TO CURE “SHIMMY”

Timaru Herald, Volume CXXXVII, Issue 19308, 8 October 1932, Page 12

TO CURE “SHIMMY”

ORIGINATING CAUSES. One of the most curious and dangerous results of excessive axle tilt is exhibited when the front wheels yank violently and rapidly from side to side as the car is in progress (says “Motor Transport”). This condition continues as long as the speed of the truck and the road conditions remain unchanged. It is commonly called shimmy. Such a condition is initiated by an unbalanced wheel or by some irregular road surface which swings the wheels slightly to one side. They then tend to roll in this new direction. The car, however, tends to continue straight forward. The wheels, due to excessive caster effect, are almost instantly straightened, with such force that they pass slightly beyond the straight ahead position. During that brief moment while the wheels were pointed to one side the car has received through the springs and axle the slightly sideways motion. The car, forced to alter its direction, does so just as the wheels are whipped into the opposite direction. A deflate lag in the change of direction which can only exist at certain speeds is required in order to have the side lunge of the car occur at the same instant that the wheels are whipped into the opposite direction. With the correct amount of caster effect the wheels will not be whipped back with sufficient violence to cause shimmy. The exact speed at which the condition exists is controlled by the flexibility of the springs, the air pressure in the tyres, the play or spring in the steering mechanism, the weight of the car, but the basic cause is nearly always excessive axle tilt. Another cause of shimmy is an unbalanced wheel. This is more likely to show its effect when balloon tyres are used, and usually occurs only at high speed. This condition can be corrected by jacking up the wheels and attaching a weight to make the wheel balance perfectly. This is preferable to putting more air pressure in the tyres, as it corrects

the fault without sacrificing ridinpqualities. A badly warped wheel will sometimes cause a condition very similar to shimmy. Improper adjustment of r.he drag link will sometimes cause shimmy.