Tom A. wrote:
Might be time to "upgrade" to hollow CroMo steel rails...probably weigh the same as solid Ti rails, and if designed correctly, basically infinite fatigue life.
Nice work out there in any case :-)
Well cromo steel rails would not have an infinite fatigue life. While at a low enough stress, mild steel has a flat fatigue limit, it would not really apply for saddle rails. First there is a knock down for the heat treatment (and if you are not heat treating, then you might as well go with mild steel). Second, you are going to have a stress concentration where the saddle clamp attaches to the rail. This concentration where the saddle clamp ends and where the rail is free will be another knock down that cause the fatigue life not to infinite. I am guessing that is the stress concentration that drives the design, maybe where the rail is attached the saddle is another concentration. Third, you will have to deal with corrosion causing a fatigue failure in steel, since you may wear away any sort of protection where the saddle clamps. Carbon fiber may actually be a pretty ideal material for this kind of clamped joints. Most of the stress concentration will be in the polymer matrix, so it take the most of the load preventing from sliding. Polymers are pretty good for this sort of stress concentration. Even if it really stresses some of the carbon fibers and they break, their load will be transferred to fibers not being bent by this clamp.
My random guess for failure of the rails is fretting. This joint, especially if the saddle is taken on and off, will be subject to fretting. Basically some of the oxidized coating the saddle rail is rubbed off in the clamp joint. Since this oxide is so hard it just slowly wears away at the softer titanium creating a sharp stress concentration.
I am trying to get the parts back (tech support still has it), but what's weird is that that the saddle did not SEEM to fail at the location of the clamp. Obviously my exact recollection is not super reliable as I was in the midst of an Ironman, but I remember where we took off the saddle shell (it was no longer attached to anything) and started loosening the bolts to pop the saddle off, I remember thinking, "huh, that's a weird place for the rails to snap..." It was basically halfway between the clamp and the insertion point into the shell, at all four locations.
I actually agree with you on the carbon. I think carbon is a pretty good material for saddle rails, especially if you do it how Fizik does it where it's a carbon rail and then they wrap the rail with a carbon overwrap that takes the stress of the clamping, so you separate the clamping forces from the stresses on the rail. One thought I had was to try wrapping the titanium rails with a thin wrap of carbon. Not that this is likely to ever happen again, but "semper paratus..."
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