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cycling computer(7 posts)

cycling computerby666
Jul 31, 2001 11:46 AM
i just changed the battery on my cycling computer i had to reprogram the tire size in it. my question is why does the width of the tire matter. ther are different setting for 650c and 700c i can understand that but why different settings for 700cx20 or 700cx23 or 700cx25
re: cycling computerLen J
Jul 31, 2001 11:55 AM
The computer measures speed by how fast the magnet moves by the sensor. In order to translate this into speed, it has to know how much ground is covered during that one revolution of your wheel. A 650 wheel (being smaller) will not cover as much ground as a 700 wheel, Therefor the computer can't give you precise speed if it doesn't know wheel size. In addition, there are small differences in wheel diamater (read ground covered) between a 700X20 vs a 700X23. They may not seem like much but over a long distance they can give you erronous speeds and distances. Hence the different entrie.

Hope this helps.
Tire width affects the rolling diameter ...Humma Hah
Jul 31, 2001 12:21 PM
The wider tires also cover slightly more ground per revolution.

The more uptight of us actually measure wheel travel per revolution directly, by rolling the bike, while sitting on it, exactly ten revolutions, measure the distance traveled to the millimeter, then divide by 10!
That is usually known as the circumferenceCliff Oates
Jul 31, 2001 12:31 PM
My tape measure isn't long enough to stretch 21+ meters. I just do one revolution, walking alongside the bike while leaning heavily on the bars, and figure it's close enough.
Frankly, the last time I just wrapped the tape measure around ..Humma Hah
Jul 31, 2001 4:43 PM
... the front wheel and measured that way. A subsequent test ride on a measured course showed I'd almost nailed the correct calibration. But Doug Sloan and a couple of others here are really nit-picky about using the multi-turn technique.

Interesting observation: I've been running the bike for a couple of months on a set route, consistently getting 23.0 miles per ride. Last weekend I did a century, and in prep for that I upped the tire pressure a little. Now that same ride is showing 22.7 miles! So even adjusting tire pressure does affect the calibration of a computer.

The problem with saying "circumfrence" is that there are a couple of circumfrences to the torus that is your wheel. The way I described it, there's no doubt.
The world goes round and roundCliff Oates
Aug 1, 2001 3:11 AM
Bear in mind that Doug clips the little rubber mold nubs off his new tires. One turn leaning heavily on the bars provides more than adequate accuracy. To tell you the truth, I always get the same number as the vendor provides in their calibration guide, give or take a fraction of a centimeter (computer only takes integers, so the number has to be rounded anyway).

I keep my tires at a fairly consistent pressure and trips over the same route usually only vary by several hundredths of a mile. I guess I need to be cutting those corners closer on a consistent basis. On that cruiser, you have a wider range of potentially useful inflation pressures than we do with our skinny tires, plus the tire itself comprises a greater fraction of the wheel's diameter and so affects circumference to a greater degree. I would probably see similar behavior in a narrower range of variation with the 32mm tires on my CX bike.

Finally, while it is true that wheels have more than one place to measure circumference, it never occurred to me to measure anything but the outer circumference of a mounted tire for the purposes of a sensor measuring distance (and time). Some folks do have trouble with core concepts though, no doubt about that.
The joys of inaccuracyjtolleson
Jul 31, 2001 6:37 PM
I know my computer's inaccurate. I tend to lose, say, maybe 1/10 or so every twenty miles, even. Love it! In my mind, I always forget the math and overblow my own rides. "Gee my computer says 43. Must've been 50." Gee, my computer says 72. Must've been 80.

Back when I had pinpoint accuracy, I'd beat myself up. Getting to the end of a so-called century showing 98.3???? What's with that? Or showing 102?? Hey, where's the finish line? The ballpark has somehow brought me to a zen like state where everything I do is FABULOUS!