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Old 01-02-08, 07:12 PM   #1
pismocycleguy
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20 inch wheels

Where does the size designation come from for 20 inch wheels? Where do you measure to obtain the 20 inch size measurement? Is it the rim size, the tire size, or the outside diameter of the tire when mounted on the wheel? I need some clarification!
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Old 01-02-08, 07:17 PM   #2
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Read Sheldon Brown's Tire Sizing article.

Keep in mind 20" typically refers to either 406 or 451 wheels.

The total diameter of the rim + tire will vary quite a bit from tire to tire so it is best to measure it.
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Old 01-02-08, 07:22 PM   #3
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Originally Posted by pismocycleguy View Post
Where does the size designation come from for 20 inch wheels? Where do you measure to obtain the 20 inch size measurement? Is it the rim size, the tire size, or the outside diameter of the tire when mounted on the wheel? I need some clarification!
Thanks,
PCG
The 20" size designation must come from the marketing department.

Nominal tire sizes in inches seem to be closest to the ouside diameter but this varies with the tire you fit and the actual sizes are sometimes so far off that even this can't explain it.

The metric sizes seem to be more meaningful. I assume that they are the diameter in mm of the rim surface on which the tire bead is intended to sit. This would be a bit smaller than the outer diameter of the rim.

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Old 01-02-08, 09:15 PM   #4
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Not the rim bead seat, the outside diameter of the tyre (whne the correct tyre size is used). This also applied to British tyre sizes (16", 20", 26", 28").

The bead seat diameter is the only sensible identifier 622 = 700C, 559 = MTB 26", etc.
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Old 01-03-08, 09:28 AM   #5
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The bead seat diameter is the only sensible identifier 622 = 700C, 559 = MTB 26", etc.
Yep ... just go by ERTO sizes.
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