Calibrating Schwinn Bike Computer Wheel Value

Do you have a bike trip computer that makes you enter a 4 digit number when you power it up?

Does that number correspond to the number of mm circumference your bike tire is?

Did you also loose the manual like I did?

Well, you're in luck then.

I got a different road bike for this year, and the tires are clearly a different size than my Mountain Bike's. What that meant was I'll need to remember a new number for when my trip computer battery dies.  But what's that number in the first place?

For my trip computer (A Generic Schwinn one) I found an online scanned version of a similar manual. That manual gave my bikes wheels (28.0" Dia.) a nominal circumference of 2237mm per rotation.



I did a 1km test run (1.024km) and it came out as 1.093km. That's not too bad, if I wasn't too sure how accurate my 1km stretch was, I'd say the 6% error is pretty good. But we can do better.

So, here is what you do if you want something on the order of >99.9% accuracy.

1. Set a value into your bike computer (this really doesn't matter which one) (Ci)
2. Bike a known -preferably straight- path for some known distance (dk), longer the better up to a point. 
3. Mark down what distance was reported (dr). 
4. Use this simple equation to calculate your calculated circumference (Cf)
5. Bam! That should be a pretty accurate measure for your bike tires at that tread level.  I realize it sounds a little silly to go through that much effort for maybe 6mm/rotation error. But if you bike than 3000km/year it's the difference in the odometer by over 10km. 

The real key to this is finding an accurate length of road/trail that is measured accurately.  The distance doesn't need to be far, as few as 20 wheel rotations. Just so long as the stretch of road you were on is accurate. 

...although if you have a tape measure you'll get a pretty accurate result (>5%error). But this way you have an excuse to go biking!