Resonant whirling of two-piece propshafts in rear-wheel-drive cars

Abstract

Many modern cars with rear-wheel drives have a two-piece, rather than the traditional one-piece, props haft (or tail-shaft). The two-piece shaft requires less floor clearance, and hence a smaller floor tunnel, and does not suffer from the high-speed twisting resonances to which the one-piece shaft is susceptible.

The two-piece shaft is, however, susceptible to a lower speed, high load (high torque) phenomenon in which the propshaft whirls with increasing amplitude until its restraining rubber bush is fully compressed in its mounting bracket, causing a thump sound. The whirling amplitude builds up periodically, so that a regular series of thumps is heard at a frequency of about 5 per second.

The effect is very sensitive to the properties of the rubber bush and to its temperature. Different designs of bush are used by different makers, each with the purpose of minimizing both the whirling problem and the transmission of other vibrations to the car body.

GMH has a particular design which it would prefer not to vary, because of retooling costs. It can however vary the properties of the rubber material itself. By trial and error it has achieved rubber properties which control the whirling problem to an acceptable level under normal conditions. GMH posed the question, to the MISG, of whether one can give theoretical guidelines for optimal rubber properties. This might lead to improved control of the whirling problem. It would also be useful for new and modified propshafts, if it could replace the tedious trial and error process previously employed.

GMH were also anxious to gain a better understanding of the whirling phenomenon, and felt, with us, that a mathematical description of the phenomenon was a prerequisite for the study of the rubber properties. The thumping phenomenon is observed at both 40 and 80 kph approximately, although the phenomenon had a different quality at these two speeds.