Q: Is wheel circumference that 3 or 4 digit number that I must program into my computer every time I change the batteries?
A: Yes. That 3 or 4 digit number represents the exact circumference of your wheel in centimeters (3 digit) or millimeters if it is a 4 digit number. This number MUST be entered in millimeters or centimeters; inches will NOT work.
Q: What is the relationship between circumference and diameter?
A: Circumference divided by pi(3.14) equals diameter. A wheel with a circumference of 112 cm will have a diameter of 35.6 cm. The math is: 112 cm divided by 3.14 = 35.6 cm.
Q: What wheel sizes will bike computers accept?
A: Cateye computers will generally accept wheel circumferences from 112 cm to 255 cm. Put another way, that represents wheels from 14 inches (35 cm) to 32 inches (81 cm) in diameter. Sigma Sport and Topeak computers will accept a wheel size from 1 to 50 inches ( 2.5 cm to 127 cm) in diameter. Topeak and the Sigma Sport BC600, 800, 1200, 1400 and Targa all use the more precise 4 digit, wheel setting numbers.
Q: What is the best way to measure wheel circumference?
A: The most precise method is to use a 6 foot (2 meters) piece of single strand, 16 to 20 gauge wire. Bend a small "L" on one end and then tape that end to the center of your tire's tread. Be sure to bring your tires up to the normal pressure that you ride with first. Then, lay the wire lengthwise along the outside of your tire's tread and use electrical tape to hold it in place. When you come back around to your starting point, bend another "L" in the wire. Now carefully remove the wire and precisely measure it. This measurement will be the exact circumference of your wheel.
Different width, brands and types of tires will have different circumferences. If you race on one set of wheels and train on another we reccommend precisely measuring both sets. If the circumference difference is more than 5% we suggest using a computer that allows for two wheel settings. Your speed and distance readings will be more precise and make more sense.
The calibration chart below gives approximate values for the circumference of various
size tires. For an exact measurement and precise readings we suggest using the method detailed above.
For the following Cateye computers use centimeters; AT-100, Mity, Mity 2, Mity 3, Enduro, Velo 1 & 2, Dual, Mate, Micro, Vectra, and Cordless.
For the Cateye HB-100, Cordless 2, Kosmos, Astrale, Solar II and ATC, use millimeters.
If you have the original Solar and are using it for miles per hour,
the "Solar (mph) column is for you. If you are using an original Solar for kilometers per hour then use the
Millimeters column.
The Cateye calibration chart may be used for any computer that requires a tire circumference size in centimeters or millimeters.
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