I like the idea of a toothed tensioner, but I can see where alignment issues could be a problem. The chain runs pretty much straight from the rear wheel sprocket to the engine sprocket, but the cheap crappy tensioner that comes with the kits clamps onto the chain stay, which puts it at an angle to the chain. The teensioners that come with the kits have a ton of problems. First, they clamp to the round chain stay, allowing them to rotate on the frame, unless you weld them in place. Second, they are made of super soft flimsy metal that you can bend your hands. Third, the cheap plastic roller is just that, cheap plastic that easily gets chewed up. It is also not nearly wide enough. The bearings? are pretty much worthless. The chain will beat one of these wheels to pieces in short order. Part of the reason for that is the chain, which is not uniform from one end to the other, and the other reasons are that the rear sprocket is simple stamped steel, not "hobbed" like a motorcycle sprocket, and then there is the rag joint, which is almost impossible to get perfectly straight. All of this together results in a chain with tight spots and loose spots. And when you have a non spring loaded tensioner that can't follow the chain as it constantly gets tight and loose as it turns, it causes the chain to "whip" This is what causes it to destroy the tensioner, or to come off the tensioner altogether. It can also pull the tensioner into the rear wheel, which is what happened to me.
Derailleurs would make excellent tensioners if it were not for 3 things. One, they are too small for the motor drive chain. Two, they are not robust enough to handle the stress put on them by a motor driven chain. And depending on where they were mounted, they would have alignment problems just like the toothed rollers. They do make excellent tensioners for regular bicycle chain. I have converted multi speed bikes to single speed, and left the derailleur on it simply to tension the chain.
A heavy duty tensioner, with a strong non bendable bracket and a roller made out of something tough and wide with good bearings, like a skateboard wheel, would be a 100 times better than what comes in the kits. But the problem of chain whip caused by the stationary roller would still exist. The best commercially available solution I have seen is a combination of a clamshell rear sprocket mount, which eliminates the rag joint, attaches the sprocket to the hub so it is centered, and does not put any stress on the spokes. Combine this with an engine mounted spring loaded tensioner, and that will solve 99% of the problems with the stock drive chain/sprocket/tensioner
Engine mount tensioner
http://www.ebay.com/itm/49-66-80cc-...hash=item2a6b531cc3:m:m-b3lt30KT8w_ql7lB4QDvw
Clamshell rear sprocket mount
http://www.mmbikeparts.com/Sprocket-Adapter-only-p/a-cb110l-only.htm
I don't know why parts like this don't come with the kits these days. The Chinese are still using the same old junk. My guess is price. Just remember, you get what you pay for.
