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.
Step 2: Test Fit Tensioner Components, Initial Installation
Remove the rear wheel, and thread in the 10mm bolt from the left side of the derailleur hanger.
Install the Baja Sports tensioner, so the rubber wheel will be in line with the chain. Add the washer and bolt. Experiment with tension on the bolt to see what lets it have free enough movement that it doesn't bind.
Install the wheel and chain, and move the tensioner wheel up against the chain, to get a feel for how much tension is needed to remove the slack, and to check for spoke clearance.
If the rubber wheel rubs against the spokes, it may be remedied by the next step, or by adding a thin washer between the derailleur hanger (frame) and the tensioner bushing.
Step 3: Create Aluminum Bushing Shim From Cola Can
If needed or desired, you can create a small shim from a coke can, which wraps around the bolt 1 time, between the bolt and the tensioner bushing. This will prevent side-play of the tensioner, and prevent wear on the threads of the bolt, preserving this level movement and preventing binding.
Measure the width and depth of the tensioner bushing area (may vary a little between parts sold) and cut a shim to match this, so the shim wraps the bolt one time and just overlaps enough to prevent deformation. This may require a couple tries, or cutting it shorter to fit.
Install the shim inside the bushing, and install onto bolt, to test. The tensioner should move well in its arc path but not have side play.
Step 4: Drill the Tensioner Arm for the Spring Connection
Remove the tensioner and shim from the bolt, and mark with a sharpie pen, to approximate this design (see photo). The spring must not rub on the rubber wheel. Select a small bit that will drill a hole large enough for the spring wire to insert through. A slightly bigger hole will facilitate installation.
Using a needle nose plier, carefully bend out the spring loop, install through hole in the tensioner arm.
Loosen the bolt in the der. hanger, add blue threadlock, or nail polish as a thread adhesive. Tighten securely but do not strip the threads.
Step 5: Reinstall the Tensioner Components Permanently
Add a dab of axle bearing grease (lithium tractor grease, automotive grease, marine grease, vaseline, etc) to the inside of the tensioner bushing, and a dab on the threads exposed near the derailleur hanger. Reinstall the shim into the tensioner arm bushing, Install the tensioner onto the bolt, add the washer, and nut. Use threadlock or nail polish to prevent the nut from ever loosening unexpectedly. Tension the nut only enough to prevent play. The spring will hold the tensioner in place if it ever is lost, but it is needed to ensure proper movement and tension of the bushing.
Step 6: Create the Tensioner Spring Strap and Hook
You will need a rubber shim that fits around the seat stay, a hose clamp that can tighten down onto that, and a spring hook made from a piece of stiff coat hanger wire. If you use a quality steel hose clamp, you can install the spring onto one of the worm clamp slots, but you won't be able to quickly remove the spring for tire changes.
Using needle nose pliers, fashion a section of coat hanger wire to duplicate this arrangement.
Pull the tensioner spring up and take up the chain slack. Note the position of the spring on the frame. Move your tensioner strap and hook (wire goes inside of hose clamp, outside of rubber shim) about 1/2 inch above this. Tighten the hose clamp securely over the wire and shim, leaving the hook hanging on the right side of the seat stay of the frame. The first time you tighten it hard, the clamp will deform a little over the wire (good). Be sure to check this clamp after a test ride to make sure it is still tight.
Step 7: Test System and Proceed to Test Ride
Install the spring end loop onto the hook. With the bike in a stand or hanging by a rope or strap from a convenient rafter or tree, test the bicycle by pedaling it slowly, to check that the system operates without binding, loud clicking noises, chain derailments or other unwanted movement. When the rear tire is lifted and dropped from 1 foot, the chain should not slap or bounce. It of course will move or vibrate, but should not wave or strike the chain stay. The bicycle should be able to be pedaled forward or backwards without complication. If using a coaster brake (I do), the chain will droop slightly on the top run, and the tensioner will be pulled down by the bottom run, but it should resume it's normal duty instantly as soon as brake tension is released (pedals moved forward).
Update. This bike is still working great, and has been converted to a motor bicycle with a Dax Friction Drive kit.
It is crucial that the wheel stays located in the same exact spot, and is secure at high speeds (20-30mph). This design has allowed a very solidly done steel multispeed bike to be made into a commuter bicycle with one gear to take-off or assist the motor.