Re: Aluminum or Steel? Which do you prefer?
Cafe Racer, Chromoly steel frames are the best. The steel is stronger and dampens vibration better. The tubes are smaller diameter which helps with mounting the engine kit. Here is another thought about aluminum frames. In most cases they aren't lighter than a steel frame or at least any substantial amount. Now let me back this up. If you buy a cheap Walmart or Target aluminum bike the aluminum is a heavier poor quality aluminum. Usually thicker walled too. You won't see any weight savings going aluminum until you get into higher quality bikes. I bought a Specialized Stumpjumper for 1500.00 (not for a motorized bicycle project) and the frame is maybe 1.5 pounds lighter than my GT chomoly frame on my mb. The Walmart bikes that are aluminum are a marketing ploy that tricks the consumer into thinking it is a better bike because it is aluminum.
All that being said, you aren't going to loose 5 pounds on your bike unless you spend more then two thousand dollars on a high end bike. So I would recommend the chromoly since you can find them on craigslist really cheap and they work much better for a motorized bicycle application.