A wheelchair includes a rigid upholstered seat removably mounted on a crossbrace-type chassis. The seat includes a generally square-shaped frame having a clevis assembly depending from each of the four corners of the frame. Each one of a first pair of the clevis assemblies disposed at diagonally opposed front and rear corners of the seat frame is formed with aligned generally pear-shaped openings. Each one of a second pair of the clevis assemblies disposed at the other diagonally opposed front and rear corners of the seat frame is formed with aligned generally horizontal elliptical-shaped openings. An inverted generally U-shaped mounting bracket formed with aligned openings is disposed on each end of each one of a pair of chassis sideframes. Each clevis assembly is engageable with a respective one of the U-shaped brackets so that the respective openings of the engaged brackets are aligned. A quick release pin is passed through each set of aligned openings to secure the seat to the chassis. The pear-shaped openings allow vertical articulation of the chassis relative to the seat in certain instances when a bump, depression or other irregularity on a travel surface is encountered by usually one of the wheels of the wheelchair. This articulation enables the wheelchair to insulate its occupant from shocks caused by such irregularities, and also maintains all wheels of the wheelchair in continuous contact with the travel surface to preserve occupant steering control and stability of the wheelchair.