Evening all,
Interesting discussion...
I think the tendency towards custom stainless steels, rather than custom Ti is down to a few factors:
- -Good cheap Ti frames are available, even full custom, from China and Taiwan, but not stainless, so there's some differentiation in the market if a builder uses stainless.
-Similar material costs for stainless and Ti make the extra hassle involved with Ti combined with the diminishing Ti premium (see above) make it less attractive for small builders these days
-Because of the contamination risk, Ti fabrication does not mix well in a workshop which also builds in steel. Stainless and cro-mo are far happier bed fellows. The current 'steel-is-real' and artisan building boom does not combine well with the clinical cleanliness needed for Ti.
-More steel tube choice, butting and shaping now available than in Ti, especially for smaller builders who can't afford to have tube runs made for them. Custom Ti is inevitably mostly plain gauge mill spec tube, except for the bigger or more exclusive players
On modern steel frames:
The EN rules don't actually specifically test or require a minimum stiffness as such. However, they do have strict yield and fatigue requirements, which, of course, go hand in hand with stiffness. View a copy here: [https://www.dropbox.com/s/gb8flhrixib2jha/EN%2014766%20MTB%20.pdf?dl=0]
This, combined with tapered headsets, BB30, dropper posts, slack head angles, curvy stays, disc brakes and fatter, longer frame tubes have easily added 500g to the average production steel frame.
All steels are (pretty much) the same stiffness for a given tube dimension. High end bike steels these days have far higher yield and tensile strengths, so can be drawn with thinner walls and still spring back. However, they also need to be drawn to fatter diameters to get the basic structural stiffness back to reduce the fatigue/yield risk from the increased flexing strain. This runs them very close to the buckling limit (Coke can failure).
As an example, the current 2017 Dedacciai Zero Uno MTB 44mm down tube might have very fine 0.7/0.5mm butting but it still weights in at 390g and still has a marginal fineness ratio of 1:62.
The 31.8mm Columbus Cyber OR downtube on a decent 90s Kona is a chunkier 0.8/0.5mm wall, but weighs in at 272g and has a reliable 1:40 fineness ratio.
The modern tube will be approximately 28% stiffer and 30% heavier than the older one, but more likely to dent or buckle. So no obvious gains there. The old tube is less stiff (mainly from the smaller diameter), so will flex more, making it a little riskier (as per the EN tests) on fatigue and yield failures, but is also less likely to dent or buckle, even taking into account the lower yield strength of the old (Nivacrom) steel.
Plenty of tubes are still around and being newly made to the older style specs...it's just that the tube manufacturers and larger builders are playing it 150% safe now that there is a legally enforceable standard, so they list them as cyclocross or road only.
There's been plenty of research into the ride benefits of less stiff, light steel (road) frames from the guys over at
Bicycle Quarterly.
All the best,