The easy answer is to download one of the sprocket calculators eg http://wareseeker.com/free-sprocket/
Usually one is fairly constrained with a kart
but: the two centres (crankshaft and axle) should be between 20 and 30 chain pitches apart. (Normally it is 30 to 50 cp, but under a pulsating load reduce to 20)
For normal karting purposes, the distance between the two centres must be larger than the diameter of the large sprocket - diameter of the small sprocket
There must be 120 degree minimum contact on the driving sprocket (more isn't bad, but 120 is best)
The number of links is found by the formula
2C+S/2 + K/C
Where C= distance between centres in chain pitches S= number of teeth in Small Sprocket + number of teeth on large sprocket K=number of teeth on large sprocket -number of teeth on small sprocket
It should be fairly easy to use excel to make up a table that shows for any combination of sprocket the chain lengths (only available as even numbers)
Oh yes, when the chain elongation exceeds 1.5 percent, then it is time to scrap it (that equals a link and a half over the average chain) or if you get extra wear on any one link.
Have a look at: http://www.misumi-europe.com/www/catalogues/FAEnglish/eucatalogue/pdf/45/E_FA_08_2543_2544.pdf
Does that help?
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