My bike frame flexes when I stand up to pedal. Will this eventually brake my frame?

I'm riding an old steel Schwinn LeTour. It's probably got a couple thousand miles on it, and just recently I converted it to fixed gear.

noticeable flex in a steel frame is not uncommon, but it should feel "springy" not "mushy". mushy would indicate something is already broken, or improperly adjusted. steel frames (when well cared for) are very durable--but in a technical sense this flexing will eventually lead to a broken frame...although probably not in the time you own it. steel frames can flex like that millions of times before anything will actually break.

most steel frames will flex whether or not being butted joints, but no some people get them for those reasons

No. The flex is keeping it from BREAKING. Now if you do mean BRAKING, watch how much your front fork gives when you hit the front BRAKES hard.

Everyone's on the same page here. Flex is not a concern with steel frames as far as breakage. If the welds or lugs are done right and in good shape the frame will last a long time.

NO! Ussually steel flexes and that is the beauty of it. If it was an aluminum frame it will be another thing. Just try to keep rust away and you'll be fine.

PS: Make sure the flex doesnt come from underflated tyres or lose spokes. Many people check frame flex by pushing the BB of the pedal and what they are actually doing is flexing the tyres or the wheels.