I realize the rear sprocket should be concentric with the hub, but even then, a beefy spring loaded tensioner would do the job of the idler, without breaking. There are several problems with the stock "idler" one it it's flimsiness. The bracket bends very easily, and the plastic wheel is prone to breaking. Another is the method used for holding the bracket in place. simply clamping it around the chain stay will allow it to rotate on the frame. Yet another problem is the narrowness of the wheel itself. It requires almost perfect alignment with the chain. If it is even slightly off, the chain will either break the wheel, or come off of it. Plus the idler is mounted at an angle to begin with, as the chain stay is not straight from the bottom bracket to the rear drop out.
There is another thread on here several pages long, about using a spring loaded tensioner from Tractor Supply mounted to the seat post. To me that seems to be a major improvement over the stock oem tensioner, but the parts don't seem to be available anymore. It used a much wider heavy rubber wheel with ball bearings. A zillion times better than stock, but I still prefer the rubbing block idea over the roller. With no moving parts other than the pivot, it should be bulletproof. And if it were made wide enough, there would be no danger of the chain slipping off of it from normal side to side chain slap. I have also considered using such a device without the spring. Then it would serve the same purpose of the factory part, as an idler, without being a tensioner. It would be much easier to build and install, and the rubbing block would last forever.
The first time I installed one of these kits, the tensioner failed twice, the first time the cheap plastic wheel broke, and I was ablle to stop in time to save it. The second time the bracket bent into the wheel spokes, destroying the wheel. At that point I abandoned it and went to friction drive, which has worked beautifully here in southern AZ, where it never rains. But I now find myself wanting to go with a frame mount engine again, maybe 2 stroke, but I am seriously attracted to the $G 4 stroke setups. But both require the same tensioner, unless you can get the chain at just the right tension without it. That is almost impossible to do, because of the pedal chain. You can't get them both properly tensioned at the same time. Another idea I had would be to install the motor chain at the proper tension, then use a spring loaded tensioner (probably a derailleur) to tension the pedal chain. But this would only work on a bike with adjustable drop outs, which many multi speed bikes don't have. The bike I plan to use is a former multi speed, converted to a single speed, and has vertical dropouts.
My main reason for wanting to go with a frame mounted engine is looks, as the friction drive works fine. Yes, I admit it, I want to build something that looks like a motorcycle, specifically an antique motorcycle. But it has to be reliable. I also want it to have clean details, which means no threaded rod heated and bent and turned into a clamp. Functional yes, but not pretty.
Anyway I am still working on some ideas, and have access to both a welder and CAD software. I am determined to come up with something that will work well and look good at the same time. I have come up with several designs that will work, my problem is how to build them. Sure would be nice to have a CNC mill or a laser cutter.
