This article describes how to calibrate the wheel sensor of a bicycle computer or GPS unit in the easy way -- or in harder, more accurate ways. . A companion article, the cyclecomputer database on this site, links to owner's manuals for cyclecomputers and GPS units. Another article serves up some history of distance measurement on bicycles, and the limitations on its accuracy.
Calibration from a tire-size table
The simplest method is to set the wheel sensor for a nominal tire size. We have provided tables of tire sizes for this purpose. We have classified cyclecomputers into six groups according to the number used in calibrating them.
||Circumference in inches
||Circumference in inches X 2.727
||Circumference in centimeters
||Radius in millimeters
||Circumference in millimeters / 1.609344
||Circumference in millimeters
Different manufacturers have used different brands of tires to calibrate -- or have done calculations based on nominal dimensions -- so there can be a slight inconsistency between the numbers in the tables and the most accurate number for your bicycle.
The chart below doesn't list all possible tire sizes, but does list the most popular ones. If your marked tire size falls between two sizes shown on the chart, interpolate the appropriate calibration number between those above and below, or for greater accuracy, do a roll-out test (keep reading...).
|700 X 56
|700 X 50
|700 X 44
|700 X 38
|700 X 35
|700 X 32
|700 X 28
|700 X 25
|700 X 23
|700 X 20
|27 X 1 3/8
|27 X 1 1/4
|27 X 1 1/8
|27 X 1
|26 X 2.125
|26 X 1.9
|26 X 1.5
|26 X 1.25
|26 X 1.0
|26 x 1/650C
|26 X 1 3/8
|24 x 1
|20 X 1.75
|20 X 1 1/4
|18 x 1.5
|17 x 1 1/4
|16 x 1 3/8
|16 x 1.5
Deriving tire size from ISO/ETRTO numbers
The I.S.O. tire size consists of a tire width and a bead seat diameter. Both of these numbers are in millimeters. For example, a 28-622 (700 x 28C) tire has a nominal width of 28 mm on a rim with a bead seat diameter of 622 mm
To get an approximate diameter (in mm), add the bead seat diameter to twice the tire width (since the tire depth comes into the diameter twice: 622 + (28 X 2) = 678. Multiply this by pi (3.142) to get the circumference in mm (F) 2130. Appropriate calculations will yield calibration numbers for computers in other groups.
(Thanks to Chris Ziolkowski for suggesting this.)
However, the actual rolling diameter will be about 1% smaller for a road tire, and smaller yet at low inflation pressure. A deep tread, on the other hand, can increase the effective diameter.
If you require greater accuracy than the chart or nominal tire size provides, do a rollout test or measured distance test (OK, next...)
Roll-out test for high accuracy
Values read from a chart or derived from ISO/ETRTO numbers will generally be accurate to within one or two percent, which is good enough for most cyclists, and more accurate than most automobile odometers.
If you require more accuracy, you can do a "roll-out" test.
Unless you need to count "miles" ridden on a stationary trainer, it is best if you measure the roll-out of the front wheel and mount the computer sensor there. The rear wheel "creeps" on the road surface as you pedal, and can skid during braking, so it gives a less-accurate readout.
Since the effective tire size is affected by tread thickness, tire pressure and rider weight, the rolling circumference should be measured by rolling the bike with the rider aboard. Run the test on a paved surface: most floors are slipperier, and that will affect the reading too. It is possible to do a roll-out test while lightly scooting along while bearing weight on the handlebars and one foot on a pedal, but it's better to have an assistant holding the bicycle upright and pushing it along.
You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line, and ending when the valve is back at its low point.
Another approach is to put a small dot of paint on the tire and measure the distance between the marks that the paint prints on the road. With either approach, the rider must hold the handlebars straight while an assistant balances and pushes the bike. Otherwise, the wheel may not follow a straight path.
Use an accurate, metal tape measure. You may measure for one wheel revolution, or for greater accuracy, for three or four -- whatever your tape measure can span -- and divide by the number of revolutions.
If the tape measure is divided in inches, multiply the measured circumference by 2.54 for centimeters or 25.4 for millimeters. For cyclecomputers that require a diameter value, divide the result by 3.1416 (π), and for those which require a radius value, divide the result by 6.2832 (2 x π).
Once you have measured the rolling circumference, use the formula indicated to find the calibration number for the cyclecomputer involved.
If the greatest accuracy is important to you, ride with the calibrated tire at the pressure you used for the roll-out test.
Using a measured course to fine-tune your setting
A comparison of a cyclecomputer's distance reading with a mile markers over stretch of road, or with GPS readings, can dial in the accuracy even closer. Divide the actual distance by the cyclometer's mileage reading, then multiply your calibration number by the result to get a corrected calibration number. It is best to measure over a distance of 10 miles or more to reduce round-off error and to avoid using inaccurately-placed mileposts. (The highway department generally avoids placing them in the middle of driveways and intersections...)
|X Old Calibration Number = New Calibration Number
Correcting calibration errors after the fact
Even after calibrating a bicycle computer against a measured course, there will still be small errors due to loading, tire inflation etc. -- but also, a cyclecomputer's calibration can only be adjusted by steps, and so it must be rounded to the next higher or lower step. Rounding error for a cyclecomputer calibrated in 1/100ths inch or millimeter steps is very small, but some cyclecomputers are calibrated in centimeter steps. Automotive odometers can't be calibrated at all, and neither could mechanical odometers used on bicycles.
Still, all calibration errors can be corrected with a bit of math. Multiply any recorded distance by the factor you derived from your ride on the measured course,
Cyclometer or odometer Reading
Let's give an extreme example. Let's say that a car has been retrofitted with low-profile tires, and the odometer says that it has gone 11 miles, but mile markers say it has gone 10. The car is taken out to survey a route for the bike club. Multiplying the car's odometer readings by 11/10 will give fairly accurate readings.
It is easiest just to record the raw odometer readings while surveying. You could carry a small voice recorder on a lanyard around your neck when on the bicycle, or on the seat next to yourself in the car, and record the distance at each turn along the route. When home, enter the readings into a computer spreadsheet, which lets you correct all of the readings at once.
If you are out riding and you know that your cyclecomputer's calibration is off, you can correct for it mentally. The mental exercise is easy if the cyclecomputer reading "drifts" away from the distances given on a cue sheet. The error accumulates bit by bit as you go along -- so it is easy to track. And, don't jump to conclusions: more of the error may be in the cue sheet than in your cyclecomputer!
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by John Allen