My bike computer arrived in the mail at work today! The first thing that struck me is that it is much much smaller than I thought it would be from the pictures I looked at. This is a picture from their website:



Because there is nothing to compare it too I expected it to be about the size of a digital kitchen timer.

For comparison, this is what it looks like in my hand:



Big difference.

I have already installed it on my bike and it works great. It is really clever how it works. There is a sensor that gets attached to the fork. There is also a small magnet that gets attached to one of the spokes on the front wheel. You have to position everything so that the magnet passes five millimeters from the sensor.

A wire attached to the sensor gets snaked up the fork and then along one of the brake cables to the handle bar. At that end of the cable is a bracket that gets attached to the handlebar. The computer can slip in and out of the bracket easily so in case you are going to leave your bike locked up in a public place you can just slip it into your pocket.

Before you can use it you have to enter the outside circumference of your tire. In my case it is 205 centimeters (80.7 inches or 6.73 feet).

Every time the sensor detects the magnet passing by it knows that it has just traveled 6.73 feet. Remember the elementary school equation rate x time = distance. The computer knows the distance of one revolution is 6.73 feet, and because it has an internal clock it knows how long it was since the last time it detected the magnet. Therefore, to calculate your speed it just divides the distance by the time of the revolution.

So the 9.73 feet circumference of the tire is equal to 0.00127381094696970 miles.

If it takes the tire 0.5 seconds to make one complete revolution that is the same as 0.000138889 hours.

Finally 0.00127381094696970 miles / 0.000138889 hours = 9.2 mph!

It doesn't matter where along the spoke you attach the magnet even though it will travel faster the farther away from the axle it is placed. This is because it will still take the same amount of time to make one revolution no matter where along the radius of the wheel it is placed. Pretty clever, huh?