What's this?

[CopperCyclist]

Saw this on a bike today. What is it, and what's it purpose? Bike was a single speed if that helps.

I'm talking about the strange extender thing that seems to keep the chain away from the bottom of the chainring.

 

[Scilly Suffolk]

It looks like a chain tensioner, used when converting a bike without suitable dropouts/track ends to fixed gear or single speed.set-up.

Having said that, I've only seen them mounted at the rear where the rear derailleur normally sits, so I may be wrong.

 

[TheSandwichMonster]

Seen them up front on DH mountain bikes before, the principle being that they stop your chain from bouncing off on particularly rough ground/landings...

 

[biggs682]

chain tensioner normally fitted on downhill bikes , but should work on other sorts

 

[Recycler]

Is that an off-centre chainwheel, or is that just a trick of the camera angle?
If it is off centre, then that may be why a tensioner is needed.

 

[mickle]

It's a chain retention device, designed to stop th chain falling off the ring.

And it's been assembled wrong.

 

[Cubist]

It is indeed a chain device. It keeps the chain in place on a bike without a front derailleur, or keeps tension in a chain on a double MTB , preventing chain slap and chain suck.

As Mickle points out, it has been put on wrongly. It should be at a bout 8 O'Clock on the chainring, and should actually lift the chain slightly rather than lowering it. The chain is on the wrong side of the lower pulley.

Here's a proper example.
http://www.google.co.uk/imgres?q=bl...ndsp=25&ved=1t:429,r:1,s:42,i:168&tx=78&ty=56