Originally Posted by
Sy Reene
Thanks! Makes sense and that was my guess.. that somehow width measurement in ETRTO system was contingent upon some sort of assumed rim width, while the 700x?? system maybe not so. That said, even this link offers maybe some outdated into.. ie suggesting that even a 25mm tire is incompatible with a 19mm rim width (internal), when the flavor of the month seems to be to run 21mm(25mm external width) with 23 to 25mm tires. Their chart seems to imply that to run a Hed Ardennes rim, you need a minimum of a 35mm tire.
ETRTO is based on actual rim diameter. Unlike other, older standards.
Chart on rim widts is conservative - on the safe side. If one really knows what they're doing, it can be exceeded. Some rim producers, like Mavic for example, note on their rims what minimum and maximum width tyre rim takes, and what are maximum pressures it can take - the widest tyre having the lowest pressure allowed for the rim to hold.