ydm9 top-posted:
"In general", if you ride a 2003 Trek 2100 which has an aluminum frame
and DOES NOT have carbon seat stays and compare it to a 2004 Trek 2100
which appears to be about the same bike WITH carbon seat stays, how
much difference is there in ride quality?

The only definitive way to tell would be to swap wheels, seatpost and saddle
between the two models (with identical bar tape, too!), and even then fit
would probably be the most important "ride quality" issue.

My bike has carbon seat stays and it rides very smoothly, TYVM, but I really
doubt I'd notice if someone magically switched 'em to aluminum. Maybe a 1-2
% difference in road vibration? Who knows...

Bill "but they look cool, and of course that's what REALLY matters" S.