DIVIDING A RIM INTO 20 EQUAL INCREMENTS

1) Begin by measuring from the center of the wheel to the inside of the rim. See below.


In the above, the radius is 12 3/8 (12.375) inches

2) Use the formula: Spacing = 0.313 x Radius. For the above, 12.375 x 0.313 = 3.87 inches, or 3 7/8 inches.

Marks will be 0, 5, 10, 15, 20, 25, 30, 35, 40……90, 95, 0. If you read 128 on your cyclocomputer and 35 on the rim your complete reading is 128.35.

3) Mark a zero point.

4) Mark 25, 50, 75 also. These are easy, as they are quarter points and need no fancy calculation.

5) Lay out the marks between the quarter points as below. You will be dividing that interval into 5 equal lengths, using 4 marks. When you are done, the remainder will be the same length as the other intervals.





6) Repeat for the other three-quarters of the wheel.

When you are done your wheel will have decimal markings.

Commentary invited.

This message has been edited. Last edited by: Pete Riegel,

Thanks Pete. As usual, very clear, illustrative, and helpful.

AN INQUIRY FROM JUSTIN KUO

Pete,

In the recent article, DIVIDING A RIM INTO 20 EQUAL INCREMENTS, the text
calculated the 1/20 rim diameter as 3.87 inches, or 3 7/8 inches.

Yet, in the third of the three photos, it appears the increment is 3 5/8
inches.

It's a small difference unless someone measures all 20 increments
consecutively. Did the divisions in quarters and then in fifths result
in the shorter measurement?

Just curious.... Justin

Pete replies:
I noticed this too, and attribute it to various measurement errors and photographic parallax. It could cause a problem if someone indeed laid out all the increments sequentially. That’s why I recommended measuring inward from the quarter points. The resultant error would be very small. For example, if I used 3 5/8 inches instead of 3 7/8, the first mark would be ¼ inch off, and the second ½ inch off. ½ inch error would cause a reading error of about 0.006 revolutions. If I stopped at 382.37 revolutions, the correct reading would be either 382.36 or 382.38 revolutions. This is tiny and can be ignored.

I didn't use this method to lay out my wheel. I laid mine out years ago using Neville's instructions. I used my wheel only as a photographic example.

This message has been edited. Last edited by: Pete Riegel,

I just had an experience with a novice measurer and looking at his numbers for calibration it was clear there was a problem. He apparently marked a 36 spoke rim only into 4 subdivisions.
I found that he was not aware of doing further subdivisions.

For our Regional Certiifiers, please ask how one has set up their rim. It may be best to explain the process to anyone using an electronic counter.