A spacing bar of a wheelchair frame comprising an elongated structural body having a pair of opposite ends, each of the opposite ends having a C-shaped clamp configured for being clamped to respective lateral frame members of the wheelchair frame, wherein the elongated structural body and the C-shaped clamps are integrally made of a monocoque composite construction, and a tightening assembly for each of the C-shaped clamps, the tightening assemblies being arranged in the spacing bar to loosen and tighten the C-shaped clamps to the respective lateral frame members. A wheelchair frame comprising: a pair of lateral frame members configured to support at least a seat, and front and rear wheel units, the lateral frame members each having an elongated frame portion in a seating zone of the wheelchair frame, and a spacing bar having an elongated structural body and a pair of opposite ends, each of the opposite ends having a connector configured for being clamped to a respective one of the lateral frame member of the wheelchair frame, wherein each of the connectors forms a joint with the respective one of the elongated frame portions constraining the spacing bar to movement of the spacing bar axially along the pair of the elongated frame portion, a tightening assembly for each of the connectors, the tightening assemblies being arranged in the spacing bar to loosen and tighten the connectors to block or allow movement of the spacing bar along the pair of the lateral frame members.