Which is more accurate?

[User16625]

Trying to setup my speedo and need to measure my wheel circumference. I measure the wheel diameter using a tape measure to be 680mm (including tyre obviously), so the circumference based on this should apparently be 2136mm.

However if I measure the circumference by rotating the wheel once and drawing 2 lines on the floor to mark the start and end of the wheels rotation, I get a measurement of 2115mm. This is the one im gonna use for now but which do you think will result in a more accurate result? I double checked each of the measurements including rolling the wheel again to make sure I had it strait.

 

[Rasmus]

The wheel rotation method should be more accurate. Remember also that the result will be affected by tyre pressure, and the weight of the rider.

Personally I wouldn't care too much, though. The difference between the two numbers you quote is less than 1% - do you really want accuracy better than that?

 

[User16625]

Dont suppose theres much need for that kind of accuracy, was just curious tho cheers.

 

[Biker Joe]

Rolling the wheel.
Though I tried both methods and got as near as damn it the same numbers. Not enough to make a difference.

 

[User6179]

Trying to setup my speedo and need to measure my wheel circumference. I measure the wheel diameter using a tape measure to be 680mm (including tyre obviously), so the circumference based on this should apparently be 2136mm.

However if I measure the circumference by rotating the wheel once and drawing 2 lines on the floor to mark the start and end of the wheels rotation, I get a measurement of 2115mm. This is the one im gonna use for now but which do you think will result in a more accurate result? I double checked each of the measurements including rolling the wheel again to make sure I had it strait.

Just put the tape measure around the tyre, no need for chalk lines .

Most computers have preset sizes and I find these to be pretty accurate(within 1%).

 

[hoski]

I'm not sure either is more accurate than the other - I don't think one could measure a wheel to the nearest millimetre...

But in terms of process - measuring the circumference directly is likely to be more accurate. It's not easy to measure the diameter of a wheel as there are a whole load of spokes and a hub in the way. Measuring the circumference also means that any error in measurement won't be magnified (measuring the diameter and the calculating the circumference multiplies any error by pi).

 

[paul04]

At 1st mine was set at 2130 but the bike computer was out after a ride. so I set it at 2100 (on a 700c wheel) on sunday I did a 20.8 mile bike ride (according to the bike computer) and strava said 20.7. so happy with that.

 

[Pumpkin the robot]

I put a mark on the wheel and rotate it 3 times and measure the distance travelled, you will be pretty accurate doing it that way. A mm doesnt sound like much, but in the job I do it is a country mile and I find that it crosses over into "every day" life!