I have a 2005 Trek 6700 Hardtail MB. It's an aluminum frame. I ride it mostly on easy dirt / gravel paths, as well as on pavement ... it's my basic work horse. I have used it for some single track, but maybe only 20 hours worth. For this type of riding, I doubt the frame will give out. If you're a more hardcore mountain biker, then other materials may be more suitable.