My 2000 Rockhopper Pro has a Surley Rigid front fork and 26 X 1.5 Nimbus Armadillos. Also have a 2003 Specialized 24" TR-X for MotoXing with my kids. And a 2002 XR400 which I don't ride nearly enough
Derailleur rubbing on the chain ring when I pedal hard
When pedaling lightly no problem. But when I start to put pressure by either going up hill or a sprint they rub. I was thinking it may be the cranks flexing under the stress.
The bike is a 2000 RockHopper pro. Alum frame and cranks
You didn't say what crank you have now! Problem is that any good step up crank for a MTB is expensive like the Shimano XTR at $450; you could try the Shimano XT unit and see if that works. Another problem is that if it's frame flex and you spend money on a new stiffer crank-your still going to have the problem. You should take the bike into your LBS and have it checked to make sure frame flex is not the issue.
But I don't know that much about MTB's so I'm just guessing.
Actually I think it's the bottom bracket that's flexing, not the cranks ... or maybe a combination thereof.
I found the new Shimano XT M760 cranks/bb combo set to be extremely stiff ... if there's any flex at all, it's the frame (it's an old Trek 7000 mtb frame). I also weigh about 135 lbs so I'm not that hard on equipment.
Some flex is normal. If it's really bugging you, I'd go for the new set.
I wouldn't use the XTR cranks if you have concerns about longetivity ... the chainrings are all aluminum (for weight savings), so they wear faster.