# Bicycle Mechanics - what is tire size code for my cheapo computer

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cvcman
07-09-12, 11:54 AM
I have a cheap computer on my bike, when I chg the battery I loose the 4 digit code for my 700 x 35 tires, next time I get it I'm going to engrave it on the top of the unit....

02Pilot
07-09-12, 12:04 PM
You can just measure it yourself. Rotate the front wheel to have the valve stem at the bottom, mark the ground with a piece of tape, roll the bike forward enough to rotate the tire back to the same position (valve stem at the bottom), mark with a second piece of tape. Measure the distance between the two (in millimeters) and you're done. This method takes care of variance between different tires as well.

oldbobcat
07-09-12, 12:08 PM
The 4-digit code is the wheel circumference (inflated) in mm. 700Cx35 is 2168mm, according to Cateye. http://www.cateye.com/files/manual_dl/0/415/tire_size_chart_v2.pdf

Bill Kapaun
07-09-12, 12:27 PM
Some different computers use different "calculations".
Often if you look at the chart provided, they don't even agree with themselves vs different tire sizes.

As a "rule of the thumb" try-
(622+(2*35))* PI
That will give the "theoretical" tire circumference.

Spld cyclist
07-09-12, 01:10 PM
Since there can be substantial differences between the actual sizes of tires that are nominally the same, I would never just use the code that's in the instructions. Measuring as described in the first response takes just a few minutes. If your measuring tape is in inches, multiply by 2.54 to get centimeters or 25.4 to get millimeters. If you want to be really accurate, first inflate the tire to the normal pressure you use and remember to put some downward pressure on the handlebars as you move the bike through one rotation.

cvcman
07-09-12, 01:12 PM
Some different computers use different "calculations".
Often if you look at the chart provided, they don't even agree with themselves vs different tire sizes.

As a "rule of the thumb" try-
(622+(2*35))* PI
That will give the "theoretical" tire circumference.

If I do it in mm's it comes out to 2197. But that doesn't sound like what it was ???

DiabloScott
07-09-12, 02:25 PM
If I do it in mm's it comes out to 2197. But that doesn't sound like what it was ???

If it's four digits, it's almost certainly the circumference in mm. Not the diameter or equivalent value in inches. 2168 as noted below is as good as any for a start. If you have a measured mile somewhere you can ride it to check and then adjust according to the results.

I have one computer that only allows 0.5cm increments, so you'd set it at 216.5 or 217.0 but it should be obvious if that's what you have.

fietsbob
07-09-12, 02:28 PM
Still got the information flyer that came with the computer?

no? ... you may find another copy online thru the manufacturer's website.