Hi Graham,
Having already lived through the demise of the 27in touring tire -- and the first death of the 650B* -- nothing really surprises me anymore.
I don't mean to sound cynical, but as the market share decreases, designers often seem to do something to obsolete established old standards in favor of generating interest in and sales of the "next best thing" -- even though that often means dredging up innovations that have already come and gone decades before. Sometimes these things work better, sometimes the same or worse. In any case, it all seems new and has the effect of being backwards non-compatible, which stimulates the purchase of new stuff -- and the unfortunate retirement of the old stuff when maintenance and replacement parts become hard to obtain.
Among such things...
Rear dropout width...I have bikes ranging from 120mm OLN (Over Lock Nut) measurements to 124, 126, 130, 135 and 145mm (the last my tandem). I cannot swap rear wheels between these bikes.
Bottom bracket shell dimensions.
Rear derailleurs that used no cable housing so the frames produced for them had no cable stops (the last was the SunTour Superbe Tech L:
http://www.disraeligears.co.uk/Site/SunTour_Superbe_Tech_L_derailleur_4800.html ).
A whole slew of handlebar stem clamp dimensions -- 25.2, 26.0 and now more commonly 31.8mm. Not a stopper for frames, but can make interchanging parts a bit more difficult when changing stem length or handlebars.
Quill stem/threaded steerer versus clamped stems/threadless steerers....and threadless steerers in various diameters. Anyone remember 1in threadless? Kinda hard to find a headset now...almost as difficult as 1-1/4in threaded oversize. Now we have tapered steerers and sometimes head tubes as well.
Various kinds of integrated headsets...and the specialized frame head tubes meant to house them. I recall some that used the frame's head tube as the mating bearing race, a notsogood idea.
French component frame dimensions (headset, BB, quill stem diameter)...and the rear derailleur hangers intended for them.
U-brakes...and replacement pads for them; they have slightly oversized shafts and the posts are too high to retrofit with cantilevers or v-brakes.
Steerers with a flat instead of keyed kerf for cable stops. Anyone seen a front brake cable stop with a D-opening for flats recently?
"Big" and "Little" 20in rims and tires...one is 451mm BSD (Bead Seat Diameter) while the other is 406mm and though both are nominally 20in, they don't interchange. I have the same problem with my late father's 1938 Hercules coaster...it has 26in tires (571mm) that do not match contemporary (559mm) 26in diameters...or bead design. I use tires intended for period Schwinn cruisers and their old Leviathan tandem.
Chainring and cog width...and the width of the chains intended for various sizes...and the quick-links for them. Not a stopper in itself for frames, but the number of speeds correlated with rear dropout spacing and that can make for upgrade problems.
...and so on.
Any thoughts on what is happening, or will happen with 26" tyres? Will they go extinct? Harder to get? Less range of available types and widths?
Fortunately, there is a large installed base of fat road-oriented tires in the trekking bikes market and I think this will ensure a steady supply for some time, likely the same for knobbly MTB tires.
I haven't seriously started stockpiling 26" tyres...well not yet. Well may be just a few extra tyres ordered just in case.
I tried that with 27inchers with mixed luck. I stored them in cross-matched plastic garbage bags with the air pulled out using my ShopVac, then stored them in a cool, dark closet. I found they were still good after 20-off years but with something odd: They deteriorated much sooner once out of their little nests than if they'd been new. It was like accelerated aging. Gum/Tan/Skin sidewalls were where it showed first, with the surface becoming cracked very quickly from age, UV, and ozone exposure once they were mounted. Tires with black sidewalls (really, the tread material molded onto the sides) were much more resistant. A wholebuncha Avocet Duro 20s and 30s did fine until ridden on rainy days and then the sidewalls just sort of dissolved, turning white very quickly and then appearing to lose any sort of latex coating before the threads in the weave came apart. Most disappointing. Some old 700C silk sewups (tubulars) did pretty okay, but the rubber tread glazed much more quickly than the clinchers did once released from captivity. You might have similar luck or better or worse. The outcome depends on a lot. I still have some 27inchers in storage and this is a good reminder to take a look at how they're doing. Some have been sleeping for over 25 years in my gear lockers.
All the best,
Dan.
*In 1995, the Brotherhood of the 650B was founded in an attempt to resurrect this tire size which had fallen from favor among mass-production bike manufacturers and so were sometimes difficult to get as replacements. I remember placing a 650B tire order at a local shop for a friend's bike in the early 1980s. We were only able to get some really old, sun-rotted Wolbers after a nearly 8 week wait. Pitiful. I followed the Brotherhood's mission with some interest. You can go back through their archives here:
http://confreriedes650.org/nos-randonneuses/Google English translation here:
https://translate.google.com/translate?hl=en&sl=fr&u=http://confreriedes650.org/nos-randonneuses/&prev=search...and a bit of translated history and a mission statement here:
https://translate.google.com/translate?hl=en&sl=fr&u=http://confreriedes650.org/nos-randonneuses/&prev=search