# Proper chain length?



## apoint (Nov 22, 2010)

I took off my compact 50x34 crank and put on a DA 53x39. The chain looks a bit shorter now and I was wondering how do you know when you have a proper length? Thanks Herb


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## rx-79g (Sep 14, 2010)

Depends on the system. The chain length for Shimano derailleurs can be found in their technical PDFs for rear derailleurs. For SRAM derailleurs the method is found in the SRAM technical PDFs for chains.


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## andulong (Nov 23, 2006)

If you can shift to big/big and still have a bit of play in RD cage...should be okay.


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## DrSmile (Jul 22, 2006)

Shift to big/big, if the derailleur doesn't explode, you should be fine!


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## C-40 (Feb 4, 2004)

*well...*

The method for determining the proper chain length is really the same for all brands. In a nutshell, the chain must have enough length to wrap the big/big and also not hang loose in the little/little combination. The only exception to the little/little hanging loose is with certain MTB drivetrains, where a RD is used that deliberately has less wrap capacity than needed. Road bikes with triple cranks can also use this type of setup, when a smaller little ring is used. The little ring and several of the smallest cogs would never be used, so the chain could hang loose in those combination and not cause a problem.

If you put the chain in the little/little combination (with the chain through the RD) and make it as long as possible, without hanging loose or rubbing on the RD cage, then you have a length that produces the maximum wrap capacity. That length would always wrap the big/big, unless you've put on a cassette that exceeds the RD's wrap capacity.

The big/big method suggests the shortest possible length. Wrap the chain around the largest cog and big ring, then add 1 inch is the ends that come together can be joined. If the ends that come together are the same and can't be joined, add 1-1/2 inches or 3 links, instead of two.


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## apoint (Nov 22, 2010)

Thanks Guys, and C40 thanks for the in depth procedure, That's a Great help


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## looigi (Nov 24, 2010)

Doing the math, going from 50 to a 53 adds one and a half chain links (since each link occupies two teeth) around the circumference of the ring. Of course, the chain only covers about 1/2 the ring so the total potential increase in chain length needed is 3/4 of a link (one half of 1.5 links). So you may need to add a link if the chain was near the short end of the range before.


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## rx-79g (Sep 14, 2010)

C-40 said:


> The method for determining the proper chain length is really the same for all brands.


No, it isn't. The Shimano method is designed to give you the right chain length in consideration of the servo pantagraph spring design. While it might not matter if you're at full capacity which method you use, best shifting with a Shimano derailleur isn't going to happen from the Big/big: The Shimano method locates the jockey pulley closer to the cogs.


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## DrSmile (Jul 22, 2006)

Since this is getting technical, I follow this:

http://bicycletutor.com/calculate-chain-length/


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## C-40 (Feb 4, 2004)

*wrong*



rx-79g said:


> No, it isn't. The Shimano method is designed to give you the right chain length in consideration of the servo pantagraph spring design. While it might not matter if you're at full capacity which method you use, best shifting with a Shimano derailleur isn't going to happen from the Big/big: The Shimano method locates the jockey pulley closer to the cogs.


You're sure fun to play with. If you really understood the subject, you'd know that what you've written is nonsense.


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## Blue CheeseHead (Jul 14, 2008)

When in doubt about anything to do with a chain, defer to C-40.  His method is the same as outline by Leonard Zinn.


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## rx-79g (Sep 14, 2010)

C-40 said:


> You're sure fun to play with. If you really understood the subject, you'd know that what you've written is nonsense.


If you use Big/big on a straight block or with a long cage Shimano derailleur, it will make the average cage angle further foward than it needs to be. Since Shimano top pulleys are located aft of the cage pivot, that means that the pulley will be rotated down and away from the cogs more than it would be with a shorter change. Longer distances from pulley to cogs = less shifting accuracy. 

That's how Shimano derailleurs have been since 1971 - it's a method to get the pulleys to track closer to the cogs that Shimano used before the slant parallelogram patent died. 

SRAM derailleurs have neither B-tension pivots during shifting nor chain tension variable pulley position, so their shifting performance isn't affected by chain length. Unlike Shimano, they position the pulley purely via slant angle.


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## cxwrench (Nov 9, 2004)

i'm always surprised that people who have obvious knowledge of how the interwebs work will ask questions like this on forums, rather than going straight to the manufacturers techdocs, and using the method that their no doubt college educated and experienced engineers have decided works best for their system. odd...


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## C-40 (Feb 4, 2004)

*wrong again...*

Maybe you should take the time to read the installation instructions for both brands before you put your foot in your mouth. Both Shimano and SRAM have B screws to adjust the clearance between the upper pulley and cogs. It is NOT dependent on chain length.

The same principles of adjusting chain length apply to all brands. I always use the longest possible chain length. That way, I can put any cassette within the RD's wrap capacity and not have to worry about changing chain length. Since I'm also smart enough not to use a setup that exceeds the Rd's wrap capacity, the little/little method works perfectly.

