I am considering building a friction drive setup, powered by a 18 cc weed eater engine. I plan on gearing down the rpm with a simple sprocket set up. It would probably be around a 2:1 gear ratio. I have read that the range of weed whacker rpm is about 3000-12000. When you say 7000, is that about where the max power tapers off?
I have also been thinking about what to use for a roller. The main reason i didn't want to build a friction drive was that i was very worried about shredding up tires. I have a hybrid bike ( tires skinnier than a mountain bike, but wider than a road bike) pretty smooth tires. Researching this seems to show that with smooth tires and well tuned setup, tire wear is a much smaller issue. I have used the Gear Rat caculator (somewhere one this site) and figured out that I should use a 1.65 inch to a 1 inch diameter roller. Thats based on my engine producing 12000 rpm WOT, then geared with a 2:1 ratio =6000rpm (then simple math i don't feel like explaining)= 26.8 MPH with a 1.65 inch roller or 17 mph with a 1 inch roller. So probably aiming at a 1.25-1.5 inch roller would be good, I am worried about the torque needed to climb steep hills.
I'm also wondering if i should have a centrifugal clutch ( I have one, but there was not a stock one on the weed eater) or if i should rig up some kind of control to en-gauge the roller on the tire. Leaning towards the latter as it would be easier to adjust the pressure of the roller on the tire. I would use some skateboard bearings to reduce friction from the roller supports, requiring a little machining. I going to post more info on a separate thread if your interested, though this will probably be re-posted there.
