The present invention refers to a wheelchair (10a) comprising left and right side frames (42, 44 242, 244 342, 344), each side frame having a lower side frame member (46, 50) and an upper side frame member (48, 52 248, 252 348, 352 452) and a folding mechanism (40) for connecting the left and right side frames (42, 44 242, 244 342, 344) to each other in a folding and unfolding relationship, the folding mechanism including: cross struts (56 256 356 456) rotatably connected to the side frame members (42, 44 242, 244 342, 344), the cross struts including one or more cross struts from the left side frame pivotally connected to one or more cross struts from the right side frame hinge links (66, 68 266, 268 366, 368 466, 468) connected to the side frame members (42, 44 242, 244 342, 344), the hinge links each having a first end (70, 78) pivotally connected to the side frame member and a distal end (72, 80), wherein pivoting the hinge links relative to the side frame members causes the left and right side frames to move closer together in a folding movement or farther apart in an unfolding movement, and a lock (100 - 124 200 - 208 300 - 310 400 - 412) for locking the hinge links (66, 68 266, 268 366, 368 466, 468) together when the folding mechanism is unfolded to form a rigid truss (126) between the left and right side frame members (42, 44 242, 244 342, 344), thereby fixing the distance between the left and right side members at a widest point of rotation established by the hinge links.
