EI Consultants (formerly The Engineering Institute) has been studying solid rear axle tramp for well over a decade, and contributed several publications to the literature outlining recommended test methods and their results. Throughout the history of EI’s research, sustained tramp inputs have been achieved by use of a tire featuring affixed lumps of rubber to induce wheel hop at one end of the axle. The principal methodological guide for studying the vehicle response to this input has been the test methods and data analysis recommendations of test standard SAE J266: Steady-State Directional Control Characteristics for Passenger Cars and Light Trucks. More specifically, past testing has been patterned almost exclusively on the circle test (constant-radius/slowly-increasing-speed) method discussed in J266. Historically, the J266 recommendation for data analysis and presentation, i.e. understeer/oversteer gradients derived from a wheel angle versus lateral acceleration plot, were principally used. Recent research, along with fresh analysis of previous testing results, revealed limitations of the circle test and the J266 recommended manner of data analysis in the context of tramp resonance testing. During a constant-radius/slowly-increasing-speed test, a single control variable (speed) has the effect of changing both the lateral acceleration and the tramp input frequency simultaneously. This effect results in a non-steady-state test event where only a narrow portion of each test run expresses the resonant axle tramp phenomenon that is the intended object of the observation. To provide a wider view of vehicle response characteristics during sustained axle tramp, EI Consultants selected and evaluated expanded test methods in a recent testing project. These methods included performing circle tests at multiple radii, performing continuous tests modeled after the J266 constant-speed/variable-radius method, and performing path-following tests modeled after the slowly increasing steer method. Expanded data analysis and presentation methods were developed to quantify and understand the vehicle oversteer response in more effective ways than those recommended by J266. Due to the abrupt discontinuity in the vehicle’s response upon reaching the resonant tramp frequency, novel methods of data presentation were shown to be more useful in assessing vehicle characteristics during resonant tramp. Of particular value was examining the steering input delta in the vehicle speed and tramp input frequency domains during the phase of resonant axle response; and examining the difference between the actual yaw rate and the theoretical Ackerman yaw rate derived from the measured steer angle. This paper will detail the data analysis techniques that were developed to overcome the limitations of the J266 standard’s steer gradient methodology, and thus introduce a more useful approach to evaluating understeer/oversteer characteristics during non-steady-state test events. This paper is the first of two companion papers presenting theory and results from EI Consultants’ most recent axle tramp testing. This paper focuses on new understandings of test data analysis theory, while the second paper will summarize the results of numerous tests and their application to various suspension design strategies for improving solid rear axle tramp control, with a motivation for enhancing vehicle controllability and highway safety.

Tire tread separations on light trucks and SUVs have resulted in numerous catastrophic highway accidents over the past two decades in the United States. These accidents frequently involve single-vehicle rollovers or deviations of the impaired vehicle into oncoming traffic, where high speed frontal collisions may ensue. On light trucks and SUVs equipped with a Hotchkiss rear suspension, one explanation for the loss of driver control during an in-process rear tire tread separation is solid axle tramp response to the imbalanced separating tire. This explanation has met with some controversy. The present study will demonstrate that the imbalance forces generated at highway speeds from a partially detreaded tire are sufficient to induce continuous cyclical axle tramp, and can even be sufficient to completely elevate rear-axle tires out of contact with the paved roadway. This imbalance-induced tramping action may be exacerbated during braking and the vehicle’s terminal yaw, when rear traction is crucial to avoiding a catastrophic accident. In addition to test data, several field examples of such events are presented. A key metric of solid axle response to an imbalanced, partially detreaded tire is shock absorber motion. In the present study, shock absorber displacement on the test vehicles, as measured during highway speed tread separation axle tramp events, is found to oscillate through a stroke generally less than one inch (2.5 cm) in length at a frequency in excess of 10 Hz. Peak instantaneous velocities of the shock absorber have been observed as high as 40 in/s (16 cm/s) or more during straight driving under axle tramp conditions. Confirming several previously published findings, the present study shows that increasing shock damping force at the higher operational velocities of the shock absorber reduces the magnitude of axle tramp and assists in keeping the rear axle tires in contact with the ground. Additionally, increasing the distance between the shock absorbers by moving them closer to the wheels provides the same advantage.

In Part I of this two part series, the mechanisms of occupant ejection through automotive side glazing during rollover collisions were analyzed. It was shown that partial or complete ejection can occur through centrifugal acceleration based motion through an open portal, or due to the changes in velocity (ΔVs) developed during corner impacts. A new rollover angular velocity model was also developed and validated. In this second paper, recent literature discussing occupant retention side glazing is discussed, and the new angular rate model is applied. New dolly rollover testing of two XC90s is also presented and compared to previous testing.

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.

The mechanisms of occupant ejection through automotive side glazing during rollover collisions are analyzed. It is shown that partial or complete ejection can occur through centrifugal acceleration based motion through an open portal, or due to the changes in velocity (ΔVs) developed during corner impacts. Aspects of vehicle kinematics, effective mass, impact velocity, window design, rotational velocity, and injury are examined. Analysis indicates that the dominant ejection mode is rotational acceleration induced exit motion at a low velocity relative to the center of gravity of the vehicle facilitated by vehicle body flexure based fracture of tempered side glass. In this first part of a two paper series, a new rollover angular velocity model is presented and given experimental validation, and the concept of the two ideal ejection modes is developed.