Here is an inexpensive and durable way to convert a multiple gear bicycle to a single speed, using a bicycle with a frame made with no horizontal adjustment for chain tension. This is showing how to make the tensioner. For tips on squeezing the frame or re-spacing your hub, you should see other how-to articles from Sheldon Brown, or message me. (*** In short, if you do re-space a frame, you should use a steel frame. You should lay the bike on its side on a carpeted surface, without the components on the frame (bare frame). You should slowly and very carefully press on the airborne dropout while someone heavy stands on the top-tube and head tube of the bike. Make a small adjustment at a time, and flip the bike over to bend the other stay. You may have to "tweak" the dropouts with a heavy duty crescent wrench to make them straight toward the rear as they were previously. Adding threaded nuts, or washers, to the axle stack can help control your chain-line, and avoid having to pinch the frame as much or any.)
This tensioner is made from:
1. A 10mm bolt, long enough to go through the derailleur hanger, the tensioner arm, and a washer-and-nut or a 10mm axle nut.
2. A Baja Sports go-kart tensioner, about $10-15 from Tractor Supply Co, hardware stores, go-kart and mini bike dealers. It includes the arm and welded on bushing, and a neoprene wheel with sealed hi-speed bearings
3. An extension spring. (C-163 from Century Springs Co, available at many common hardware stores for $3-5 a pair. Other longer springs can work if they are about the same strength. It takes an "educated guess". )
4. A small bushing made from an aluminum can (see photo) to insulate the bolt threads from the brass bushing of the tensioner arm. I made it with scissors and a coca cola can.
5. Blue thread locker or nail polish,
6. A dab of grease.
7. A small hose clamp to fit around the right seat stay
8. A rubber shim made from a bicycle tube (or electrical tape) to insulate the frame from the hose clamp
9. A spring hook made from a piece of strong coat hanger wire. (makes changing the tire easier)
Enjoy! This tensioner is fairly simple in design and adjustment, works with a variety of chains, and is expected to last for decades.
I made mine for less than $20.
P.S. If you are using an unmodified frame, and a cassette style hub, this should work well. If you are using a cog (and single-speed spacers) that is not near the right side of the freehub body, you might have to install this so that the bolt is installed through the OUTSIDE of the derailleur hanger, and the tensioner is inboard! You can position the spring catch accordingly, so the spring runs to the left of the drive-side chain stay. Same goes for using a BMX freewheel on a hub made for a thread-on multi-speed freewheel, unless you re-space your axle to move the hub shell right and re-dish. Then the assembly shown above should work fine. You have lots of options that don't require compressing your frame.
Step 1: Test fit your components
You need to build the bicycle up so you can ride it, including shortening the chain as much as practical, and testing chain alignment. The bicycle should function without a tensioner, but you may have some "chain slap" or slack. This device will just take up that slack including chain wear, and prevent chain derailment over rough terrain.