The difference in weight between carbon and alloy isn't that great. I think carbon has more benefits than just weight which has lead to its widespread adoption. However, you realise that Ultegra and Centaur cranks are not going to fit on a mountain bike? What do you call 'kinda heavy'. Mountain bikes are generally heavier than road bikes and it probably isn't a such a great issue if it's a good bike. For example, a friend of mine has just bought a Voodoo mountain bike (Aizan, I think) which weighs over 15kg but gets very good reviews. You could spend a small fortune on carbon bits and pieces and make very little difference to the overall weight.
Last edited by jgwilliams; 12-15-20 at 03:54 AM.
Reason: Fix bike name