tony, galvanized frames are cheaper and easier for most to build, so i would imagine that has alot to do with why skookum and others tend to use galvanized steel tube. but why are the bumpers,under the plastic skins, on most new cars aluminum. there are quite a few aluminum subframes in cars also. the main reason they dont build aluminum frames is your car would cost alot more. also if i remember correctly airplanes are built out of aluminum......they seem pretty strong