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)
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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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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 |
Originally Posted by squirtdad
(Post 22557869)
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 |
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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Originally Posted by Bill Kapaun
(Post 22558081)
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Originally Posted by smd4
(Post 22558124)
To me, this is also overthinking it.
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Originally Posted by Bill Kapaun
(Post 22558131)
Everybody has their limits.
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Originally Posted by HillRider
(Post 22557695)
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.
thankx for the heads up on this.:thumb: |
Originally Posted by smd4
(Post 22558134)
At least I don't need to use a pair of tiny screwdrivers to know if my chain is the right length.
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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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Originally Posted by cxwrench
(Post 22557985)
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 almost think such posts which use math instead of simply measuring stuff are intended to be ironic? |
Originally Posted by ronnydee
(Post 22558156)
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.:thumb: 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. |
Exactly @Camilo, this is why not any drivetrain manufacturer uses a formula of any kind.
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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. |
Originally Posted by Iride01
(Post 22559066)
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. |
Originally Posted by Iride01
(Post 22559066)
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. |
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. |
Originally Posted by Camilo
(Post 22558283)
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. |
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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Originally Posted by ronnydee
(Post 22559690)
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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Originally Posted by smd4
(Post 22559345)
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. |
Originally Posted by ronnydee
(Post 22559690)
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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Originally Posted by SoSmellyAir
(Post 22559927)
Having both RD pulleys "perfectly vertical" also depends on using a cassette that is officially within specifications and the B screw adjustment.
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.:rolleyes: |
Originally Posted by smd4
(Post 22560153)
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.:rolleyes: |
When using a drivetrain specced as compatible by the manufacturer I agree; do as the manufacturer says. When using unspecced mixes then some other guidelines are needed (big-big seems the foundation of these). I've a drivetrain with components from Shimano, SunTour, Campag, KMC, Stronglight and Simplex. The Shimano RD-M953 is (I think) specced with a wrap of 42 and max sprocket of 34. With larger jockey wheels, reversed and lengthened B-screw and the chain just the right length this is pushable to a wrap of 53 and max sprocket of 40 (no drop-link).
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Originally Posted by smd4
(Post 22560153)
The B screw adjustment? Seriously? That has absolutely nothing to do with sizing a chain.
Originally Posted by smd4
(Post 22560153)
It almost sounds like you don't know what the B adjustment screw is for.
Originally Posted by smd4
(Post 22560153)
And of course the cassette has to be within the limits of the derailleur capacity. Duh.
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Originally Posted by SoSmellyAir
(Post 22560523)
I did not say that the B screw adjustment has anything to do with sizing a chain.
Originally Posted by SoSmellyAir
(Post 22560523)
I only said that it affects the angle between the ground and the line between the two RD pulleys, especially since (as you correctly pointed out) the Shimano Dealer Manual calls for them to be lined up "perfectly vertical."
Originally Posted by SoSmellyAir
(Post 22560523)
B screw adjusts the distance between the guide (i.e., top) pulley and the cassette. Depending on the chain tension, it also affects the aforementioned angle between the ground and the line between the two RD pulleys.
Originally Posted by SoSmellyAir
(Post 22560523)
And if the cassette is not within the official specifications of the RD (e.g., GS model with a 12-25 cassette) but still shifts fine, it may not be possible to have the two RD pulleys perfectly vertical.
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Originally Posted by smd4
(Post 22560994)
Then I don't know why you'd bring up the B adjustment in a thread specifically about sizing chains, in response to a post specifically about sizing chains for a specific drive train.
Originally Posted by smd4
(Post 22560994)
My delay in responding to this post is because I wanted to be absolutely sure about this before I called out BS, so I had to wait until I got home before working on my bike. Now that I've done so, and run the B adjustment screw--all 11 mm of it--all the way in and all the way out, with my chain on the big ring and smallest cog, I can safely say, and without reservation, BS! Moving the screw those miniscule 11 mm one way or the other does not move the derailleur cage at all off the vertical.
But please feel free to insist that the B screw does not affect the angle of the derailleur cage under any circumstances whatsoever. |
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