How to measure the center distance
- Put the rear axle roughly in the middle of its adjuster travel.
- Measure from the center of the countershaft (front sprocket nut) to the center of the rear axle.
- Measure straight, not along the chain.
The formula
links = 2C/p + (N1 + N2)/2 + ((N2 − N1) / 2π)² × p/C
C is the center distance, p the chain pitch (½" for 420/428/#40/#41, ⅝" for 520/525/530, ⅜" for #35), N1 and N2 the tooth counts. The result is rounded up to an even number, because an odd number needs an offset (half) link, which is weaker.
Quick check when changing sprockets
Every 2 teeth added to the rear sprocket needs about 1 more link; changing the front by 1 tooth needs about half a link. Because chains come in whole pairs of links, small changes are often absorbed by the adjusters.