EXPERIMENTAL work done to ascertain the influence of frame and body structures upon front-end stability of the automobile is described by the author and definite means of preventing the phenomena of wheel wabble, shimmy and vibratory movements of the radiator, head-lamps and fenders are set forth.
Early investigation showed that the problem involved not only the unsprung portions of the car but also the structural arrangement of the frame and the body.
Chassis-dynamometer tests revealed a nodal point of zero torsional vibration approximately at a plane through the front seat but varying with different cars and body types, the forward portion of the chassis vibrating torsionally about the longitudinal axis in opposite phase to the rear portion. Experiments rather conclusively proved that damping is needed in the frame and body. Many tests with stiffened frames gave disappointing results on the road as compared with laboratory tests, and it became apparent that the frame and body must be treated as a single structure. A more rigid method of bolting the body to the frame, bracing and reinforcing to increase the body rigidity and securing the radiator more rigidly gave greater front-end stability.
Addition of weight at the front, remote from the neutral axis of the frame, as by placing spring-mounted weights in the ends of the front bumper, proved very effective, the weights being given a frequency substantially the same as that of the torsional vibration of the chassis.
Spare wheels and tires mounted in the front-fender wells, and unsprung weight well out from the center of the axles, have a very beneficial effect, and the author believes that we have gone too far with many cars in reducing the unsprung weight.
Open and convertible cars call for different treatment than is most effective with sedan models, which are stiffer in the mid-section.