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.