I want to build up an Xtracycle, but am unsure what type of donor bike frame material I should look for. I've heard that a lot of people like chromoly or steel frames, but then I also hear that aluminum is lighter and stiffer. What's the low down?
It comes down to personal preference and whether or not you're touring abroad with your Xtracycle. Steel is repairable, so it's great for touring applications where you may need to fix something in far away lands (or just far from the closest bike store).
Ride quality of an aluminium frame with a FreeRadical installed is a bit stiffer than a steel/chromoly frame. More road vibrations will be absorbed by a steel frame, but you'll already get some bump softening by riding a longtail anyway. Join RootRadicals Yahoo! group and delve into the archives for this very, very longwinded debate.
I agree with Rick - you can't go wrong, it's purely a matter of preference. I have really come to like the combo of the aluminum front end (preferably a gusseted mountain bike frame) with the chromoly FreeRad tail. I feel like I get max stiffness with enough cushion to keep the ride comfortable. That said, if I was doing a longer tour, I'd stick with steel to make on the road repairs viable. I believe that carbon fiber and Xtracycles make poor bedfellows, but titanium, beryllium, scandium, all good.