Trailing axle suspension system

A trailing axle suspension system for suspending a trailing axle with steerable wheels from a motor vehicle having a vehicle frame that is normally supported by primary axles with wheels and in turn supports a load-carrying vehicle body with a tailgate wherein the vehicle frame is supported for rolling movement relative to a vehicle frame roll axis and the maximum allowed load supported by the vehicle body has a center of gravity located below the vertical height of the vehicle body and substantially at or above the vertical mid-height of the vehicle body, said suspension system comprising

• a carriage from which the trailing axle is directly suspended wherein the carriage is pivotally connected to the tailgate and is adapted to locate the trailing axle in a stowed condition and in an active condition with the tailgate closed wherein the trailing axle in the active condition is located at a substantial distance behind the vehicle with the steerable wheels engaging a road surface and the trailing axle is supported for rolling movement relative to a trailing axle roll axis,
• a pair of hydraulically operated actuators pivotally connected at one end to the carriage and pivotally connected at an opposite end to the tailgate adapted to establish the trailing axle in the active condition wherein the actuators apply force on the trailing axle to help support the vehicle frame,
• gas springs adapted with the trailing axle in the active condition to allow a prescribed range of swinging movement of the trailing axle relative to the vehicle frame to accommodate differences in road grade between the trailing axle wheels and the wheels of the primary axles when the road grade at the trailing axle wheels is substantially below and above the road grade at the wheels of the primary axles, and
• the interrelationship of the carriage and the actuators and the gas springs being such that the minimum possible distance in height between the trailing axle roll axis and the actuator pivotal connections with the tailgate is always greater than the maximum distance in height between the vehicle frame roll axis and the center of gravity of the maximum allowed load supported by the vehicle body when the trailing axle is in the active condition and within the prescribed range of swinging movement of the trailing axle allowed by the gas springs.