ljsense

I broke a seat tube right underneath the seat post lug, and it was no big deal. Like someone else who broke a seat tube mentioned, the bike felt really springy afterwards. I pedaled it home, nice and easy.

If you break your chainstay, your rear triangle will be less rigid. There's a good chance you'll drop your chain if you're putting a lot of pressure on the pedals, either to slow down or speed up. If this happens when you're going down hill, maybe you can just stick your sneaker against your fork crown and front wheel and be fine. Or maybe you have a hand brake to slow you down.

Even if you're thrown to the pavement at 35 mph, odds are good that most of your injuries will heal within about 6 weeks.

Two other notes. Fresh welds are very susceptible to rust. And it's easy to repair cracked carbon -- you just lay up fresh layers of carbon and epoxy. You don't even have to fire up a welder.

DCB0

It is certainly possible that a cracked chainstay will not result in an immediate crash... but you never know. I had the NDS chainstay of a GT MTB frame suddely break at the dropout and I was able to coast home.

The odds of an intact frame causign a sudden and uncontrollable wobble or veer into the path of an oncoming car is practically nil. The odds of this happening to a frame with a crack is significantly higher. You can parctically eliminate this risk by not riding a cracked frame.

Your choice. I wish you the best of luck.

A frame builder can likely replace the entire cracked tube for less than the cost of a replacement frame ande with greater durability than welding over the crack.

mikezs

My brother has been riding a steel frame with a crack in it from rust at the chainstay for about 3 years, and he's fine. The bike got stolen, so I am unable to tell you how long the frame lasted before breaking!