THE PREVENTION OF SHIMMYING 250016

Shimmying, although known for many years, did not become a serious problem until the arrival of the balloon tire and the four-wheel brake. Apparently, shimmying is of two kinds: the low-speed variety, which is merely a persistent front-wheel wabble without an abnormal bouncing of the axle, and the high-speed species, which is chiefly a persistent bouncing of the axle accompanied by wabbling of the wheels. The two most obvious effects are wheel wabble and axle bounce.

As low air-pressure seemed to be the cause, the attention of the tire makers was first devoted to stiffening the body of the tire in various ways, but the results obtained were not satisfactory; and the conclusion was reached that the solution lay in making the car control the tire rather than attempting to control the car through the design of the tire. These considerations led to a search for mechanical means of control. A form of stabilizer appeared to be efficacious in checking axle bounce, and an hydraulic damper, similar in principle to that of an ordinary door-check, in preventing wheel wabble. Although successful in preventing shimmying in open cars, these devices eliminated only from 60 to 70 per cent of the axle bounce in closed cars. This is believed to be a resonance effect between the tires with their unsprung weights and the body springs with their loads and should be worked out mathematically. The author concludes that relief is to be found in the mechanical features that control the tires.