When rolling on an uneven road green frame, wheels of the vehicle are kept always vertical in traversal plane (perpendicular to the vehicle motion direction) thanks to:
1. Green, blue and pink bars create a parallelogram mechanism.
2. Center of mass of the green frame (with or without load (user weight)) is below its revolution joints with the blue bars.
It means that the seat is kept always horizontal in traversal plane.
Orange part represents the vehicle load.