On a couple of my bikes I can see the chain move around a bit inside the FD when I start cranking on it. Is this bottom bracket flex, or flex elsewhere? Everything seems tight (grab it by hand, no movement; no clunking either) but in certain gear combos I can get derailler rubbing. It's enough that I'm considering switching to friction front for this one MTB I have, as it doesn't have a trim feature like my Tiagra brifter does (on my road bike that is).

I've been tempted to mess around with my old Panasonic road bike, make a utility bike to tow a trailer; but I recall it being quite flexy down there--I can't imagine it being any better with stronger legs and more loading.