Stair climbing is necessary to improve the convenience of electric wheelchairs. An important issue in developing a climbable wheelchair is stability and speed during climbing. In this work, tri-wheel and supporting leg were adopted in the stair climbing method for high speed and stable posture. Based on the mechanical and static analyses of the climbing mechanism, the design method of the wheel cluster system was expressed as equations. These equations were proposed considering the design parameters that ensure the continuity and stability of the climbing motion. Moreover, the collision information between the stairs and the cluster system during the climbing motion was included in those equations. A validation test of the stair climbing mechanism was performed by manufacturing a prototype of 750 ( $W$ ) x 800( $D$ ) x 1,000( $H$ ) $mm^{3}$ and $120 kg$ . The experimental verification proved that the electric wheelchair can stably go up and down the stairs under extreme conditions (stair width = $300 mm$ , stair height = $180 mm$ ) at a speed of 10 steps/min.
