Originally Posted by Cafe Racer
I have an aluminum framed bike right now and this was going to be my first build. But before I begine I was wondering what you floks feel is best.
If you plan on welding anything at all to your frame in the course of your conversion, use steel. Aluminum is dramatically weakened by welding, and it takes a special (expensive) heat treatment process to bring the strength back. So if you want to weld on motor or tank mounts, racks, braze-on fittings, etc., use a steel frame.
I have never installed a frame-mounted engine, though I have fixed some bad installs at my bike shop. From reading kit installation instructions and the postings on this forum, it appears to be a lot simpler to mount an engine on a frame with small diameter tubes than one with large tubes. Steel frames can be of small or large diameter, but aluminum frames are all large diameter.
Something like a mid-1980s MTB will offer the best combination of characteristics for an uncomplicated conversion-- rugged construction, smallish diameter tubing, level top tube (for ample space inside the frame), stable handling qualities, good clearance for drive sprockets and fat tires, strong brakes, and a relatively long wheelbase. In this category, look for bikes that are high-quality but less collectible like Miyata, Ross, or Univega, because mid-'80s Specialized, Mongoose, Ritchey, Gary Fisher and other sought-after bikes can be very expensive.