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Hang on, this is melting my brain.
...Oh, now I see it - the lower pulley slides sideways across a shaft.
How does a Brompton normally do its derailing? I'd love to see an explanation of the reasoning behind this weirdness... Wonder how well it shifts.
Kimmo-
That’s pretty much how a standard rear mech works on a Brompton. They are more like a front mech on the back than a traditional derailleur. The tensioner is separate as it has to take up the slack when you fold the bike. If it was part of the mech it would try to change gear as you fold. The shift is ok (ish) in the same way a front mech is ok (ish) moray of the time.
Chaley
here