Balloon tires have caused the points at which the greatest trouble formerly occurred to be reversed; the greatest wear hitherto has occurred on the rear tires; now it occurs on the front. The carcasses of old tires were heavy and thick and carried a large part of the load; balloon tires are flexible and will support very little load. The air-pressure carries the load and gives greater cushioning effect. Four-ply tires have proved to be the most satisfactory and have none of the disadvantages of high-pressure tires. Balloon tires steer harder than high-pressure tires because of lower air-pressure, which necessitates a greater area of contact. Steering resistance is caused by the load on the tire and the increased area of contact. Many designers, in making the steering gear free to overcome steering resistance, make wheel-twitching possible.
Shimmying may be classified as low-speed and high-speed, the latter occurring at speeds between 35 and 40 m.p.h. and being very violent. By a chronograph, the movement of the cycles of shimmying for different air-pressures was studied. These were found to increase with the air-pressure and with the speed of the car. The shimmying under given pressure and load conditions appears to repeat at the same speed, and to be confined strictly to cars having front-wheel brakes. The axle and wheel mass, greatly increased by the addition of front-wheel brakes, floats between two springs and in that position is thrown into violent agitation. The forces that produce this agitation are believed to originate in the geometry of the steering-gear, the angle of inclination of the king-pin and the change in the velocity of the wheels. Investigation has shown that the path of the ball connection of the steering-arm is not a truly neutral path so far as steering is concerned and that any action between the frame and the springs will cause the wheels to steer alternately to the right and to the left. The author believes that shimmying can be overcome by correcting this error in the steering linkage. With a view to introducing a flexible steering-connection, a hydraulic gear was tried. This served to eliminate front-wheel shimmying and the twitching of the steering-wheel. Because the axle is not in its lowest position when steering straight ahead, on account of the inclination of the king-pin combined with the caster angle and the wheel camber, the wheels tend to turn in other directions. With the axle-mass throbbing up and down the tendency is to throw the wheels around so that the axle can assume its lowest position. Shimmying, consequently, is produced. Making the king-pin vertical would obviate this tendency. Tables showing the results of tests are appended.