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Earlier today, I saw this bike with an absolutely bizarre drive-train. It looks like a single speed, but it has a derailleur on attached to it. The derailleur does not move horizontally, but only along a vertical axis. The bike is a Mongoose Alta, but I could tell that it was heavily modded. The bike is not originally a single speed, and has a different derailleur.

Does anyone know why this would be a desirable modification? I'm really curious.

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    Good question - next time grab some photos while the opportunity is there.
    – Criggie
    Nov 9, 2017 at 23:56
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    Yeah, what you saw was a chain tensioner. Would not be used on a "fixie", but wouldn't be unusual on a single-speed with freewheel/freehub. There are purpose-made tensioners, or the cyclist may have adapted a rear derailer. Nov 10, 2017 at 1:40

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The derailleur is there to act as chain tensioner. In a frame intended for derailleur the rear wheel can't be moved to tighten the chain, so a separate spring-loaded chain tensioner is needed. The cheapest and ugliest way is to just use whatever derailleur was on the bike before conversion.

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