Here is something you guys might be interested in.
This company called "White Industries" makes a rear hub that is unique. It is designed to convert a mountain bike from a multi speed bike to a single speed bike. Traditionally there are two types of dropouts on a mountain bike, horizontal or verticle. If you are lucky enough to have a mountain bike with horizontal dropouts then you won't need a chain tensioner because you can slide the wheel back and forth in the dropout to adjust chain tension. But if you have verticle dropouts then you would have to buy a spring loaded tensioner that mounts where the rear derailer use to mount. To make this as clear as possible I will add this: In a multispeed mountain bike the rear derailer has a spring loaded chain tensioner built in so when you convert to single speed and remove the derailer you are also loosing the chain tensioner.
So White Industries has invented a hub that eliminates the need for a chain tensioner on mountain bikes with verticle dropouts.
This hub is built with an offset kind of like a cam. The center line of the exel is offset so that when you rotate the hub it moves back and forth then you lock down the nuts and it stays there. This allows you to adjust chain tension. See the picture in the link for better understanding.
In doing this mod you will end up having a single speed driveline on your pedal side. This is ok since you rarely use that driveline anyway.
You will use this hub to adjust the motor chain tension. If you are lucky the pedal chain will be the same length and tension perfectly at the same time. If not you can try a half link in the pedal chain. If it still wont give you proper tension on the pedal chain when the engine chain is right then you will have to use a spring tensioned tensioner to keep the pedal chain tight. This is still far superior to the chain tensioner on the engine chain. It mounts where your derailer used to be.
I hope all of this is clear ;-)
Also I want to say I bought the hub with the disc brake mount. Then I use the top hatter (sprocket adapter to mount to a disk brake hub) and their sprocket from Kings Sales. This setup is bulletproof. The hub is super heavy duty and the bearings are sealed and heavy duty, along with the tophatter and sprocket the sprocket is bolted right to the hub which is solid and perfectly true.
Here is the link: http://www.whiteind.com/rearhubs/singlespeedhubs.html
Let me know what you guys think!