I used to sit on the top tubes of all my old bikes with no bad effects, but today I found myself waiting for the pack to catch up, and for one brief second I leaned my derrière and some of my 158 lbs of avois dupois on the top tube. Then I thought, "Could this possibly collapse?" These tubes are very thin and not designed to take a load in this manner. I recall seeing an old picture of Eddy Merckx leaning on his top tube. Why don't they just bend? Not recommended I know, but within the limits of structural design? Garth In Surfside Fl.