A mounting unit for adjustably mounting an add-on piece to a frame, in particular of a wheelchair, includes at least one drum receiving unit having at least one aperture and female thread. The drum receiving unit is an integral part of the frame or adapted to be attached to the frame. At least one drum assembly has at least one hole and a male thread. At least one bolt is part of the add-on piece or adapted to be mounted therewith to hold the add-on piece. The drum receiving unit is adapted to receive the drum assembly. The female thread mates with the male thread and allows an alignment of the position of the hole with respect to the aperture, such that the bolt can be inserted through the aperture at least partly into the hole. The drum receiving unit with the drum assembly and/or the drum assembly with the bolt comprises a detachable form fit connection. At least one of the drum receiving unit, the drum assembly and/or the bolt comprises a detachable force fit connection with respective mating component.
