Originally Posted by RockyMtnMerlin
Please also note that the test Tom B refers to was done in 1997. Moreover, don't stop reading after the results section. Go down to the Interpretations section and note the testers stated limitations of their own test. Finally I'll bet that in the intervening 10 years that most quality frames can do more "cycles" than they did back then due to improved fabrication techniques (et al).
I've talked about this sort of testing before. Those test results are both informative and yet not. Because most frames will last almost forever under the nornal loads of cycling, they force them to fail by turning the loads up to sometimes bizarre levels.
Any frame material will fatigue given enough force.
But here's the problem - carbon fails because of invisible flaws in the layups whereas steel only fails the fatigue test if you turn the loads up above steel's quite high fatigue limit. You can never tell when a carbon frame will fail simply because although the fatigue limits on a well made frame are essentially infinite, these frames are being made progressively lighter and they're pushing the limits of the material to the point where the maximum STRENGTH limits are being reached when you hit a pothole or a high manhole cover. Failures of this sort aren't predictable by fatigue limit testing.
Titanium frames are fine, a good design will last as long as any other frame and they aren't any more flexible than a steel frame if you buy conservatively. My Eddy Merckx EX Pro (built by lightspeed) is strong enough to carry my 200 and some odd lbs of all up weight and it still weighs 3 lbs less than my Eddy Merckx Corsa and a lb lighter than the steel Basso.
I have a couple of carbon frames and I trust them because I've never gone for the lightest you could find. So I don't worry about my Look KG241 or my Colnago C40 and when someone tells me that their street ride weighs under 16 lbs I wence. Personally I value my face and teeth and so ride something that isn't likely to break at the worst possible time.