Determining your bike's wheel circumference
Old ID
Symptoms
We're installing the Specialized Speedzone Wireless Computer and I want to know what the correct circumference setting should be.
The little itty-bitty manual lists a 20 X 1.8 as 1589 setting but a "1.8" is not the same as a "1-1/8."
Causes
Solutions
The circumference setting will depend on what type of tire you are running. You can consult the instruction manual we've put together for the Specialized brand Speedzone computer (see link below), or you can use the following method to determine what number to put in.
- Take a piece of string and lay it out on the floor.
- Take your bike, and put the front tire next to the piece of string so that the valve stem is next to the end of the string.
- Slowly roll the tire forward along the string, until the valve stem is back to the original position.
- Mark the string, and measure in millimeters (the conversion from inches to millimeters is inches x 25.4 = millimeters).
- Enter this number into the computer, and you are ready to go!
Remember that the Specialized brand computer has two bike settings. If you are only using the computer on one bike, be sure to set both setting to the same thing to avoid confusion later on.
We've put together a complete instruction manual for calibrating the Specialized brand Speedzone computer for Bike Friday-sized wheels. You can find this in our Document Library. Please refer to the link below (Instructions for setting the Speedzone wireless computer).
Peter and Merin Storey (pstorey@kslaw.com) adds:
This works better if a friend can help you roll the bike while you sit on it. Your weight does compress the tire, reducing the circumference. On a 26" wheel at 85psi, I found I was off by about 4% when checked against known distances.
David Smith (david.g.smith@akerkvaerner.com) adds:
Well I've been out of school for quite some time but the one thing I do remember is that pi (3.14...) x Diameter should give you the circumference of a circle. If getting the diameter is difficult because of the hub getting in the way of the tape then try the old 2 x pi x radius. To metrify the equation multiply inches times 25.4 to get millimeters. Also, as far as deflection changing the circumference, I'm just not a true believer. After all the deflection caused by your weight causes a higher pressure in the tire which will cause the rest of the tire to become more rigid. This in turn should change the circumference to compensate for the deflection. Remember that the bike computer can, at best, give only a good approximation of the distance and speed you travel as it does not compensate for tire and internal air temperature which will cause the air within the tire to expand (hot) or contract (cold) due to ambient temperature change and road friction.