Originally Posted by
HTupolev
It was a popular clincher size on US and British road bikes from the 60s through the 80s. 630mm bead diameter; slightly bigger than the 622mm on 700c.
There was some overlap. 700c clinchers started becoming a thing in the US by the late 70s.
I've seen theories that 700c clinchers were introduced to the market to sell high-pressure tires. Many 27" rims didn't have hooks to secure the bead, and using a tire at high pressure on a rim without a hook could be dangerous. 700c looks almost identical to 27", but the difference is large enough that tires aren't interchangeable (although tubes certainly are).
While what you say is mostly true, there are a few places to quibble. I would not say that 700C became a "thing" until the late 80s. There may have been some around in the 70s but, for the most part, they were rare. By about 1988, 700C equipped bikes were mostly standard with a few hold outs.
I would also question the theory about high pressure tires. I had a few bikes with 27" wheels on them in the mid to late 80s and high pressure tires were certainly available for that size. I see a lot of old bikes with old 27" tires on them at my local co-op and many of them have as high a pressure rating as modern tires. Bikes from the 70s might have lower pressure ratings but not a decade later.
I would say that the switch to 700C was driven by the manufacturers. They didn't want to make 700C for the rest of the world and 27" for the US market.
Originally Posted by
HTupolev
650b uses a much smaller bead diameter, just 584mm. With 650b, it's possible to use a very wide tire while still having an inflated wheel diameter comparable to a narrow 700c.
Compared with wide 27", this makes for reduced mass (mostly rotating mass) and ability to fit frames that can't use a wheel larger than 700c.
There's a couple of problems with the 584mm rims, however. First is the issue of brakes. Many of the bikes from the 1980s had the ability to easily change from a 630mm rim to 622mm rim by simply adjusting the brake pad height. But even that adjustment is rather limited. Going to a 584mm rim requires a much longer reach brake which will also result in a spongier brake because the arms of the brake are longer. If the bike is equipped with cantilever brakes, the ability to go to the 584mm rim is further limited because of the fixed brake boss point.
Also the reduced mass gain goes out the window when you start using wider tires.
Originally Posted by
HTupolev
Wide 700c tires also exist; and when they get REALLY big, we call them 29ers. (Same for 650b and 27.5".)
...to the utter confusion of nearly everyone
You would not believe the looks I get from people when I tell them that a "29er" is the same as a 700C or that you can use a "29er" wheel in the place of a 700C wheel. We really didn't need that extra confusion.
Originally Posted by
HTupolev
More relevantly today: 27" is now a legacy support market, more or less. There are a few alright tires that you can get in the size, but nothing amazing. If you want to enjoy high-end current-gen clinchers, 650b and especially 700c have a much better selection.
One thing you missed which most people do is not just that the tire selection is poor. Replacement rims and wheels for a 27" wheel are rare as well. What replacements that are available are of poor quality.