Originally Posted by Rocky_Motor
...The real reason I wanted to use rubber was to prevent the mounts from vibrating and slowly working the aluminum frame down till it brakes. Maybe that won't happen at all or not for a very long time. Talking with mdlee he warded me against aluminum frame because of that but there aren't many steel frame mountain bikes.
I've found the anti-aluminum bias to be completely unsubstantiated, I've yet to see an aluminum frame broken as a result of motorizing - what failures I have seen were a direct consequence of poor mounting/build technique, such as "rubber" mounting and/or improperly torqued/substandard fasteners, which would ofc be problematic regardless of what material the frame is made from.
Having said that, the frame's quality should be examined thoroughly - regardless of if it's steel or aluminum it should have quality, uniform welds & a good design... this doesn't mean "expensive" but just not shoddy like the very cheapest of the box store bikes, which while many have used w/success, they can't be assumed to be all constructed the same, which is why there's some that have failed and some that haven't - like the Cranbrook's upper chainstays *shrug* (steel bike BTW).
As an example, my Schwinn Aluminum Comp mountain bike. While not a "quality" bike by any serious bicyclist's standards and was a mere $200 - the frame's design is exemplary (strength/gussets etc) with simply gorgeous welds, which is why I bought it instead of a cheaper steel bike. While the other components are pretty much junk (derailleurs wheelset etc) - the frame itself has proven incredibly rugged, surviving over 10,000 miles, years of unrelenting abuse (rough roads @ stupid speeds) and even a major, "cartwheeling" crash (a friend & he's ok lol) without even a sign of fatigue or any damage at all to the frame - despite the fact it's aluminum.
As an example of something that should be examined no matter the material, here's my (yea, filthy) Schwinn's welds & gusset vs an inexpensive steel mountain bike's complete crap welds... and that isn't near as bad as I've seen on some o_O (click to view detail);
(I don't actually advise this bike as a candidate for motorizing, not because it's aluminum - but because the engine doesn't actually fit w/o a LOT of mount mod/fab that isn't worth the bother)
It's not the material it's made from, but how it's made.