ABSTRACT
Steered axle trucks exhibit modes of dynamic behaviour which are unique. Among these are a set of modes in which the steering linkage directly couples axle and body yaw motions. These modes typically occur at quite low speeds with moderate to low conicity wheels (con = 0.05 to 0.10). These modes are usually susceptible to modification by changes in either the steering geometry or the secondary suspension parameters. A rail vehicle was built in which the secondary suspension stiffness was considerably greater than had been designed for, with the result that it exhibited low speed instability with substantial body yaw motions. Analytical predictions showed that this effect should be expected and modification of the suspension stiffness eliminated the instability thus verifying the analysis. At a higher speed the modified car showed excessive motions in an upper centre roll mode. This was not expected from the eigenanalysis results. A hypothesis is proposed, which is supported by all analysis and test results obtained so far, that the effect of the truck spacing on the car is to filter the random track inputs and reinforce certain frequencies. The carbody suspension then responds in resonant fashion to these forcing frequencies even though the modal damping is sufficient to avoid the possibility of unstable behaviour.
