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What is the clutch on a modern rear derailleur actually doing, and should I ever turn it off?
New-to-me bike has a rear derailleur with a small switch labelled on and off, which I am told is a clutch. It is currently on.
What I can observe: with it on, the cage is much harder to pull forward by hand, and the chain is noticeably quieter over rough ground. With it off, the cage swings freely.
What I do not understand is the mechanism, and whether there is a cost to leaving it on all the time:
- What is it physically doing — is it a brake, a one-way thing, or something else?
- Is shifting worse with it on? It feels slightly heavier at the lever but I might be imagining that.
- Is there any situation where I should switch it off, other than taking the wheel out?
@shop_vac_shay · 2d ago · 2 replies
It is a one-way friction device, and the one-way part is the whole trick.
The derailleur cage is sprung to pull the chain tight. Over bumps, the chain slackens for an instant and then snaps back, which is the slapping you hear and, occasionally, how chains come off. The clutch adds friction that resists the cage being pulled forward — the direction it moves when the chain goes slack and then whips — while adding much less resistance to the cage moving backward, which is what happens when you shift onto a bigger sprocket.
So it is not a brake in both directions. If it were, shifting would be genuinely awful. It is closer to a damper that is stiff one way and soft the other, usually a wrapped band or a roller clutch on the cage pivot.
That asymmetry is why you can leave it on permanently, which is what it is designed for.
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@derailleur_dee · 2d ago
The asymmetry explains why it felt heavy pulling the cage forward by hand and normal when riding. I was testing it in exactly the direction it is designed to resist.
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