Join Date: Aug 2008
Location: Ann Arbor, MI
Bikes: '15 Kinesis Racelight 4S, '76 Motebecane Gran Jubilée
Mentioned: 9 Post(s)
Tagged: 0 Thread(s)
Quoted: 461 Post(s)
The basic question is, "Do carbon fiber frames and components visibly flex?" The answer is Yes, they do.
I don't ride CF frames, but I have observed CF flex in forks, seatposts, and cranks. The amount and type of flex the OP describes in their chainstay sounds very unusual to me and would be a point of serious concern if the bike were not purchased new from a LBS.
Regarding the seat post, as has been said, mind the minimum insertion mark and it should be fine, some ultralight boutique posts aside, like MCFK, which do have max rider limits, though those are usually at 100kg/220lbs. I wouldn't worry on that count, but if you're concerned, a quick web lookup should satisfy.