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Inconsistent chain length calculations

Old 05-05-22, 08:02 AM
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nPn
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Inconsistent chain length calculations

I updated my recently acquired size 54 2010 Roubaix Comp's rear drive train to be a little more climbing friendly. The original rear drive train consisted of a rd-5600 derailleur with a 12-27 cassette, 27 teeth being listed as the max for that derailleur. I switched to a rd-5701-gs derailleur along with a 11-32 cassette which is spec'd to work with the 50-34 front chainring set. I was planning on using the chain length equation L = 2 ◊ C + F/4 + R/4 + 1, which when I applied it to the original setup gave 105 for the number of links This matched the chain on the bike before I swapped the rear cassette from a 27T to 32T on the largest cog. Applying the equation to the new setup I get 107 links. Instead of just cutting 9 links off of my new KMC X10 that came with 116 links (including the "missing link") to get it down to 107, I tried to use the Park Tool Largest to Largest method. When I did this I ended up only cutting 6 links out so I end up with 110 links which is 3 links longer than the equation. I finished adjusting the H/L limits and the indexing and shifting is perfect both on the stand and on the road. So my question is should I leave it as is or trust the 107 number and remove 3 links? I did see other sizing advice that says a chain is too long if when on the derailleur and on the largest chain ring and cog, you can "fold" a link over. I can definitely fold a link when in this state, but probably not more than 1 link. Any other advice?
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Old 05-05-22, 08:11 AM
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Old 05-05-22, 08:13 AM
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I've never used an equation to put a chain on a bike. It's either too loose or too tight...just go by feel and add or remove links as necessary.

Sometimes people over complicate this stuff...

Last edited by prj71; 05-05-22 at 09:58 AM.
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Old 05-05-22, 08:24 AM
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This is why I don't follow the "repair by numbers" methods as the sole way to determine stuff. You test fitted the chain on the stand, confirmed that the length was not too short (the "dangerous" length) and then test rode on the road. Exactly what I was referring to in this recent thread Is My Chain The Right Length? - Bike Forums I will; say you did the right thing. Andy
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Old 05-05-22, 08:59 AM
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Page 28 of this .pdf

https://si.shimano.com/api/publish/s...001-26-ENG.pdf

Note that when you have bigger than a 27 toot cog on the rear you do not run the chain through the DR while determining it's proper length.

Big thing is just that after putting the chain back through the DR cage, when in the big big combo that the DR cage not be stretched to the it's absolute limit for forward travel. And when in the small small combo that your chain not go slack and droop.

Other methods for sizing chains work too.
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Old 05-05-22, 09:39 AM
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Does the chain smack the chain stay when in the lowest cassette tooth & smallest front ring?
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Old 05-05-22, 09:42 AM
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Originally Posted by nPn View Post
Any other advice?
Forget mathematical formulas. Install the chain and size with the manufacturers guidelines OR if everything is within spec use small/small. Doing math and counting links is a waste of time.
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Old 05-05-22, 11:38 AM
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As long as the chain is not too tight in the big/big combo and not too loose in the small/small combo, you're fine. Some gear combinations allow you a range of acceptable chain lengths, and if anything I'd rather have the longer of those options just in case I want to get an even larger cog in the future.
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Old 05-05-22, 11:55 AM
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I feel there is a little droop, in small - small?
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Old 05-05-22, 12:26 PM
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Originally Posted by nPn View Post
I feel there is a little droop, in small - small?
Looks fine to me.
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Old 05-05-22, 12:27 PM
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Looks OK to me. Is there still tension on the rear derailleur? How's it look in big-big?
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Old 05-05-22, 01:09 PM
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without putting hands on it & going by the picture, I'd shorten it a link.
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Old 05-05-22, 01:25 PM
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Big/big and add a link
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Old 05-05-22, 01:35 PM
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Thanks all! I think I am going to stick with what I have for now. I just realized I would need to remove 2 links one inner and one outer to shorten it with the missing link (unless I am missing something), and I think that would be too tight. If I hear any chain slap, start noticing any more droop, or start dropping the chain, I will re-consider.
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Old 05-05-22, 01:38 PM
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yes there is still tension on the rear derailleur, big big looks good, tho I might be able to fold one link.
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Old 05-05-22, 01:40 PM
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what does it look like when the chain is on the Big & Big?
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Old 05-05-22, 01:47 PM
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Here is a photo of the chain on the big-big set.
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Old 05-05-22, 01:48 PM
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I wouldn't remove any links. But that's just me.
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Old 05-05-22, 02:05 PM
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Yeah, unless it's not running smoothly in small/small (which you shouldn't be using anyway), leave it be.
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Old 05-05-22, 02:16 PM
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fwiw I like small/small sizing works vintage and modern

no calculations

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Old 05-05-22, 03:10 PM
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Yep, unless I'm working on SRAM Eagle or full suspension, small-small sizing has never given me issues. Chain length looks perfect nPn .
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Old 05-15-22, 12:22 AM
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Originally Posted by nPn View Post
Thanks all! I think I am going to stick with what I have for now. I just realized I would need to remove 2 links one inner and one outer to shorten it with the missing link (unless I am missing something), and I think that would be too tight. If I hear any chain slap, start noticing any more droop, or start dropping the chain, I will re-consider.
Post a picture of the derailleur when it's on the big ring in front and big sprocket in back. That will tell you if the chain can be safely shortened or not. Anything else, including the formula, is just speculation.
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Old 05-15-22, 01:10 AM
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Originally Posted by Camilo View Post
Post a picture of the derailleur when it's on the big ring in front and big sprocket in back. That will tell you if the chain can be safely shortened or not. Anything else, including the formula, is just speculation.

Her already did, look at post 17 of this thread
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Old 05-15-22, 07:54 PM
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Originally Posted by JoeTBM View Post
Her already did, look at post 17 of this thread
mea culpa. I agree with the advice given!
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Old 05-15-22, 08:03 PM
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BIG:BIG
You want 1-2" of slack.
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