Old dirt bikes had a spring loaded tensioner mounted to the swing arm. There was a shaft welded to the swing arm that it attached to. There was a round spring at the mounting point. the tensioner itself was not a pulley that moved, it was a round piece of rubber, kind of like a hockey puck, with a hole it the middle. It attached to one end of a short arm made of steel, the other end of the arm attached to the swing arm mount. You could push it down against spring tension, and it would come back up when released. the rubber part pressed against the bottom run of the chain, keeping it tensioned. It never required adjusting, because if the chain became looser, it just moved farther up. The chain did not cut through the rubber. It would cut grooves in the rubber, but as soon as they got deep enough to where the actual rollers rubbed against the rubber, that would stop. It worked great on an actual motorcycle, it should work fine on a motorized bike, as long as the mounting point was welded to the frame, not clamped on. The biggest weaknesses of the Chinese tensioner are that they clamp on to the frame, so they can rotate, they are made of very thin flimsy metal which can bend, and the pulley/roller itself is made out of cheap plastic that will soon break. It is also not self adjusting, and no chain, especially a cheap one, is going to be perfect. It is going to have tight spots and loose spots. A self adjusting tensioner will maintain constant tension on the chain as it becomes tighter and looser as it turns.
http://www.cheapcycleparts.com/oemparts/a/yam/50041cadf8700209bc786185/rear-arm
Look at parts 16, 20, and 18. Part 16 is the arm, part 18 is the spring, and part 20 is the actual rubber rubbing block that presses against the chain. These parts mount on a pivot shaft welded to the bottom of the swing arm that you can't see in the diagram. It works a lot like a derailleur, keeping constant tension on the chain. But it is a lot heavier and stronger. Most of the motorized bike chains I've seen pictures of are way to long. they only need to be long enough so they are not too tight at any point in their rotation. Maybe an inch of play at the tightest point. The spring loaded tensioner would then keep tension on the chain, making sure it is not to loose in any spot as it turns.
I currently ride a friction drive bike, mostly because it eliminates that tensioner and the rag joint rear sprocket, and it works just fine if it is dry. But I would like to have an engine mounted in the frame, mostly for looks. I'm getting more and more interested in fabricating my own solutions to the tensioner and rag joint rear sprocket.
