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Hello All,
I am running into a unusual issue with my Cateye Micro Wireless Computer. I put it against my Garmin GPS today and rode for 15 miles. The Garmin said 15 miles and the Cateye said 16 miles. So I went back and redid my roll measurement. I am running a 29x2.55 WTB Wierwolf LT which rolled out to 89.625 inches. I multiplied this by 25.4 and came up with 2276 mm. My interest is that the wheel chart is saying a 29x2.1 is 2288mm and a 29x2.3 is 2326mm. So how is my 2.55 less then both of these listed on there chart? Has anyone else run into this? I will surely be testing it again a few times with my GPS. By the way my previous setting was 2330mm.
Thanks
 

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since 4/10/2009
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34,314 Posts
Thanks for triple posting this.

The reason the rollout method is better than using the preset options in the computer is because the presets assume a tire pressure. With the rollout, it accommodates different tire pressures.
 

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BM and PQ Trail Rep
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1,934 Posts
Go back and edit the two posts you want out. Justs retitle them "double post" or something like that and delete the body of the post, You are not the first nor will you be the last. Chit happens. It just irks some more than others.
 

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Derailleurless
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9,122 Posts
You've done the right thing by entering a real roll-out number. Bike computers are pretty simple devices and as long as each wheel rotation is being counted, it's difficult to muck up a distance measurement.

In fact, the only time I'd trust a GPS measurement vs. a calibrated bike computer is straight-line, and even then, I'd be suspicious.

Google Earth has a distance measuring function. Pick a section of street flat and relatively straight, and pick two easily identified points as a start and finish line. Ride it, compare your computer and GPS distances to what Google Earth spits out, and you'll have a pretty good idea of which device is more accurate.
 

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The GPS will almost always give you a shorter distance than a bike computer or the real distance. That's because it measures straight line segments between trackpoints, rather than following a curve as your wheel does. The curvier the course is, the more error you will have, as each curve distance is slightly underestimated as a series of straight line segments.

Your computer, using actual roll out circumference, is very accurate.
For ultimate nit pickyness, you can add some weight to your front end while you do the roll out to simulate the amount of tire compression your bodyweight will cause during riding.
 
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