Tag Archives: Bicycle

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.

Cool bicycle Persistance of Vision project

As an introduction to computers, the kids and I have been building the SpokePOV. The kids have been into bicycle projects this summer, so this is a great crossover project.

The Spoke POV is a project I first saw on www.ladyada.net. There is a kit available from www.adafruit.com.

Photo of completed project from ladyada.net:

We are using green LEDs and hope ours will be this cool!

We ordered the kit with the extra ROM so we animate the images. We picked up some 96/4 tin/silver solder from Radio Shack. It is lead free, so I don’t mind the kids using it.

The SpokePOV uses an ATMEL ATtiny2313 microcontroller and and has two rows of 30 LEDs. The LEDs blink out a pattern as the wheel spins that the eye perceives as a complete picture (follow the above links for action photos). There is a hall-effect sensor, so the processor can sense the wheel rotation and calculate how fast it is spinning, to create the proper pattern.

So far, we have a completed circuit board. There is a Windows program to create the images and download to the Spoke POV. We’re having a bit of trouble with that.



Goodwill Bicycle luggage

I’ve always wanted bicycle luggage. The kind that you can load up with lots of gear for a long trip.

When I was at Goodwill, found a shaving travel kit for $2.00. I bought it because I have a thing about travel bags and just couldn’t pass up the deal.

The shaving kit was just the right size to attach to the top of my luggage rack. I strapped it down with three zipties. It worked great.

Later, I noticed a small laptop case I had. It had been part of my bag collection for years and never really found right use. With a few more zipties I had a bag for the side. The last bag arrived today. My wife found a Targus laptop bag at a garage sale for a $1.00. How could I resist. Now this bag was large enough that I didn’t want to attach it with zipties. I wanted it to be removable. I was able to wind the shoulder strap around the luggage rack so that it held the bag in place, and left the clips available, so the bag could be removed.

A complete set of bags for $3.00.