Using linear tyre and vehicle models, the effect of rear-axle steering can be shown in the vehicle response to steady state steering inputs. A simple countersteering relationship between the rear-axle and the primary front-axle-steering input is suggested. When this rear steering input is proportional to the respective wheelbases, the vehicle effectively turns about the rigid second axle. Thus, steering the third axle of a three-axle vehicle improves low-speed manoeuvrability by decreasing the effective wheelbase. This improvement is predicted to be greater as the tires become non-linear. At higher speeds, rear axle steering improves manoeuvrability by decreasing the understeering tendency of a three-axle vehicle. This high-speed improvement is greater on low-friction surfaces. Three-axle vehicles are typically commercial vehicles and, as such, have two primary components of their duty cycle: low-speed manoeuvring with large steering inputs, and high-speed lane keeping with small steering inputs. This work addresses handling improvement in the first portion of the duty cycle. As a practical matter, when a rear-axle steer control law such as that suggested in this work is implemented, its authority at highway speeds can be reduced to minimize an oversteering tendency added to the vehicle. The high-speed effect of rear-axle steer on lane keeping requires a different vehicle model with dynamic steering inputs. This future work should also address transient obstacle avoidance manoeuvres. The work presented cannot be used to predict the absolute minimum steady state turning radius for a given speed. In that operating condition the front tires will be saturated, and the linear tyre model will not apply, although the findings of this work suggest that, as the tires saturate, the improvement due to rear-axle steer is even more pronounced.
Handling benefits of steering a third axle
2010
11 Seiten, 6 Bilder, 2 Tabellen, 11 Quellen
Article (Journal)
English
Handling benefits of steering a third axle
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