pivot point 68 fixed with respect to platform 12 while link 70 couples arm 66 to seat 34 link 72 substantially parallel to link 70 supports seat 34 in a substantially horizontal orientation in its central position 54 the third side of trilateral arrangement 64 is formed by link 74 link 74 as shown in greater detail in fig5 b may be a rigid member or a suspension member as shown containing a gas spring 76 which serves to cushion seat 34 with respect to vibration transmitted from the road surface through platform 12 link 74 is coupled to cam follower 78 which rides on cam surface 80 in such a manner as to provide a well 82 of stability at the center of motion of seat 34 if seat 34 moves fore or aft cam follower 78 is driven up cam 80 thereby increasing the height of seat 34 relative to platform 12 relative to its position of stability at the center this reduces the effort required of the rider to maintain a centered body position thereby advantageously allowing the rider to maintain a more relaxed position and momentarily to remove his her hands from the handlebar it is to be understood that other mechanical configurations providing for seat 34 to rise as seat 34 is moved either forward or aft of its central position 54 wherein the center of gravity of the vehicle and rider is substantially over the region of contact of the vehicle with the ground are within the scope of the present invention as claimed in the appended claims the.