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Crankset Tolerance

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Crankset Tolerance

Old 07-15-19, 04:02 PM
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son_of_clyde
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Crankset Tolerance

I have a Campagnolo C-Record Pista crankset that is out of round by enough to noticeably affect chain tension. The gap between chainring teeth and a magnet that I placed on the chainstay varies from 0.0 mm (tooth touching magnet) to ~0.5 mm at 180 degrees rotation. This is enough to cause the chain tension to go from "just right" to too tight (IMO). The chainring (45T Dura Ace) is not the issue, as I tried mounting it in different positions and the results didn't change. Does this seem normal? I can live with it, but if other cranksets have better tolerances, I might go that route.
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Old 07-24-19, 01:41 PM
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Bat56
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Interesting test to determine the precision of the bolt circle diameter. I have not paid that much attention to the condition of my crank and chainring combos, but I can say that if my chain was just right in the loosest spot, that rotating the crank would find a too tight spot. My highest quality crank/ring is my All-City 612 and it's also the most precise. I would expect the C-Record Pista to have pretty tight tolerances.

but based on the ~.5mm estimate I think that your "too tight" is not actually too tight and that you can just back off the tension a bit. Make it just right at the tightest spot and as long as the chain does not fall off in the loosest spot you'll be good.

You can also loosen the chainring bolts and tighten them as you turn the crank. Take a look at Sheldon's article. Scroll down to section "Centering chainrings."

https://sheldonbrown.com/no-derailers.html
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Old 07-25-19, 08:28 AM
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I was taught that regardless of the condition or quality of the ring-cog combination there will be a tight and a loose spot. Make sure there is still a small amount of movement in the tight spot and that there is not so much movement in the loose spot that you can force the chain off the ring. Anything between those two should be good.

I used to run everything too-tight but I got sick of the noise. I haven't dropped a chain in ten years so apparently a bit looser is fine.
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Old 07-25-19, 08:57 PM
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As suggested, I ended up adjusting the chain to proper tension in the "tight" position. It never gets loose enough to come close to falling off, so all is good. I used a feeler gauge to measure the runout again (previously had just eyeballed it), and it's actually 0.4 mm. Thanks for the replies.
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