Originally Posted by
Dirtdrop
You can't expect to get decent advice on the safety of the frame without posting pictures of the damage, but the top tube is under compression and riding the bike is like stepping on an upright dented beer can.
Crumpling a tube only occurs if you've exceeded the 50:1 diameter-to-wall-thickness ratio. No bike tubing is made that thin and the only thing that'll happen is you'll bend it. But you'll also have to bend the downtube at the same time. The only bent downtubes I've ever seen were from front-end impacts like riding into a parked car. In which case, the stress on the top-tube is in tension; a small dent won't make any difference in that case.