Originally Posted by
Kuromori
To be pedantic, steel frames do fatigue, so they do "wear out" in a sense. People often don't get this because steel frames can often last a lifetime or two without any failure, and they often die some other kind of death before fatigue causes cracking. The idea that steel frames don't fatigue because steel has a fatigue limit is a fantasy fueled by people who have the romantic notion that steel is forever. So is the idea that steel doesn't fatigue because it isn't bent past the elastic limit, due to a misunderstanding of how breaking paperclips applies to bicycles. However this doesn't really change the material properties, it's just the source of frame cracks that show up and not caused by crashes or whatnot. The stronger the steel (in fatigue strength), the stiffer the frame (thicker tubes, smaller frames), and the lighter the usage (smooth roads, not being a pro sprinter), the more lifetimes a typical steel frame will last....
Originally Posted by
Salamandrine
True, the reality is slightly more complex than steel doesn't fatigue unless it is bent past its elastic limit. Regardless, for practical purposes this fuzzy line isn't terribly pertinent to the OP's question. An old bike isn't going to get more flexible. As you point out, the properties aren't going to change... .
It is true that steel has a stress limit that, if not exceeded, will not result in fatigue. However, this fatigue limit can be lowered substantially by heat. Overheating a frame during brazing can result in the fatigue limit be lowered to the point where normally experienced stress will exceed the limit and cause fatigue failures. This is why high grade frames are often manufactured with silver solder. It has a lower melting point, reducing the probability of overheating. It is also why lightweight frames should be built by experienced craftsman. The thinner the tubing, the easier it is to overheat, so you want somebody with a lot of experience in controlling the heating. You can't tell if a frame has been overheated until it starts to fail, so you're putting your faith in the reputation of the frame builder.