A tube assembly for a wheelchair frame comprises a tubular member having at least a first portion and a second portion angled relative to the first portion, the first portion configured to be in a seating zone of the wheelchair frame. The second portion is configured to be in a bottom front zone of the wheelchair frame, the tubular member defining a surface concavity in its peripheral surface delimited by a peripheral step, the surface concavity located at a junction between the first portion and the second portion of the tubular member. An interface member has a peripheral contour shaped and sized for the interface member to be received in the surface concavity and contoured at least partially by the peripheral step.