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Chain Length calculation?

Old 06-29-22, 05:51 AM
  #1  
ronnydee
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Chain Length calculation?

Thought it would be interesting in using the chain length calculation and compare to sizing the chain using Largest cog and largest chainring method.

The calculation is:
L = 2 (C) + (F/4 + R/4 + 1)
  • L = Chain length in inches
  • C = Chain stay length in inches
  • F= Number of teeth on biggest front chainring
  • R= Number of teeth on biggest back cog.
My question does the chain length include the master link? For example if the chain length result is 55, is this include the master link? Was wondering if the "+ 1" in the calculation is the master link?
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Old 06-29-22, 07:03 AM
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Yes, a master link is included in the chain length. Without a master link the chain would be joined by reusing a standard pin (older 5/6-speed) or a special joining pin (7+ speed). The "+1" accounts for the extra chain length required to allow the chain to clear the cog or chainring teeth height while shifting.
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Old 06-29-22, 09:13 AM
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I never calculate.... what works well for me for both modern and vintage (friction) setups is small/small sizing

and of course Big/Big combo has to work

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Old 06-29-22, 10:34 AM
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Originally Posted by squirtdad View Post
I never calculate.... what works well for me for both modern and vintage (friction) setups is small/small sizing

and of course Big/Big combo has to work
Yup. No need for a calculator, and the numbers put into the calculation are subject to operator error in measurement. Small/small gives the right length and if you are cheating by needing more chain wrap than the derailleur can handle then you need to double check with a big/big confirmation and expect the chain to hang slack in the small/small.
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Old 06-29-22, 10:43 AM
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Calculating chain length is about the most stupid way to figure it out. Put the chain on the bike and use the recommended method for the particular drivetrain you have.
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Old 06-29-22, 12:03 PM
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No need to overthink this-

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Old 06-29-22, 12:24 PM
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Originally Posted by Bill Kapaun View Post
No need to overthink this-

To me, this is also overthinking it.
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Old 06-29-22, 12:26 PM
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Originally Posted by smd4 View Post
To me, this is also overthinking it.
Everybody has their limits.
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Old 06-29-22, 12:28 PM
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Originally Posted by Bill Kapaun View Post
Everybody has their limits.
At least I don't need to use a pair of tiny screwdrivers to know if my chain is the right length.

Last edited by smd4; 06-29-22 at 12:37 PM.
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Old 06-29-22, 12:45 PM
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Originally Posted by HillRider View Post
Yes, a master link is included in the chain length. Without a master link the chain would be joined by reusing a standard pin (older 5/6-speed) or a special joining pin (7+ speed). The "+1" accounts for the extra chain length required to allow the chain to clear the cog or chainring teeth height while shifting.
aaah ok then my chain is a bit long. I wasn't sure if the master link was part of the calculation so i didn't cut the chain and added the master link. So, I will attached one side of the master link, and re-measure the chain from end to end.

thankx for the heads up on this.
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Old 06-29-22, 12:51 PM
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Originally Posted by smd4 View Post
At least I don't need to use a pair of tiny screwdrivers to know if my chain is the right length.
Ignore list so I don't have to deal with your pettiness.
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Old 06-29-22, 01:22 PM
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The master link is 0.5 in the +1 of the calculation. All the above approaches work but testing big-big in the stand is critical.
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Old 06-29-22, 02:12 PM
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Originally Posted by cxwrench View Post
Calculating chain length is about the most stupid way to figure it out. Put the chain on the bike and use the recommended method for the particular drivetrain you have.
I was going to write the same thing, so thanks.

I almost think such posts which use math instead of simply measuring stuff are intended to be ironic?

Last edited by Camilo; 06-29-22 at 02:18 PM.
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Old 06-29-22, 02:13 PM
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Originally Posted by ronnydee View Post
aaah ok then my chain is a bit long. I wasn't sure if the master link was part of the calculation so i didn't cut the chain and added the master link. So, I will attached one side of the master link, and re-measure the chain from end to end.

thankx for the heads up on this.
So you actually size the chain based on the calculation rather than just sizing it the way it's supposed to according to every authoritative source? (derailleur manufacturers, professional shops, etc.) Wouldn't you actually test the chain length after cutting it according to the calculation, and if so, why would you not do it the easy/correct way first?

Yeah, this sounds critical, but maybe there's a good reason to use a calculation that I don't understand? Can you explain if so.

