(this is a repost from the mechanic's forum that I thought might also be of interest here)

I would like to share with you an excellent technique I stumbled upon for tensioning single-cog drivetrains (ie with horizontal dropouts or eccentric/hubs).

First off, you are probably wondering that if we are talking about bikes with horizontal dropouts then isn't the only thing we need to do to tension the chain is pull the wheel back with the left hand while tightening the axle nuts with the right hand? Actually no, because one of the primary benefits of single cog drivetrains are that you can run the chains much much longer by really cinching down the chain tension to prevent severely stretched chains from jumping the cogs. By doing this you can forget about measuring chain stretch with a micrometer (and throwing out perfectly good chains) because no matter how stretched a chain gets you never have to discard it until it literally snaps in half! However, once the chain gets stretched to the point that it's more inclined to jump off the cogs than be driven around them, then you require a much higher chain tension than can be achieved with springs or arm strength and, until now, the only way I knew to achieve it was with a rigid tensioner/tug like this:


But chain tugs are annoying because:
1 Bikes almost never come with them...you have to buy it separately and many bike stores don't carry them.
2. They always come with the wrong kind of nut for tightening...you need a wing nut so you can cinch down the chain on the road without tools after it starts jumping off the cogs in the middle of your 30 mile winter commute on slushy salted roads. Of course you can get a wing nut, but it's yet another thing you have to buy separately and it's not like every store carries metric wing nuts.
3. The quality of many of the chain tugs is not good. I bought a fancy MKS chain tensioner for $30 and the tensioning bolt snapped in half before my chain did...kinda defeats the purpose of extending chain life when it would've been cheaper to just replace the chain.
4. There are many axle standards for single cog drivetrains...so whatever you end up getting probably won't fit without some filing. Even this is just a partial list of bike axle standards:
M9x1.0- Front "fixie" (fixed gear road bikes) hubs as well as front dynamo hubs (front generator hubs for bicycle head lamps)
3/8"x26T - Most axles including most department store bikes; Shimano Nexus and Alfine Series; 8FUN e-bike front hub motors. Very commonly used on road or bikes purchased at department stores that are not fitted with traditional quick release hubs.
M10x1.0 - European brands, SRAM, NuVinci, Rohloff; Bionx and some e-bike front hub motors
3/8"x24T - Rear coaster brake hubs found commonly on beach cruisers, Huffy brand, older department store bicycles.
13/32"x26T - most Sturmey-Archer hubs.
M12x1.25 - E-bike hubs - either 1.0 or 1.75 thread pitch.

So what's the alternative? Well, the technique is quite simple:
Step 1- Tension the chain with arm strength by adjusting the axle position with your horizontal drops or eccentric BB/hub. Carefully observe the axle position so you can remember it.
Step 2- Take the chain off the cogs, move the axle to a slacker position so you can derail the chain or whatever you gotta do to get it off.
Step 3- Set the axle position or eccentric a couple mm tighter than you had it in step 1.
Step 4- Put the chain around the rear cog and over a few teeth of the front chainring. You won't be able to get it all the way around because there's not enough slack.
Step 5- Turn the cranks with your hands so that the chain is forced to climb itself all the way onto the chainring. You might have to also push the chain laterally hard with your other hand to help guide it just as a derailleur guides a chain towards climbing onto a bigger cog.

So, in summary, the technique is to set the axle to a position that would be a little tighter than you can tension by hand (without the chain on) and then force the chain to climb onto the cogs like a derailleur does. With this technique you can get the chain just as tight as any rigid mechanical device.