Derailleur On A Single Speed 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.
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.