# Tire Size and Computer Help? WTF mate!



## silkworm13 (Apr 17, 2008)

Picked up a Cateye Mirco Wireless for my new MTB. I'm setting it up and realize that my tire size is not listed in the setup chart. I've got 26x2.25 inch tire up front, but the chart skips me going from 26x2.125 to 26x2.35. So I dialed in the 26x2.125. Will this adversely affect the data reported?

And what about them suckers with 29" tires, they're not listed at all!

Let me know what you guys think. Thanks in advance.


----------



## slocaus (Jul 21, 2005)

Here, post 2 has this link.

Sheldon Brown, 15 ways to calibrate your cyclocomputer,
http://www.sheldonbrown.com/cyclecomputer-calibration.html


----------



## primoz (Jun 7, 2006)

Ok I don't know for Cateye, but every other cycle meter I had, had bike size in millimeters (or inches), and never in tire size, and to be honest, I doubt Cateye is any different.
So I'm really sorry to be nasty now, but is it so hard to calculate bike size, when you measure radius of your bike? So suckers with 29" tires, or with tires, others then those 5 specified in manuals, normally do this... or they bring computer back to store, exchanging it for water battle, because they are too stupid to use it.
Sorry but is it really so damm hard to turn some logical thinking on?


----------



## RJend (Jun 28, 2005)

silkworm13 said:


> Picked up a Cateye Mirco Wireless for my new MTB. I'm setting it up and realize that my tire size is not listed in the setup chart. I've got 26x2.25 inch tire up front, but the chart skips me going from 26x2.125 to 26x2.35. So I dialed in the 26x2.125. Will this adversely affect the data reported?
> 
> And what about them suckers with 29" tires, they're not listed at all!
> 
> Let me know what you guys think. Thanks in advance.


I think you've been exposed to the public school system - MEASURE IT.


----------



## INCO (Mar 30, 2007)

Measure from your hub to your outer tire, that's your radius r. Use the following math formula to calculate your tire circumference, 2 * (3.141) * r = c

If you measure in inches and you need it in centimeters (I think that's what the cateye uses) take c from above c * 0.0254 * 10 should give you number of centimeters circumference. 

Warning use at your own risk, it's been a while since I've done these conversions regularly


----------



## silkworm13 (Apr 17, 2008)

Dude, noob here - I didn't even know you could enter a size other than the options they had in the instructions. Nor did I know what the 4 digit number even was, i thought it was some generic code or something. Again, this is my first bike since I was 10 - learning this all for the first time.

Thanks for the advice though, dick... (referring to RJend and Primoz, not INCO and slocaus, you guys are cool)


----------



## primoz (Jun 7, 2006)

silkworm13 said:


> Thanks for the advice though, dick... (referring to RJend and Primoz, not INCO and slocaus, you guys are cool)


 Dick? It really isn't my fault your logic thinking doesn't exist, or if you don't know how to read manual. In Cateye manual (which is available at http://www.cateye.com/en/manuals/66) it clearly says:


> Setting the wheel circumference
> (1) How to measure wheel circumference
> Measure the radius R (cm) with the rider on the bicycle as shown in Fig.4, and calculate the wheel circumference L (cm) using the following formual:
> L = 2 * Pi *R = 6.283 * R (cm)
> Or, paint a mark on the ground contact surface of the wheel, and directly obtain the wheel circumference by measuring the distance between the marks printed on the ground.


 So noob and first time biking or not, setting wheel size is not rocket science... at least I didn't think until now, it is.


----------



## Pistol79 (Apr 16, 2008)

I had the same problem. Its basic math. The 26" x 2.25" is equal to 2077. You just have to figure out the fudge factor from the chart to figure out in between sizes.


----------



## Pistol79 (Apr 16, 2008)

26" x 2.125" = 2070

26" x 2.35" = 2083

26" x 2.25" = =2077


----------



## shiggy (Dec 19, 1998)

Pistol79 said:


> I had the same problem. Its basic math. The 26" x 2.25" is equal to 2077. You just have to figure out the fudge factor from the chart to figure out in between sizes.


_*BUT*_ not all 2.00, 2.125, or 2.25 (or any width) tires are the same actual diameter, especially when you factor in operating air pressure and load on the tire. The ONLY accurate way to calibrate the computer is by doing a rollout.


----------



## FredNo (Aug 5, 2014)

There's no substitute for the roll-out. I did two revolutions since that's all my garage allows and divided by two. Another factor is the width of your rims (inner dimension). 

Also, with no weight on the bike the measure is 38 mm longer, so for an accurate measure you need to be on the bike. 

With my rims, weight, 26x2.35 tires at 32 PSI, I get 2045 mm. The Cateye manual says 2083, which is exactly 38 mm longer than my roll-out (with weight). Hmmm, I wonder if Cateye measures with no weight on the bike?


----------



## alphaOri (Feb 10, 2015)

The cateye chart is actually pretty good and accounts for weighted rollout. I have a 26x2.25 tire. I measured the actual diameter and did the 2*pi*radius calculation and got 216cm circumference. Then I did an unweighted rollout and got 212cm. Then did a weighted rollout and got 208cm. The closest tire size to 26x2.25 on the cateye chart is 26.2.35. Using this, the chart says the circumference is 208 cm - spot on.


----------



## Zomby Woof (MCM700) (May 23, 2004)

primoz said:


> Dick? It really isn't my fault your logic thinking doesn't exist, or if you don't know how to read manual. In Cateye manual (which is available at http://www.cateye.com/en/manuals/66) it clearly says:
> So noob and first time biking or not, setting wheel size is not rocket science... at least I didn't think until now, it is.


I have the same thing with my Topeak. It gave the same directions. The number you input is the tire circumference and that is how you calculate it. Very simple.


----------



## Harold (Dec 23, 2003)

alphaOri said:


> The cateye chart is actually pretty good and accounts for weighted rollout. I have a 26x2.25 tire. I measured the actual diameter and did the 2*pi*radius calculation and got 216cm circumference. Then I did an unweighted rollout and got 212cm. Then did a weighted rollout and got 208cm. The closest tire size to 26x2.25 on the cateye chart is 26.2.35. Using this, the chart says the circumference is 208 cm - spot on.


except what if you actually had a 26x2.35 tire? Now the circumference number you input is too small and measurements are off. It's cool that you checked the numbers, but because the chart was fine for you doesn't mean it works for everybody. Plus, not every 26x2.25 tire is exactly the same. Some are VISIBLY larger/smaller than others. What about the tire pressures people use? The chart doesn't account for the whole range of pressures people use.

The rollout method really is how it should be done. IMO, the computer companies should just abandon the preset numbers altogether and just explain the rollout - then tell them to go to a shop and pay for it to be done if they can't figure it out.


----------



## FredNo (Aug 5, 2014)

I called Cateye, and the guy said that they just go with the tire mfgr stated circumference. That can be way off, though. I inflate my front tire to 23 PSI (with tubes and no pinch flats yet). Doing a rollout, I get an accurate figure that’s much different than the Cateye user manual’s. I tested it on the highway from one mile marker to the next, and it was 1.00 miles…accurate to the hundredth! 

Using Strava, the mileage shown is about 6% less than my cyclo-computer due to the low sampling frequency. I trust my Cateye Micro Wireless more than my phone’s GPS.


----------



## gsa103 (Sep 1, 2014)

38mm out of 2083mm rollout is 1.8%, which good enough for most people. That's as good as the average car odometer, how much more do you really need? The crude tables work well enough for most people, if you want more accuracy, there's always the roll-out.


----------

