The Cyclo Standard was the definitive touring derailleur for many decades. It started life as the French 1923 ‘Le Cyclo’, designed and manufactured by Albert Raimond. The design of the mechanism was said to have been inspired by machine guns that Raimond examined during the First World War. In 1932 the British Cyclo Gear Company was established by Raimond and Louis Camillis, and the Cyclo Standard was its first product.
The Standard mounts under the chainstay and is controlled by two cables. The cables rotate a sleeve with a helical groove in it on a shaft with a pin sticking into the groove. One cable pulls the sleeve one way, the other pulls it the other way. As the sleeve rotates the helix drives the sleeve along the shaft. The pulley cage is attached to the sleeve, with the top pulley concentric with the shaft. Chain tension is provided by a spring attached to the eye on the arm that is just above the guide pulley. The other end of the spring is attached to a clamp on the chainstay just behind the bottom bracket.
The Cyclo Standard was heavy and was fiddly to set up, but once properly installed, it was strong and reliable. It can make a decent claim to being the first derailleur that was a practical, commercial, mass-produced product.
You saw these derailleurs in use on tandems in the UK well into the 1970s.
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