From the 1920s to the 1940s Cyclo dominated the high-end market for touring derailleurs with their helical drive design. Cyclo's derailleur, while widely admired, had its downsides. The twin cable set-up looked untidy on the bike and was fiddly to adjust. Also, the chain tension spring, running along the chainstay, made removing the rear wheel more complicated than it needed to be. By the late 1940s many cyclists had used a Simplex racing design (or similar), operated by a single cable and with the chain tension spring handily integrated into the p-pivot. These cheap derailleurs made the classic Cyclo look long in the tooth.

The indefatigable Raymond Bon decided that what the world really needed was a helical drive derailleur, operated by a single cable and with an integrated chain tension spring. What is more he decided that the whole thing should be manufactured and finished to the highest standards. That it would cost an arm and a leg only added to its allure. The result was Le Spirax - a beautiful object revered by French touring cycle connoisseurs to this day.

Given the, slightly weird, obsessions of the time, and given Raymond Bon's, slightly eccentric, track record, it was inevitable that Le Spirax would have its quirks. Much play is made of the fact that Le Spirax 'automatically adjusts' the chain tension as you change from gear to gear. Fine tuning chain tension was a matter of great concern in the late 1940s and early 1950s - and was of no concern at all before or after that brief period. To achieve this, seemingly highly desirable, 'automatic adjustment' Le Spirax used a clock spring at the p-pivot. The centre of this spring was attached to the pulley that operates the helical mechainsm, winding the spring tighter as you move to smaller sprockets and vice-versa. All very intelligent, subtle and cunning, but ultimately purposeless.

The clock spring in its circular housing did, however, give the derailleur a very singular look, letting other connoisseurs know, at a glance, that you were a person to be reckoned with.

I have two Le Spirax derailleurs in this collection. They differ in the details of which parts are chromed and which are black. This example is the silverier of the two.

• Derailleur brands: Spirax
• Country: France
• Date of introduction: 1950?
• Date of this example: unknown
• Model no.: unknown
• Weight: 321g
• Maximum cog: 26 teeth (Spirax says 12 teeth difference at the rear)
• Total capacity: 32 teeth (Spirax)
• Pulley centre to centre: 50mm
• Index compatibility: friction
• Chain width: unknown - but the pulleys are narrow
• Logic: low normal
• B pivot: unsprung
• P pivot: sprung
• Materials: steel