Single Speed Conversions: Do I need a Chain Tensioner?
On some single speed conversions the design of the frame means that a chain tensioner is needed in order to keep the chain taut enough so that it will not fall off.
A tensioner attaches to the bicycle frame and is a (usually) spring loaded mechanism with a cog which the chain runs through. The chain is threaded through the tensioner when it is installed and will be held in tension by the tensioner's spring.
This is a fixed position, rather than spring loaded tensioner
If you are converting a multispeed bicycle and have a rear derailleur from that bicycle you may be able to save some resources by using the derailleur as an improvised chain tensioner...but it's a bit of a fiddly job.
Whether you need a chain tensioner depends on one thing – the shape of the section on your frame in which your rear wheel fits.
If you follow this link to the sheldon brown website and scroll down a tiny bit you will see a discussion and images of the different shapes you might have on this section of the frame:
As noted on the page if your bike has 'vertical' rear dropouts you will odds on need a tensioner, whereas if you have track forks, or horizontal or angled dropouts which offer a few cm of horizontal distance you will very likely not need a tensioner.