This weird object is an LTWOO Fold derailleur - a model designed for folding bikes of the Brompton persuasion. These bikes have a rear triangle that folds under the main frame, massively changing the effective chain length. As a consequence they need a very long chain tensioning arm.

The main body of this derailleur will be familiar to anyone who has breathed on a SRAM DI.R.T. model. The uniqueness of the LTWOO Fold lies entirely in the its pulley cage.

LTWOO offer two Fold models:

• The 'Fold A', which is is for 5 to 7-speed bicycles.
• The 'Fold B', which is is for 8 to 10-speed bicycles.

I am not sure of the real differences between the 'A' and 'B' variants. The pulley wheels and pulley cage plates appear identical. The only clearly visible distinction is that the low gear adjustment screw is 22mm long on the 'A' and 18mm long on the 'B'.

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This example is an LTWOO Fold B for 8 to 10-speed transmissions.

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• Derailleur brands: LTWOO
• Country: China
• Date of introduction: 2024?
• Date of this example: unknown
• Model no.: F5010 LXX
• Weight: 302g
• Maximum cog: 28 teeth (Source: LTWOO)
• Total capacity: unknown
• Pulley centre to centre: 172mm
• Index compatibility: 8 to 10-speed
• Chain width: 3/32”
• Logic: top normal
• B pivot: unsprung
• P pivot: sprung
• Materials: aluminium knuckles and outer parallelogram plate, steel inner pearallelogram plate and pulley cage plates around the guide pulley, plastic tension pulley arm and discs.

Ref. 2196