Originally Posted by
ridelikeaturtle
Titanium is also very springy. If you could take the rear wheel out and try to flex the rear triangle, aluminum would be very stiff, while titanium would move quite a bit (and spring back). Again, the aluminum chainstays and seatstays would have a bit beefier tubing as well.
This is often (usually?)
not true. When Damon Rinard did his frame stiffness testing, a Cannondale 2.8 showed one of the most flexible rears of all (over 60 frames tested).
The problem is alu needs to be oversized to have decent stiffness, because it's about half the inherent stiffness (Young's Modulus) of Ti. But chainstays are constrained by chainring and tire clearance, so alu will almost always lose to an
optimized chainstay in Ti.
Many early Ti frames had
undersized chainstays, due to the lack of commercially-available tapered tubing, so they went with a 3/4" tube that was too big at the dropout and too small at the bottom bracket. Stiffness at the BB is almost all that matters in a test like Rinard, or in the ride feel. The tiny 3/4" chainstays is the reason for any Ti frames in the Rinard list that showed a flexy rear. At least with undersized chainstays you get the advantage of better tire/chainring clearance. No such advantage on a Cannondale.
Ti chainstays should be a bit larger than steel, due to Ti having a lower modulus than steel. Not as oversized as Al, but around 1" is good in Ti, vs 7/8" on classic steel. (Now of course there is also OS steel, a bad idea IMHO.) Back when Rinard did his testing I think there were zero commercially-available Ti frames with 1" chainstays. Maybe on the custom Merlin track frame made for professional sprinter Ken Carpenter — which I happen to know he didn't race on, because it was too flexy for him. I know this because I made the steel frame that he kept on riding when he was sponsored by Merlin. They made him a second frame (supposedly "twice as stiff") after his review of the first one was unprintable, but even the second was called "an abortion" by Ken. They painted my frame to look like Ti, and put Merlin decals on it! But I digress... Sorry I know steel is off-topic for this discussion.
Anyway, most any welder can tell at a glance whether a frame is Ti or Al, so maybe that's the way to tell — bring it to a welder or bike frame builder. The ability to tell by looking can't be taught over the internet unfortunately.
Mark Bulgier
Irony Cycles