20 x 2.25 tire mm question
#1
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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!
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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#2
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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.
(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.
#3
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thank you!
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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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#6
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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.
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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Guys, this is a BMX-style bike for a young boy. Micrometer level accuracy isn't needed.
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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.
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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Last edited by JanMM; 01-13-12 at 05:37 PM.
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#10
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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.
Unless you run at exactly the same pressure & temperature every time you ride, it's more than adequate.