Calculating Chain Length

I’m off touring in the Flemish Ardennes. :fries: :beer:

Because my poor ailing knees will be struggling, I’ve purchased a set of 46-30 oval chain rings to replace my round 50-34.

Current setup works well and I can ride in big-big no issue.

Am I correct in thinking I should be able to just shorten my chain by one inner and one outer link and I should be close?

I’m thinking 50 - 46 is 4 teeth and I’m only interested in half the circumference that actual touches the chain.

I guess the overall reduction in diameter might have a small effect but not in the direction I should be too worried about.

Anything weird about the ovalness of absolute black that should be accounted for?


Taking off one inner and outer link should be close enough. If this was a 1x system you might need to be more concerned but it shouldn’t be a problem with 2x.

The oval chainring has the same perimeter/teeth as a circular chainring so there’s nothing weird with that for chain length. When the oval is widest it’s narrower at other parts and vice versa.

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Bingo. The circumference of half an oval is the same regardless of which half you are taking.

Which chain rings are they and do they fit onto your existing crank ok (4 or 5 bolt)?

I usually size my chains for 2X by going into the small-small gear combination and having the chain be just long enough to avoid having the bottom part of the chain rub on the rd cage or the portion of the chain that rolls off the upper pulley. So I’m basically looking for something like this when in the small-small combo: