The claimed design started from the basic concept that the brake-disc (9), arranged on the upperside of the wheel (4), is fixed in its centre to the central shaft (7). To overcome however the relative large distance between the upper position of this disc (9) and its brake position (contact with wheel 4) and moreover to meet the requirement that the tube (6) must be as short as possible, the solution is found by the application of a special designed cam-disk-lever mechanism (13-24) which only needs a rather compact space arranged in the top of the tube (6).
