# Electronics, Lighting, & Gadgets - how to convert my tire size for cyclocomputer?

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tamiann
03-09-07, 07:32 PM
My tire size is 700x26c

That is not a default size on my Cateye computer. And I can't figure out how to convert it into mm. These are the closest default on the computer:

700x25c = 2105 L(mm)
100x28c = 2136 L(mm)

Can anyone help me out?

Thanks

Psydotek
03-09-07, 07:35 PM
Can you manually enter the circumference? Most computers should have that option. Just lay out a tape measure and measure how far the tire rolls with 1 revolution. Convert to mm and enter into computer. :) I had to do that with my friends cyclometer.

Sir Lunch-a-lot
03-09-07, 08:37 PM
Or Just take the diameter (in your case, 26 inches), and since the Circumference (C) is equivalent to the diameter (D) multiplied by Pie (pi) ( c= pi * d), you can figure out the circumference in inches by multiplying 26 inches by pi or 3.14159.

Apparently, there are 25.4 mm for every Inch (if my math and online unit converter is working right), so you can multiply the circumference in inches by the 25.4mm/inch, and that will give you inches.

So, if I have done my math and all else right, the total comes out to about 2075.

vpiuva
03-09-07, 08:53 PM
700c*26 is not 26". It is 622mm+26mm+26mm=674mm*pi= 2117. But if you can't enter it manually, just go with the closest 700x25. You'll be off (2117-2105)/2117=<0.6% I think you can live with this.

Retro Grouch
03-09-07, 09:08 PM
The "A" answer is to roll out your tire. Mark a line on the floor, line up your valve stem with the mark and roll your bike one tire circumference. Measure that distance in millimeters and use that value. That'll give you a real accurate answer.

The "B" answer is to interpolate between the factory numbers. In your example I'd use 2115.

The "C" answer is to use the closest suggested circumference in the computer set-up directions. Actually, to be honest, I always use the next bigger number because I want my customers to feel like they're riding a little farther and a little faster. It won't be as precise as rolling out the tire, but it'll be plenty accurate enough for any normal use.

tamiann
03-09-07, 09:08 PM
Yes I can enter it manually. I had input 2115, and that seems close enough.

Thanks so much!!! :)

Al1943
03-09-07, 09:52 PM
Yes I can enter it manually. I had input 2115, and that seems close enough.

Thanks so much!!! :)

Good choice. Actually rollouts aren't as accurate as they sound.

masiman
03-09-07, 10:05 PM
Good choice. Actually rollouts aren't as accurate as they sound.

Why is that?

AndrewP
03-09-07, 10:12 PM
Rollouts are only accurate for a particular tire pressure and weight being carried on the wheel. the figure will also change as the rubber wears off the tire. Close is good enough.

Sir Lunch-a-lot
03-10-07, 12:00 AM
700c*26 is not 26". It is 622mm+26mm+26mm=674mm*pi= 2117. But if you can't enter it manually, just go with the closest 700x25. You'll be off (2117-2105)/2117=<0.6% I think you can live with this.

Oh. Okay. My bad. I thought the 26 was the measure of diameter. I usually don't see tire sizes measured in this fasion. So... if you don't mind my asking, what does the 700c*26 indicate then?

Psydotek
03-10-07, 12:31 AM
700c is the wheel size, 26 is the tire width in mm i believe.

Al1943
03-10-07, 06:33 PM
Rollouts are only accurate for a particular tire pressure and weight being carried on the wheel. the figure will also change as the rubber wears off the tire. Close is good enough.

+1
And if the computer sensor works off of the rear wheel there will be a bigger difference between the loaded bike and the unloaded bike.

HillRider
03-10-07, 08:27 PM
The "C" answer is to use the closest suggested circumference in the computer set-up directions. Actually, to be honest, I always use the next bigger number because I want my customers to feel like they're riding a little farther and a little faster. It won't be as precise as rolling out the tire, but it'll be plenty accurate enough for any normal use.

That's funny. I use the next smaller number for my own bikes. A 700x23 is recommended by Cat-Eye at 210 cm and calculates at 209.8. I set my cyclometers at 209 because I want to be sure I've ridden at least the indicated distance.

operator
03-10-07, 08:38 PM
Who cares. Just use the number and re-verify with GPS or gmaps. It's not important to have millimeter accuracy only that it's consistent (within reason).

vpiuva
03-10-07, 09:12 PM
Oh. Okay. My bad. I thought the 26 was the measure of diameter. I usually don't see tire sizes measured in this fasion. So... if you don't mind my asking, what does the 700c*26 indicate then?
700 is just an approximate tire diameter when they came up with this crazy system. The actual rim diameter for a 700c wheel is 622mm. The tire size notation is the tread height above the rim, in this case 26cm, which you have to add twice (ie. both sides of the wheel) to obtain the actual tire diameter.
a 700x20c tire would be 622+20+20=662cm in diameter.
a 700x39c tire would actually be 700cm diameter

Retro Grouch
03-11-07, 06:11 AM
700c is the wheel size, 26 is the tire width in mm i believe.

It is the width. If you think of a tire as being a round tube, as it gets wider it also gets a little taller. Consequently the circumference isn't the same for all 700c tires.

Retro Grouch
03-11-07, 06:13 AM
That's funny. I use the next smaller number for my own bikes. A 700x23 is recommended by Cat-Eye at 210 cm and calculates at 209.8. I set my cyclometers at 209 because I want to be sure I've ridden at least the indicated distance.

So if we rode side-by-side all day, by the end of the day I would have ridden farther and faster than you. Wimp!

bhtooefr
06-10-07, 05:36 PM
Myself, I just used the cyclocomputer calibration chart (http://sheldonbrown.com/cyclecomputer-calibration.html) that Sheldon Brown has. More accurate than certain owner's manuals. (My Schwinn cyclocomputer's manual listed 27 x 1 1/4 (32-630) as 2155 mm circumference, the same as 700x32C (32-622), when it was really 2161...)

ax0n
06-10-07, 05:46 PM
When the only person you're trying to beat is yourself, consistancy trumps accuracy any day. Just hop on your bike and enjoy it. Give yourself some goals of riding further and faster than the month or week before if you want, and you don't need to worry too much about it.

If you want brutally honest accuracy, go get a Garmin Edge. Even GPS can be off by a bit but it'll probably be more accurate (although arguably less consistant) than a regular wheel-rotation-counting cyclometer.

Winter76
06-10-07, 06:45 PM
Myself, I just used the cyclocomputer calibration chart (http://sheldonbrown.com/cyclecomputer-calibration.html) that Sheldon Brown has. More accurate than certain owner's manuals. (My Schwinn cyclocomputer's manual listed 27 x 1 1/4 (32-630) as 2155 mm circumference, the same as 700x32C (32-622), when it was really 2161...)

+1 for this, I used his chart to calibrate my schwinn wireless computer, worked great!