Triumph sprung hub
The Triumph sprung hub is a motorcycle suspension unit contained within a rear wheel hub. It was designed by Triumph engineer Edward Turner to give Triumph's existing rigid frames the option of rear suspension. It was one of the first motorcycle components to have a safety warning cast into its housing.
Design and development
Edward Turner designed Triumph's sprung hub in 1938. The design was inspired by Dowty hubs with suspension within them, as used in the fixed landing gear of Gloster Gladiator fighter aircraft. Turner's design used a plunger-type suspension made small enough to fit inside the wheel hub. A significant difference between the two is that the Dowty hub was an overhung bearing, supported from one side. Provided that the overall height of the suspension unit, plus the travel, was smaller than the iner diameter of the wheel hub, it allowed a fairly simple design. The Triumph hub though was mounted on a small diameter fixed axle between the sides of the frame, carrying the suspension unit and springs inside the hub. The two bearings were mounted on this suspended casing. The hub itself was a hollow drum with two end discs bolted to it, which housed the bearings. The limitation of this design was that the small axle and the suspension travel needed to fit through the centre of the bearings, which required a much larger bearing than usual, yet still only gave a small travel. With one spring above the rear axle and two below, the sprung hub provided about two inches of vertical travel and weighed more than a conventional hub. It was designed to allow rear suspension to be offered optionally without altering Triumph's existing frames. The sprung hub did not provide a drive gear for the speedometer, so the design of the transmission was revised to provide the required drive gear.Frank Baker, the head of the Experimental Department at Triumph, tried to convince Turner that the handling of Triumph motorcycles at the time the sprung hub was available was potentially dangerous at speed, to the point of having himself filmed while riding one of them on a test track at speed. Turner ignored Baker's efforts and continued with the sprung hub.