1. i purchased an odometer for my bicycle and need to calibrate it.
it's a magnetic type that counts tire revs by a small magnet mounted on the spokes.

okay, the chart that came with it gave a set of numbers for various tire sizes but when i entered my tire size the odometer gave too many miles.
i think what is happening is that the tire squishes when you ride it so that the circumference is essentially smaller.

the figure given for my tire is 2117 mm (millimeters). i tried 2105 and it seems pretty close but i want to be as close as possible.

here is what i want to do:
i will ride my bike the distance.
given the milage shown i would like to have an equation i can use to arrive at the correct number to enter into the odometer to calibrate it.

2.

3. New setting = old setting * (measured distance/old reading)

4. Well, my first question is how do you know it's giving too many miles? If you know the distance was, say, 12 miles, and the odometer is stating 200, then your setup is incorrectly done or the odometer is defective.

However, if you know it's 12 and the odometer reads 13 or 11 or some other close approximation, you may only be experiencing the differences between how your bike drove, and how the original distance was measured. (ie - measurement with a car's odometer won't match up with a bicycle odometer, usually.)

That's all well and obvious, though.

It's probably safer to assume that either your odometer is defective, or you don't have the right calculation set in your odometer. (Remember, the chart you have could be bogus, or may not count for the type of tires you have.)

Try adjusting the calculation on the odometer until things match up. Who cares if the setup of the odometer doesn't match the chart?

5. the figure given for my tire is 2117 mm (millimeters). i tried 2105 and it seems pretty close but i want to be as close as possible.
2117 wasn't good enough, but 2105 worked well? That's a difference of roughly 0.5%. Are you serious in expecting a higher accuracy??

There are various reasons why these odometers are not perfect. You named one of them: The exact wheel circumference depends on the tire pressure and load (your weight). Then there is slippage, i.e. the bike wheels are not rolling perfectly (especially the rear wheel, which is the reason why you put the odometer on the front). Furthermore, magnetic odometers sometimes skip a reading, especially when you're riding fast.

A diameter that's smaller than expected will indeed cause a high reading. Both of the other errors will lead to a low reading. As slippage and skipping take effect at higher speed, you almost *want* your reading to be a little high at low speed, so the errors partially compensate when it really counts. In any event, don't expect too much from this simple device.

6. New setting = old setting * (measured distance/old reading)
i don't need to convert any units?
the wheel circumference must be in millimeters not inches or feet.

Try adjusting the calculation on the odometer until things match up.
an equation is what i need.
the manual only gives a chart of wheel diameters in inches and the corresponding circumference in millimeters.
the only way to "get it right" with this setup is to input a number, ride the distance, then see if it's accurate, if not input new number and repeat the process.

2117 wasn't good enough, but 2105 worked well? That's a difference of roughly 0.5%. Are you serious in expecting a higher accuracy??
hmmmmm . . .
the only thing i can say to this is the .5% is for every rev of the wheel, it also amounted to a sizable difference in readings after about a mile of actual distance.

well, thanks for the responses.
i'll try the equation that harold posted.

7. Originally Posted by leopold
an equation is what i need.
I think you'll find that even if you posses an equation, the ability to measure and/or adjust your equipment to a degree in line with the equation will be limited at best. Unless you're using a precision measurement environment and electronically calibrating gages, you're limited to the physical limitations of the manual adjustments you can do.

Even further, suppose you do manage to get your odometer setup precisely on the dot. What makes you think that your bicycle, not to mention your biking distance, will result in a zero-point variation?

8. I think you'll find that even if you posses an equation, the ability to measure and/or adjust your equipment to a degree in line with the equation will be limited at best.
Guys, he only said he wanted to get it as close as possible. I think he's going about it the right way.

i don't need to convert any units?
the wheel circumference must be in millimeters not inches or feet.
No, you don't need to worry about what the units are. If your reading is 5 percent low you need to tweak it up by 5 percent. Doesn't matter what units you use, it's still 5 percent.

Incidentally, I wonder about that explanation of the wheel squishing down. The circumference will still be the same, or so it seems to me, with a flat tire as a fully inflated tire. The shape is just not quite circular. So unless there is slip, it shoud go just as far per revolution. It doesn't matter, as far as the question you are asking is concerned.

9. Originally Posted by Harold14370
Guys, he only said he wanted to get it as close as possible. I think he's going about it the right way.
Yeah, but from what we've been told, he's already narrowed things down to less than half a percent! (0.5%)

Even if he gets an equation and finds the absolute perfect setting, I doubt he'll be able to set his odometer to an accuracy any closer.

10. Incidentally, I wonder about that explanation of the wheel squishing down. The circumference will still be the same, or so it seems to me, with a flat tire as a fully inflated tire.
okay, you measure the tire at rest from the ground up through the axle to the top of the tire. in my case it was 26.5 inches.
now, you put weight on the bike and then measure the wheel. you will find, depending on the weight applied and the inflation of the tire, that the diameter will be less. this implies the circumference will be less.

Even if he gets an equation and finds the absolute perfect setting, I doubt he'll be able to set his odometer to an accuracy any closer.
i can set the circumference to within one millimeter such as 2104, 2105, 2106.

11. Originally Posted by leopold
okay, you measure the tire at rest from the ground up through the axle to the top of the tire. in my case it was 26.5 inches.
now, you put weight on the bike and then measure the wheel. you will find, depending on the weight applied and the inflation of the tire, that the diameter will be less. this implies the circumference will be less.
Only if it's a circle. If you put a tape measure around the tire when it's fully inflated, then put a tape measure around it when it's flat, there is the same amount of material there in the tread, so it should measure the same. OTOH the distance traveled in one rev should be 2 pi R where R is the distance from the center of the axle to the road. So the difference must be in the slip. I guess that's why your gas mileage and tire life go way down with an underinflated tire.

12. Originally Posted by Harold14370
Only if it's a circle. If you put a tape measure around the tire when it's fully inflated, then put a tape measure around it when it's flat, there is the same amount of material there in the tread, so it should measure the same. OTOH the distance traveled in one rev should be 2 pi R where R is the distance from the center of the axle to the road. So the difference must be in the slip. I guess that's why your gas mileage and tire life go way down with an underinflated tire.
Not necissarily true. The tire may have the same material amount in both the inflated and deflated state, but remember the inflated state has the tire expanded. Measure a balloon before and after it's inflated. Same idea.

As for the bike...

An odometer is really just a counter. It counts an incremental value, and that value is translated into something more than just a count (usually a distance measurement).

All it's doing is multiplying the number of revolutions times the circumference of the wheel, to get the distance.