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20 x 2.25 tire mm question

Old 01-13-12, 09:22 AM
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20 x 2.25 tire mm question

I am putting a basic cat eye computer on my sons bmx style bike.

Question is, the book does not have 20x2.25 tire size in its chart, so I don't know what mm to set the computer at. Does anyone know what this magical number is?

The closest the book has is 20x1.75 at 152mm

thanks!
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Old 01-13-12, 09:47 AM
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The "book" value if 152 cm (not mm) for a 20"x1.75 tire assumes a roughly circular tire cross section and ISO 405 wheels. The calculation is rim diameter plus 2 times the tire diameter (converted to mm) all times Pi and divided by 10 to give the value in cm. So for the 1.75" tire example:

(405 mm + (2*1.75* 25.4))*3.14159 = 1551 mm = 155 cm or about 2% above the book number

Using the same calculation for your son's bike with 2.25" tires it's:

(405+(2.25*2*25.4))*3.14159) = 1631 mm = 163 cm. Say 160 cm compensating for the apparent non-circular tire dimensions as above.

So your initial setting should be 160. You can confirm that with a "roll out' measurement or against a calibrated course but it should be close.

Last edited by HillRider; 01-13-12 at 09:52 AM.
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Old 01-13-12, 01:55 PM
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thank you!
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Old 01-13-12, 02:15 PM
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btw, the variance in number from the book and calculated, is because tires compress and this changes the roll out diameter ever so slightly.
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Old 01-13-12, 02:18 PM
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Do a roll-out. It is the most accurate way.
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Old 01-13-12, 03:27 PM
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2 people.. rider and spotter,
or a dab of paint you ride thru, getting on the tire then laid back down a few times,
for a roll out while riding , the tires properly inflated , like you would have them while riding.
is really most accurate.
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Old 01-13-12, 05:25 PM
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Guys, this is a BMX-style bike for a young boy. Micrometer level accuracy isn't needed.
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Old 01-13-12, 05:33 PM
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Never too young to start learning OCD behavior.

Some would say, though, that a roll out is simpler and faster than doing the math. On top of being often more accurate.
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Old 01-13-12, 06:41 PM
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Originally Posted by JanMM
Some would say, though, that a roll out is...faster than doing the math.
Perhaps in the days of the abaci.
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Old 01-15-12, 03:28 AM
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I just wrap a string around the tire and then measure the length of the string.

Unless you run at exactly the same pressure & temperature every time you ride, it's more than adequate.
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