Originally Posted by BigBlue
It's also the grade (quality) and source of the metal that makes a difference between a good frame and a not so good frame.
As far as aluminum, I am sure the more expensive mountain bikes use a different grade of aluminum than the aluminum cruiser bikes.
Most of the older steel frames are good for motorized bicycle. Just be aware that some older frames, like Huffy, had pressed fitted and spot welded chainstays. I don't think I'd take a chance without having some brazing or welding done on the chainstays.
Reinforcing the dropouts on aluminum? You could probably find someone to weld, but without heat treatment, you'll probably have more problems down the line.
Go with steel and you should have no problems with having the chainstays reinforced.
Heat treating is everything with aluminum. You should NEVER weld on aluminum bike frames unless you KNOW the alloy used and how welding heat will affect strength.
Adding gussets to aluminum frames can weaken them more than it reinforces unless done correctly by an experienced welder.
Just my opinion from 30+ years welding.
If more is better, then too much is just enough.
If you can't afford it, build it yourself.