Bike computer calibration

Back in the 80s, when I used to read my dad’s bicycle magazines, I thought bike computers were the coolest thing.  Back then they were over $100, and I just couldn’t see spending that much money to know how fast or how far I had ridden.

Not too long ago I found one at Goodwill for less than $2.00 and could not pass it up.

It installed easily, but I had to enter the wheel’s circumfrence in millimeters.  I’m enough of a math geek to know that that would be the wheel diameter in inches * pi (3.14159) * 25.4, but what is the actual diameter for 26″ x 2.125 wheels?  The computer came with a chart, but my tire size wasn’t listed.

After searching the internet a bit, the best answer is to simply mark the tire and pavement, roll the tire forward until the tire makes one complete circle, mark the pavement again and MEASURE the space on the pavement between the marks.

I wasn’t in the mood for that, so I continued looking and found this:

Sheldon Brown’s Cyclecomputer Calibration Chart

According to his chart, my tires are 2070.

Thanks Sheldon!

Update:  I took the bike for a ride and the mileage seemed a bit off.  I decided to measure the actual circumfrence.  Rather than try to measure around the tire, I choose to measure how much ground one wheel rotation covers.  I marked the tire and the ground under the tire, rolled the bike forward and made another mark on the ground where the tire made one full circle.  It was 82 inches, longer than I thought it was going to be.  82 inches * 25.4 is 2082mm.  So there was a diffrence of 12mm per rotation between the value in the table and the actual measurement.

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