[CR]Spirax setup

(Example: Framebuilders:Richard Moon)

Date: Thu, 15 Aug 2002 22:30:51 -0700
To: Paul Lee <plee1@optonline.net>
From: "Jan Heine" <heine@mindspring.com>
Cc: classicrendezvous@bikelist.org
Subject: [CR]Spirax setup

Paul,

I have disassembled (partially) my Spirax and here is what I found - it is an ingenious design, starting with the familiar Cyclo, but adding the feature of relatively constant chain tension no matter which cog is used.

The Data Book drawing (p. 116) shows only a few of the parts...

- The round "box" (p. 134) contains a spring that is responsible for the chain tension only. I haven't been able to open mine, so I don't know what it looks like inside. But the spring shown in the Data Book (p. 116) is that one.

- The movement of the derailleur (in and out) is like the Cyclo with a peg sliding on a helical groove. As the pulley (to which the peg is affixed) turns, the derailleur slides in or out. That, too, you can see in the Data Book. On my derailleur, the pulley was stuck on the rod. I had to lock the two outer nuts against each other and hold them in a wrench, then turn the pulley with force. That freed it.

- Unlike the Cyclo with its cable looping around the pulley and pulling both ways, there is a return spring. So the cable pulls the pulley only one way. The screw on the pulley (page 134) holds the cable end. There is a big flat spring that moves the derailleur toward the bigger cogs (inverse of current practice). That is the one you see on page 134. My return spring is tired, I doubt the derailleur would shift well onto the bigger cogs.

- The limit of derailleur travel can be set by moving the entire mechanism inward or outward (like the Cyclo). The rod that forms the center of the derailleur is attached to the plate that attaches to the dropout with two nuts. But moving the rod in or out, you limit the travel.

- Chain tension is adjusted by rotating the main rod in the plate that attaches to the frame.

- The ingenious feature is that the chain tensioning spring turns with the pulley that shifts the gears. Thus, as you shift to a bigger cog, you get less spring action, and vice versa. The chain tension is held relatively constant, unlike modern derailleurs where the chain flaps when on the small cogs (especially with a triple setup), while it is too tight when on the big cogs.

- The main rod that forms the center of the derailleur is internally threaded and had a smaller screw inside. I don't know what that screw does...

I hope this helps. Unfortunately, I don't have a bike to go with my derailleur. I really would like to try one and see how it shifts.

Jan Heine, Seattle