steering swivels would be reduced to a quarter of what it was before, and in running it I found that the wobble had very much diminished, being hardly noticeable until just over twenty miles an hour, and nothing like so bad as before at the higher speeds, though I did not take the car faster than thirty-five miles an hour owing to the remarkable way in which the springs on the same side as the weighted wheel moved up and down. I quite recognise that no wheel is ever out of balance as much as 14 lbs., and I used this big weight so that the observations might be clear and unmistakable, but each of the steering wheels with a single bolt valve, not counter-balanced by security bolts or by added weight, and with a wheel a little bit out of balance itself, can easily be a pound and a half wrong. If both wheels happen to be out to the same extent when they are working in opposite directions, that is to say, when the heavy side of one is half a turn in advance of the heavy side of the other, the total effect will be equal to 3 lbs. unbalanced and will produce the huge force with which I was experimenting, viz., 600 lbs., at a speed of seventy-five miles an hour approximately. These speeds are admittedly outside the requirements of most motorists, but with the four inches between the plane of the weight and the steering swivel which represents the conditions of ordinary current practice, the wobble was extremely bad at even twelve miles an hour, and with the degree of error in balance which may be met with by any motorist, viz., 1 1/2 lbs. in each wheel, the steering wobble may become really serious at less than thirty miles an hour. I think that the mysterious disappearance of wobble, and then wobble coming again experienced on some cars, is probably due to a pair of steering wheels, both out of balance, sometimes working together so as to cancel each other's wobble, and sometimes, as in the case above recited, working so as to increase that wobble. I hope to continue my experiments with more refined apparatus, so that I can get the axis of the steering swivel passing exactly through the centre of the circle in which the weight turns, and then to carry the matter further by having steering swivels made like fig. 7, where the central plane of the tyre and the rim itself (the things that can get out of balance) will absolutely coincide with the steering swivel. | ||