When a 24 isn’t a 24

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I’ve got 2 24 inch wheels but as you can see one is clearly larger. The wheel is stamped 24 1 5/8 which if I’m reading Sheldon brown’s article right was a Schwinn oddball size.
I have another 24 that looks normal, but it’s smaller than the wheel that’s stamped 24....what am I missing here?
 

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Its that new math that your kid has been telling you about.
A 24x1.75" tire does not fit the same rim as a 24x1-3/4" tire.
I have heard that tire sizes with decimals are based on bead diameter and tire sizes with fractions are based on diameter at the tread.
 
That's a 24 x 1 3/8ths. There are 2 sizes, one for skinny tires and the other for skinny and fatter tires. 24 x 1.75 would be a cruiser style while the 1 3/8ths is for lightweight racer style bikes. Schwinn had made a slightly larger diameter size so you would have to buy their tires but your wheels aren't those.

26 inch wheels are the same way, 2 common sizes with different diameters. You have to look at the size stamped on the rim. Usually you can tell by the width of the rim.

There is a lot more tire selection at the 1.75 size, with mostly basic tires at the 1 3/8ths size.

24 inches is what the diameter would be including the tire. The 1.75 rim is smaller but the tire is taller, and vice versa, so they are both right at 24" with the tire on.
 
I've built scores of single speed conversions and about 10 with 24" wheels. My preference is to use aluminum rims from 24" bso mountain bikes. They are all 36 spoke hole count in that product range so the rims will fit most old school hubs. 24" bsos are mostly free around here. $5 tops. Look for one with good rims. Those chinese aluminum rims are "acceptable". Probably the only usable part on those bikes. the target age is 10 to 14 years old. Most get outgrown before they wear out. Or because they ride so bad, they deter kids from riding them at all. Bottom feeder brakes, gears and suspension make for a *&$% ride. Rim size is 24 x decimal point so there is a huge variety of tires to fit in colors, widths, tread patterns. the decimal sized rims are usually smaller in diameter than the fractional sizes so you get more room for bigger tires. Scuff them up with sand paper and you can paint them any color. Light weight rims will improve the ride of any bike.

Any bike I've done with 24" fractional sizes, well, I toss the original rims and go decimal.
 

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