Why is the 100 mm tube the shortest?
The dimensions given are the modular dimensions of the USM system and refer to the distance from ball center to ball center. The ball and tubes are connected by a connector (double wedge) which is 40 mm long. Two connectors (2 x 40mm) are required to connect the tube, plus 2x the distance between the end of the tube and center of the ball (2x9mm), so 98 mm in total. The 100 mm tube is therefore the shortest possible.
