I've recently noticed that my Force crank flexes noticeably (enough to rub the front derailleur cage). I can flex the crank and rub the front cage in both the small chainring/largest cog and the big chainring/smallest cog when pedaling with a fair amount of force and/or when standing.

I've done some checkup to the whole BB/crank area. Crank is at the correct torque with no noticeable damage or wear. Same goes for the chainring bolts and bottom bracket. I took out the crank and checked the BB area and there is no noticeable wear or damage, and the BB cups seems fine. I'm not really sure what else to check. Could my chainrings be worn/fatigued to flex enough to cause chain rub? fwiw I've been using the SRAM Force group for about a year now, with about 10,000 miles on it. I've always replaced cassettes and chains after noticeable wear.

if everything were allowed to be heavier , then there would be more stiffness ,
but bike buyers count grams, companied respond.
so I think it is a result of the light weight frame and components being lightened .

Is the flex in the crank or the frame?

It could be that the SRAM cranks have a knack for coming loose. The axle loosens on the spider allowing movement. The fix is to buy Shimano. Here's a picture of mine.



The red arrow shows the movement. The yellow bracket shows the gap.

This happened to me on an all-aluminum Truvativ crank as well.

so i'm considering replacing the bb cups - if the bearings/bb cups are worn to the point that there is movement under pedaling force, would replacing the bb cups theoretically fix the flex if everything else is not the cause? My Alu frame can't flex enough to be noticeable can it?

I don't remember having this chainring rub problem when everything was new, and the bike is in perfect mechanical condition besides this problem.

would getting the ceramic GXP bb make it any better (or worse) in terms of robustness/stiffness? or other after-market GXP bb's like Chris King?

If the crank is moving enough to rub the chain on the fd's cage, that's usually a case of frame flex ot the front derailleur need to be readjusted to clear the chainrings more. If the frame didn't flex with the previous crank, you may have the fd set in too far. I expect changing the bottom bracket will have no benefit and a ceramic bottom bracket, for what ever it's pluses and minuses (that's another thread entirely) will certainly not significantly change the crank/bottom bracket/frame stiffness.