As an owner of cars and motorcycles I’ve always thought of “wheel size” as referring to the metal thing (often called rims) that the tyres sit on. For example my Toyota 4-Runner is fitted with 15” alloy wheels, but with tyre fitted the complete package stands at 31” tall.

So….. why is my folding bike referred to as having 20” wheels when the rim is only 16.5” and it is in fact the tyre that measures 20”

Just curious as to how / why this system came about?

Paul

When people get down to nuts and bolts they will be specific about wheel size. So, you'll see conversations about 406 vs 451 mm rims while still talking vaguely about 20 inch wheels.

I suspect that the fixation about the outer diameter vs the rim diameter is that bikers care deeply about their gears, and gears are commonly described in gear inches or development. Both of those systems require the use of the outer wheel diameter.

Speedo

Keep in mind that cars aren't exactly a model to follow. The tire sizing is completely messed up. 225/55R15 Has three different types of measurements in there. Millimetres, Percent, and Inches ...

Whoever designed that one deserves a good slap in the face.

And then there's Campi axle threading, 10mm x 26 threads per inch

I'm a car nut... the car tire size thing makes perfect sense to me ... or atleast, I understand it perfectly.

I'd agree about bike size relating to the gearing.

Also... atleast for me...

Tire = the rubber thingy
Rim = the metal hoopy
wheel = combination of the two.

No, in bicycle terms: wheel = hub + spokes + rim.

For me
Tire = the thingy wot the Alpine Classic does to you
Tyre = rubber thingy

Tire, tyre, to-MAY-to, to-MAH-to...

Ad alta per angusta: To the top on skinny rubber thingys?

Heh heh, quite good! Better than the original...