20 x 2.25 tire mm question
01-13-12 | 09:22 AM
#1
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the commutor / tourer
Joined: Aug 2004
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From: jacksonville fl
Bikes: trek 6700 turned touring machine, giant TCR2
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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"Ready to retire, just can't afford it yet!"
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01-13-12 | 09:47 AM
#2
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From: Pittsburgh, PA
Bikes: '96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '20 Surly Midnight Special, All are 3x10. It is hilly around here!
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.
01-13-12 | 02:15 PM
#4
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From: A Coffin Called Earth. or Toronto, ON
Bikes: Bianchi, Miyata, Dahon, Rossin
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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Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;)
https://sanfrancisco.ibtimes.com/arti...ger-photos.htm
01-13-12 | 03:27 PM
#6
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From: NW,Oregon Coast
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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.
01-13-12 | 05:25 PM
#7
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From: Pittsburgh, PA
Bikes: '96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '20 Surly Midnight Special, All are 3x10. It is hilly around here!
Guys, this is a BMX-style bike for a young boy. Micrometer level accuracy isn't needed.
01-13-12 | 05:33 PM
#8
rebmeM roineS
Joined: Jan 2006
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From: Metro Indy, IN
Bikes: Bacchetta Giro A20, RANS V-Rex, RANS Screamer
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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Bacchetta Giro A20, RANS V-Rex, RANS Screamer
Bacchetta Giro A20, RANS V-Rex, RANS Screamer
Last edited by JanMM; 01-13-12 at 05:37 PM.
01-13-12 | 06:41 PM
#9
Low car diet
Joined: Aug 2007
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From: Corvallis, OR, USA
Bikes: 2006 Windsor Dover w/105, 2007 GT Avalanche w/XT, 1995 Trek 820 setup for touring, 201? Yeah single-speed folder, 199? Huffy tandem.
01-15-12 | 03:28 AM
#10
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From: Mid Willamette Valley, Orygun
Bikes: 87 RockHopper,2008 Specialized Globe. Both upgraded to 9 speeds. 2019 Giant Explore E+3
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.
