Inasmuch as the use of low-pressure tires has become established, the conditions of car design affected by them are reviewed, particular reference being had to the members of the chassis included under the term unsprung weight, namely, the axles, the wheels and the tires.
Referring to the principles that underlie basic design, the author first investigates the effect on the steering of such changes and compromises from the perfect structure as failure of the king-pin to coincide with the vertical load-plane, the inclination of the king-pin toward the wheel, or the wheel toward the king-pin, or both, and the giving of a toe-in to the front wheels. Further modifications have served to reduce the car shock, to add to the strength of all the parts by increasing the dimensions, to improve the spring-suspension, and to reduce the car weight per passenger. But four things remain to be done, namely, to stop the angular rotation of the axle because of the flexing of the springs, to eliminate backlash in the steering linkage, to construct a positive-steering mechanism that will be absolutely neutral, and to divorce the steering mechanism from all influences except those that occur from the road.
In the opinion of the author, all these objectives can be reached, but then looms prominently the question of lateral stability. This is to be expected unless the width of the rim is increased. In high-pressure tires, the normal ratio of the width of the rim to that of the tire is about 62 per cent, but ratios of from 50 to 55 per cent are common. Lack of lateral stability has amplified the errors of present design until a point has been reached at which shimmying has become prevalent.
By studying the footprints of tires, it is found that the side-slip per revolution is 60 per cent greater with balloon tires than with high-pressure tires. As the forward rake of the king-pin exerts a leverage that tends to turn the wheels still farther in the same direction, these two forces are said to be sufficient to cause shimmying even though the geometry of the steering mechanism were neglected.
The conclusion reached is that, in order to avoid shimmying, all backlash must be taken out of the steering-mechanism, and that, so far as possible, all rake of the king-pin outward leading of the wheels, and toe-in should be eliminated. The introduction of an hydraulic steering-gear is suggested as a means of accomplishing these results.