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@Sharkstar
I just went from a 20 sprocket to an 18 and the position of my hub on the dropout position didn't change much at all. This got me thinking whether the received forum wisdom on this topic can be right, and, having thought about it, I reckon it's not.
This led to a some geometric doodling and a bit of algebra. The geometrical modelling isn't perfect, but it should be fairly accurate.
To cut a long story short, I've developed a little Excel based calculator (attached) that allows you to find the BB to rear hub centre distance for any given chain length and chainring / sprocket combo. More importantly, you can use it to see how much further forward or backward your hub is going to travel in your dropouts when you change gearing - and hence whether you'll need to mess with your chain.
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