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. 
I swapped cranks from the 34t to a "2x9" crank with a 28 tooth inner ring, and ended up having to use a half-link and a 1 tooth smaller rear ring (19 down to 18), to get a tight chain. This gear 28x18 lets me spin up to about 17mph and do wheelies very easily. I found the chainring doesn't run as true as the pinned steel crank with sawed off rings, though. I'll try loosening my chainring bolts and re-aligning the ring.
The practical limit of this device is a 19t rear cog, if you have fixed(vertical) dropouts. If you have horizontal dropouts, it might be redundant, unless you want it to act as an automatic tensioner (a nice function). Coaster brake cogs are about $5-15 new, usually 5, and you can get free ones off broken wheels and junked kids bikes. <br> <br>Changing from an odd to even tooth cog should help you get the optimal length if you still have any trouble, and won't change your ratio much.
<p>I still have this bike, and it works great. I would recommend using a slightly longer bolt, and a Nylock nut (the kind with the plastic insert that keeps it tight). </p>
<p>I found these chain tensioners online for under $5 each, from Alibaba or something similar. Check DX (dealtime Extreme) also. Just search &quot;Go kart chain tensioner&quot; </p>
<p>I'm still riding this bike, with the 28 tooth chain-ring. It's working great as a 1x1 mountain bike &quot;klunker&quot;. </p>
<p>I still have this. I haven't ridden it in a while, but it's still working like before. Works great with a wide BMX freewheel. </p>
Still going strong! I have used this bike frequently this year, on very rough trails and roads, with no problems. I'm using KMC single-speed chains from niagaracycle.com (inexpensive generic chains) with good luck even in muddy conditions.
The motorcycle or heavy garage-door chain can work well, but its heavy. If you can weld a nut (or sawed off derailleur hanger) to the right chain stay of your frame, this can be installed there the same way, and work even better, plus tire changes will be a breeze even with the fat chain (also takes up more slack)
I found a 410 roller chain at Tractor Supply Co (where I got the tensioner) for $17 (pre-greased, with a master link). This chain makes a bicycle chain look like a wet noodle! It does not flex side to side! You will need a motorcycle/go kart chain tool, or a pin punch or sawed off nail the size of the pins or a hair smaller. You can use a soft 2x4 (to drill a little hole for pin helps), and use the punch and a hammer to drive the pins out of the top plate on a link beyond the chain you want to use. Install with the wheel axle loose, using the master link. (Pliers help to drive home the master link clip. install it with opening facing back of bike on the top run.) http://www.surpluscenter.com/images/p1-1163-AC.jpg
Niagara Cycle has a lot of single speed cogs for coaster brake hubs (you can convert CB hubs to &quot;freecoaster&quot;), cassette hub single cogs, track cogs, freewheels, chains, and single-ring cranksets.
It's still working. The cheap KMC chain I am using was somewhat worn anyway, and now has stretched quite a bit! The tensioner has taken up the slack pretty well. I plan to find a heavy duty track bike or BMX chain.
After further testing, in off road conditions, with a trailer, this still performed silently and flawlessly.
I rode this 8 miles today with a kid trailer in tow, with no problems. I rode the bike nearly 100 mi as a single speed with no tensioner, with no problems, even on mtn bike trails. The roller device was silent and the chain stayed snug the whole time without noise.

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