Weed-eater bike conversion - Safe rpm? I've recently started working on retrofitting an old bike with an even older weed-eater engine.  The engine will sit atop a bicycle rack and directly drive the rear wheel tire (engine has a built-in clutch).  I've already constructed the engine mount and obtained other hardware. http://imgur.com/a/we3bn  

My concern is that once I get moving (maybe 15-20mph) the chain will be moving so fast around that tiny sprocket that it will come off.
My concern is that once I get moving (maybe 15-20mph) the chain will be moving so fast around that tiny sprocket that it will come off.


You can use a chain tensioner to keep tension on the chain, so some slack on the chain won't cause the chain to skip off the sprocket.