This Idéal derailleur is pleasingly elegant. In the style of the times it is a sliding rod mechanism, but its genius is that the sliding rod slopes down at an angle, to allow the derailleur to follow the profile of the freewheel. In this way it maintains a relatively constant chain gap - prefiguring SunTour's 1960s slant parallelogram that is still influential today.

I am aware of two variants of this design:

• A 'racy' model with a single pulley wheel and a flamboyant pusher plate.
• A 'touring' model with two pulley wheels and a longer pulley cage.

I do not know Idéal's model names for these variants - you could guess 'Course' and 'Tourisme' but you might be wrong.

Finally, I would note that the pulley wheels and their associated nuts and chain catchers on these Idéal derailleurs are extremely similar to those on Super Rapid derailleurs. I don't know if Idéal manufactured the Super Rapid derailleurs or vice versa or if both brands shared a third party supplier of these parts.

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This is an example of the 'racy' single pulley wheel model. In this case the main sliding rod components are manufactured from bronze - which is tougher, but also heavier, than aluminium.

This derailleur is finished with a type of industrial silver/grey paint - unfortunately not in great shape. I believe this is the original finish - which makes it extremely hard to clean without cleaning off all that dodgy paint.

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• Derailleur brands: Idéal
• Themes: Ultra-lightweight - single pulley models
• Country: France
• Date of introduction: 1938
• Date of this example: unknown
• Model no.: unknown
• Weight: 222g kind of including hanger plate
• Maximum cog: unknown
• Total capacity: unknown
• Pulley centre to centre: none
• Index compatibility: friction
• Chain width: 1/8”?
• Logic: low normal
• B pivot: sprung
• P pivot: none
• Materials: the two parts of the sliding rod system are bronze, the hanger arm, adjuster rod and pulleys are steel.