Abstract: An extension device, a seat system and a cart are disclosed. The extension device includes a frame that is detachably connected with the cart frame. The frame generally has an annular structure and consists of two rods, the frame has a length direction and a width direction, each rod has a transverse section extending in the width direction and extension sections connected to two ends of the transverse section respectively, and extension sections of the two rods corresponding in position are connected to each other. By improving the structure of the extension device, the compatibility between the seat and the cart frame is improved.

Abstract: A cart has a folded configuration and a deployed configuration. The cart includes two side frames arranged opposite to each other; and a plurality of support members connected between the two side frames. Each side frame includes a vertical synchronization rod with a front foot assembly and a rear foot assembly arranged on two sides of the vertical synchronization rod. The front foot assembly includes a front foot rod with an upper front oscillating rod and a lower front oscillating rod hinged between the vertical synchronization rod and the front foot rod. A bottom of the front foot rod is hinged to a front wheel. The rear foot assembly includes a rear foot rod with an upper rear oscillating rod and a lower rear oscillating rod hinged between the vertical synchronization rod and the rear foot rod. A bottom of the rear foot rod is hinged to a rear wheel.

Abstract: A cart has a folded configuration and a deployed configuration. The cart includes two side frames arranged opposite to each other; and a plurality of support members connected between the two side frames. Each side frame includes a vertical synchronization rod with a front foot assembly and a rear foot assembly arranged on two sides of the vertical synchronization rod. The front foot assembly includes a front foot rod with an upper front oscillating rod and a lower front oscillating rod hinged between the vertical synchronization rod and the front foot rod. A bottom of the front foot rod is hinged to a front wheel. The rear foot assembly includes a rear foot rod with an upper rear oscillating rod and a lower rear oscillating rod hinged between the vertical synchronization rod and the rear foot rod. A bottom of the rear foot rod is hinged to a rear wheel.