A rider support assembly for a bicycle, motorcycle or other multi-wheeled vehicle that utilizes a harness to suspend the rider from a support frame that is attached to a ball-type mounting structure. The support harness includes straps or a sling that is secured to the rider's torso and/or legs. A base section of the support frame includes a connecting structure (e.g., a socket) that is movably connects to and supported by the mounting structure such that the support frame can be pivoted in any direction by the rider during operation. A flexible spine extends upward along the rider's back and is secured to the rider's upper body by the support harness. An optional mounting structure pivots relative to the vehicle to facilitate a relatively stiff spine that supports the rider when sitting upright or leaning back.