Originally Posted by
3dvvitch
Ah yeah that's helpful, thanks! I just found a 1974 Bike World Magazine review of Speedwell, Flema, Teledyne titanium frames, and it seems like figuring out the appropriate level of stiffness was a little hit-or-miss for the early fabricators. Considering that the Trecia bikes were supposed to ride pretty well without using particularly oversized tubing, I think it's safe to infer that the tubes were rather thick (by today's standards).
Yes I should think getting on for 2mm as Ti is about half the stiffness of steel so you'd need to double the wall if you don't change the diameter. It's possible Ti alloys weren't as strong back then as they are now. If you need 2mm to get the strength you will have the stiffness you need without going oversize. Or maybe they couldn't draw them as thin. Ti has some strange properties and is a bit of a beast if you try to bend it or do anything with it. But Reynolds seem to able to double-butt it now!
The Ti alloys used for tubing now are similar strength to CrMo but half the stiffness. CrMo is about 800MPa strength and 200GPa stiffness. Reynolds 3AL-2.5V Ti is 900MPa and 115GPa. So you go oversize. The tubes they sell are 0.9mm at the ends (vs about 0.8mm for a typical quality CrMo tube) with diameters from 31.75mm to 44.2. A typical steel bike would be 25.4 on the TT (or maybe 28.6) and then either 28.6 or 31.8 on the DT.
The Reynolds Ti tubes are pretty expensive (about 7-8x the price of quality CrMo) but cheaper bikes probably use 0.9mm plain gauge. I saw a video about Moots bikes I think it was and they had a big rack of about 8m long Ti tubes they were just cutting up so they must have been plain gauge.