Vehicle Evaluation During Sustained Solid Axle Tramp: Part 2 — Application of Methodology to Shock Absorber Design Strategies

Abstract

EI Consultants (formerly The Engineering Institute) has, for over a decade, researched and tested methods of mitigating the controllability effects of solid rear axle tramp by optimizing rear axle rotational damping. This optimization has explored the balance between increasing the damping forces of the shock absorbers and increasing the distance between the shock absorbers positioned along the axle. Axle tramp is detrimental to vehicle handling and stability, since the reduction in normal force at the rear tires can lead to a total loss of control situation. On solid rear axles such as those common on SUVs and light trucks, underdamped tramp motion will result in an oversteer characteristic of the vehicle as the rear lateral capacity is compromised due to the tires alternately bouncing out of firm contact with the road surface. In severe cases of axle tramp, the alternating normal forces at both the input tire and the opposite tire will go to zero when each tire fully leaves contact with the ground. EI Consultants has tested numerous SUVs and light trucks and their responses to axle tramp. In order to excite the tramp mode in a sustained fashion for close study of suspension design alternatives, the test methodology utilizes one rear tire with three vulcanized rubber lumps, placed equidistant about the circumference of the tire. Throughout this research, increased effective rotational damping has been repeatedly demonstrated to have a direct relationship to increased controllability. The most recent testing included maneuvers modeled after those recommended in test standard SAE J266: Steady-State Directional Control Characteristics for Passenger Cars and Light Trucks. This testing included multiple shock absorber configurations, and the data was analyzed in multiple domains to provide insight on the effectiveness of various shock absorber design strategies. Several shock absorber design variables were evaluated, with the most significant of these being the lateral distance between the shock mounts along the axle. Other variables that were able to be observed and evaluated in the latest testing included the balance between shock absorber rebound and compression forces, and the relative effect of “staggered” shocks in side-view angle, where one shock is positioned with a rearward angle, and the other shock is positioned with a forward angle. The effectiveness of placing shocks further apart along the length of the axle was unmatched. This paper is the second of two companion papers presenting theory and results on EI Consultants’ most recent axle tramp testing. Where the first paper focused on new understandings of test data analysis theory, this paper will summarize the results of numerous tests and their application to various design strategies for improving solid rear axle tramp damping, with a motivation for enhancing vehicle controllability and highway safety.