A low aspect self-propelled vehicle (1) operated by a driver (4) sitting on top of it, has its seat (7) supported from beneath by a flange (12) which is pivoted to tilt about a horizontal axis (15) in the mid-vertical plane of the vehicle. The spacing between the axis (15) and the seat (7) is sufficient to ensure that when the seat is tilted about the axis by operation of a tilt control mechanism (21), the center of seat also displaces a significant extent in the uphill direction of the slope. This reduces the downhill movement of the center-of-gravity (10) of the combination of vehicle plus driver, so that the risk of the vehicle toppling is reduced.