The mechanical device having folding and telescopic teeth is characterized by the spontaneity of its operation the utility of its structure and the simplicity of its composition: a fixed frame (1) and a deformable chain (2) for the one part and a lock (3) and three flanges (4) for the other part. This mechanical device is able to be produced by the most basic embodiments of conventional mechanics. The mechanical device is aimed both at an accessory which is fitted and removed manually with ease simplicity and rapidity on any wheel of any vehicle (towed or motorized) notably for the wheels of a wheelchair for disabled people and at an integral wheel of any vehicle (towed or motorized). Regardless of its destination this mechanical device with automatic bifunctionality (landing/staircase) allows the vehicle to climb and descend staircases without jolts and to move over a flat surface. This mechanical device automatically changes configuration depending on the type of ground (landing/staircase). The teeth (ABC) of this mechanical device adapt automatically to the profile of the staircase step negotiated. When negotiating staircases this mechanical device is in a toothed configuration when it leaves the staircase (climb or descent) and reaches the landing this mechanical device automatically takes on a circular configuration and is locked automatically by way of an automatic locking mechanism (3) thereby allowing it to maintain its circular configuration throughout its movement over a flat surface.
