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Accurately Calibrate a Bike Computer

Titanium Rocket

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Can some of you boffin hubbers check the logic below to accurately calibrate the distance measurement on a bike computer? Here is the method, using some very hypothetical figures: <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />


1.       Measure the wheel circumference as accurately as possible 3 times (with a fabric tape or along the ground), calculate the average, say 2.865 m

2.       Enter this into bike computer

3.       Ride a straight, flat route of about 20 km, record distance on bike computer, say 21.2 km

4.       Now measure exactly the same route with a <?: prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />GPS (in car, or on bike) and record, say 19.9 km

5.       To calculate exact wheel circumference:

a.      wheel did 21200/2.865 = 7399.65 revolutions along the route

b.      correct wheel circumference should be = 19900/7399.65 = 2.689 m

6.       Enter 2.689 m into the bike computer

7.     DONE.
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i wondered myself how to get this right as my 705 is always more than my polar cs300. I will try this



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The logic is good, but you can save yourself some effort in step 1.


The procedure will work regardless of what value you use for your initial circumference, so just guess a value (no need to measure it).


For the best accuracy, try to carry the GPS with you at the same time you measure with the cycle computer. Also, try to do a route where you don't have to stop. Autopause and signal accuracy can effect the GPS distance while you're stationary.


Edman2009-12-08 06:46:18

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That is assuming that GPS is 100% correct and you take exactly the same route with your car (inlcuding lines through corners etc).


When you measure wheel circumference on your bike.

Get someone to help you.

make mark on tyre and ground.

sit on bike and roll forward 1 wheel rev till mark touches ground again.

get helper to measure distance.
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Your weight on the day, tyre pressure, all chages the radius and circumference is 2 x pie x radius. But does it really matter if you have ridden 67.8km or 68.7km? it is more than 60 and less than 80...Wink

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