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frame fatigue factors(3 posts)

frame fatigue factorssushiken
Feb 25, 2002 12:26 PM
just to add to my previous topic, am also wondering how susceptible the lightweight steels are to denting.
re: frame fatigue factorsguido
Feb 25, 2002 2:15 PM
Chromium molybdenum tubing is very stiff, that is, resistant to bending, but it is so thin, .6-.4 mm thick in the middle and .8-1.0 at the butted ends, that it will dent fairly easily if struck hard from the side. I have two dents in my top tube. That's where they happen mostly. The bike rides fine, however, because the deformed top tube is stressed front to rear, not side to side.

Steel is touted as long lasting mostly because of its superior "modulus of elasticity." Steel can flex millions of times without developing fissures and cracks, unlike aluminum. I've seen aluminum frames separate at the tube junctions, only once in a badly brazed steel frame.

I'd say steel has very good fatique life, but is still susceptable to denting because it's so thin-walled.
DentedKerry Irons
Feb 25, 2002 5:12 PM
It used to be assumed that the thinnest you could go on steel was 0.6 mm to avoid having something that dented too easily. These frames weighed 2kg or just under. In the quest for light weight, 0.4 mm is common, and I think someone is selling a tube set that is 0.3 mm in the thin sections. If you don't believe this dents easily, then you've bought the hype. Yes, steel can be made light, but this is the price you pay.