Shimmying was noticeable before four-wheel brakes began to be used, but since that time the trouble has been greatly increased. Two kinds are distinguishable; (a) low-speed shimmying, a violent wabble of the front wheels about the king-pins without a bouncing of the front axle, and (b) high-speed shimmying, a severe bouncing of the front axle during which one hub is up while the other is down. This occurs at somewhat higher tire-pressures and at high car-speed. Believing that both forms are not correctible by changing the design of the tire and are only slightly affected by changes in the steering-gear, efforts were directed toward prevention rather than correction. A dashpot, attached between the front axle and the front-wheel brake-housing was found to prevent all low-speed and a large part of the high-speed shimmying without appreciable resistance to steering and also to be effective in removing “road-shock,” “whip-back” from the steering-wheel and even the rotational vibration that accompanies ordinary driving. The door-check type of dashpot avoids many undesirable features common to other types. To be effective, all air must be religiously excluded, all joints must be tight, all fits must be good, all moving parts must be non-flexible and the liquid used must be non-freezable.