Last edited by Camilo; 06-29-22 at 02:20 PM.
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Old 06-29-22, 03:46 PM
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Exactly @Camilo, this is why not any drivetrain manufacturer uses a formula of any kind.
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Old 06-30-22, 08:46 AM
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Formulas tend to be very exact and don't take into account every possible component group, cassette range or chain ring difference.

In reality there isn't an exact number of links your chain must be and one more or less might do better or might perform the same for a particular bike.

The big thing in sizing a chain is just knowing what to look for. If your chain sags when in the small/small then it might be too long. If the rear DR cage is stretched to it's forward limit when in the big/big then your chain might be too short.
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Old 06-30-22, 09:49 AM
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Originally Posted by Iride01 View Post
Formulas tend to be very exact and don't take into account every possible component group, cassette range or chain ring difference.

In reality there isn't an exact number of links your chain must be and one more or less might do better or might perform the same for a particular bike.

The big thing in sizing a chain is just knowing what to look for. If your chain sags when in the small/small then it might be too long. If the rear DR cage is stretched to it's forward limit when in the big/big then your chain might be too short.
Not exactly true, at least not anymore. Shimano and SRAM both have more involved instructions for chain length for both road and mtb. They don't involve formulas but they're more complicated than small/small.
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Old 06-30-22, 09:57 AM
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Originally Posted by Iride01 View Post
Formulas tend to be very exact and don't take into account every possible component group, cassette range or chain ring difference.

In reality there isn't an exact number of links your chain must be and one more or less might do better or might perform the same for a particular bike.

The big thing in sizing a chain is just knowing what to look for. If your chain sags when in the small/small then it might be too long. If the rear DR cage is stretched to it's forward limit when in the big/big then your chain might be too short.
never mind where exactly a wheel might be if the rear drop outs allow for sliding position
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Old 06-30-22, 12:03 PM
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Here's how you determine chain length for my DA 7700 derailleur: With the chain on the smallest cog and the largest chainring, the two pulleys on the rear derailleur should be perfectly vertical--90 degrees perpendicular to the ground. Straight from the manual.

Works every time for me.
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Old 06-30-22, 04:07 PM
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Originally Posted by Camilo View Post
So you actually size the chain based on the calculation rather than just sizing it the way it's supposed to according to every authoritative source? (derailleur manufacturers, professional shops, etc.) Wouldn't you actually test the chain length after cutting it according to the calculation, and if so, why would you not do it the easy/correct way first?

Yeah, this sounds critical, but maybe there's a good reason to use a calculation that I don't understand? Can you explain if so.
oh i always size the chain large cog large chainring. Was just curious if the calculation works out the same thats all
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Old 06-30-22, 05:42 PM
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About the only use for doing the calculation in advance rather than actually fitting the chain would be to see if the chain you want to buy will be long enough. New chains come in different lengths, 112, 114, 116, etc links. If your calculation says you need 114 links you would want to avoid buying a chain 112 links long.
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Old 06-30-22, 08:18 PM
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Originally Posted by ronnydee View Post
oh i always size the chain large cog large chainring. Was just curious if the calculation works out the same thats all
Totally depends on how far forward you pull the derailleur cage.That's why you should follow the drivetrain manufactures recommendations. On older road bikes and mtbs that are completely within capacity spec you can use small/small and you'll get the longest chain possible, which is a good thing. Big/big is totally dependent on the chain installers preference.
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Old 06-30-22, 08:58 PM
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Originally Posted by smd4 View Post
Here's how you determine chain length for my DA 7700 derailleur: With the chain on the smallest cog and the largest chainring, the two pulleys on the rear derailleur should be perfectly vertical--90 degrees perpendicular to the ground. Straight from the manual.

Works every time for me.
Having both RD pulleys "perfectly vertical" also depends on using a cassette that is officially within specifications and the B screw adjustment.
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Old 06-30-22, 09:36 PM
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Originally Posted by ronnydee View Post
oh i always size the chain large cog large chainring. Was just curious if the calculation works out the same thats all
Thanks - did it? I'd be curious too (although just theoretically!).
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Old 07-01-22, 06:12 AM
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Originally Posted by SoSmellyAir View Post
Having both RD pulleys "perfectly vertical" also depends on using a cassette that is officially within specifications and the B screw adjustment.
The B screw adjustment? Seriously? That has absolutely nothing to do with sizing a chain. It almost sounds like you don't know what the B adjustment screw is for. And of course the cassette has to be within the limits of the derailleur capacity. Duh.

Feel free to argue with Shimano if you must:

RD-7700

God, for some reason you guys make sizing a chain the most complicated thing about bikes.

Last edited by smd4; 07-01-22 at 07:59 AM.
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