Shimano's website is all jacked up, or I'd post their RD instructions. Sram's is linked below. Check out screw #6 in the SRAM instructions.

http://www.sram.com/_media/techdocs/95-7915-010-000 Rev B - Road Derailleurs.pdf


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## apoint (Nov 22, 2010)

cxwrench said:


> i'm always surprised that people who have obvious knowledge of how the interwebs work will ask questions like this on forums, rather than going straight to the manufacturers techdocs, and using the method that their no doubt college educated and experienced engineers have decided works best for their system. odd...


 I know it is insanity to even have this sub forum at all, then name it
"componetrys wrenching". What where they thinking? We should all just bail from using this forum all together and take up rock collecting.


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## rx-79g (Sep 14, 2010)

*Congratulations, you learned something today.*



C-40 said:


> Maybe you should take the time to read the installation instructions for both brands before you put your foot in your mouth. Both Shimano and SRAM have B screws to adjust the clearance between the upper pulley and cogs. It is NOT dependent on chain length.
> 
> The same principles of adjusting chain length apply to all brands. I always use the longest possible chain length. That way, I can put any cassette within the RD's wrap capacity and not have to worry about changing chain length. Since I'm also smart enough not to use a setup that exceeds the Rd's wrap capacity, the little/little method works perfectly.
> 
> ...


Nope. Shimano has B* tension*, a screw which varies the tension preload of the spring that controls the angle of the derailleur at different chain tensions. That's what that servo pantagraph thingy is all about. If you shift chainrings, the angle of the whole derailleur and the specific location of the pulley will change.

SRAM has a screw that sets a derailleur body angle - not spring tension. Once set, the derailleur body does not move, regardless of the gear combination or chain tension. And because of the locations of the upper pulley and cage pivots, nothing else varies by chain tension, either. Unlike the Shimano system, SRAM 's RDs do nothing during shifting to change the relationship between pulleys and cogs that isn't built into the geometry of the slanted parallelogram.



What I love is when people talk about how this model rear derailleur shifts better than this other model, and the whole time they might not be getting the best shifting because they "know better" than to use the manual to set up their chain length. This is especially a problem with long cage derailleurs, where the big/big can make the chain up to 4 inches shorter than the Shimano method, resulting in a different pulley location and unnecessarily high spring tension in the derailleur.

Here's your Shimano Tech document:
http://techdocs.shimano.com/media/t...00-5501/SI-5TK0D-En_v1_m56577569830603908.pdf


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## C-40 (Feb 4, 2004)

*more ignorance...*

I feel sorry for your customers. Every time you post something it proves how little you understand.

The Shimano method that you've mentioned is just what I thought - the same thing as Campy. It's not a better method and it would rarely suggest a length that is different than the little/little method, since it does not factor in the largest cog size. The problem with it is that the angle of the RD cage can not be set to 90 degrees in all cases. Depending on the chainstay length, you may have the RD cage angled slightly back with one chain length and slightly forward with a 1-inch shorter chain (the minimum increment of change). There is no perfect length when that happens, so you must decide which length to use.

Those who understand chain length know to try the longer length first, then see if the chain hangs loose in the little/little combo. If it doesn't then you're good to go. You could have just tried the little/little method first and skipped the Shimano/Campy method.

The maximum difference in chain length between a cassette with a 21 tooth large cog and a 27 tooth large cog is only 2 inches (actually 1.5, but that can't be done). There is no way that you can get the chain length off by 4 inches, using any of the prescribed methods.

If you had any brains, you'd also know that cage length has no effect on chain length. I've changed from a short cage RD with a 53/39 to a long cage with a 53/39/30 triple crank, and the same 12-25 cassette. The same 53 inch chain length worked for both.

Check out the rigorous chain length formula at the Park Tool website. It works for any brand and the RD cage length is not part of the equation, because it has no effect on the chain length.


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## rx-79g (Sep 14, 2010)

C-40 said:


> I feel sorry for your customers. Every time you post something it proves how little you understand.
> 
> The Shimano method that you've mentioned is just what I thought - the same thing as Campy. It's not a better method and it would rarely suggest a length that is different than the little/little method, since it does not factor in the largest cog size. The problem with it is that the angle of the RD cage can not be set to 90 degrees in all cases. Depending on the chainstay length, you may have the RD cage angled slightly back with one chain length and slightly forward with a 1-inch shorter chain (the minimum increment of change). There is no perfect length when that happens, so you must decide which length to use.
> 
> ...


Well, let's see here:

You've suddenly stopped talking about how the derailleurs work and have switched over to insults.

I think it is pretty clear what that means. 


Customers?


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## cxwrench (Nov 9, 2004)

apoint said:


> I know it is insanity to even have this sub forum at all, then name it
> "componetrys wrenching". What where they thinking? We should all just bail from using this forum all together and take up rock collecting.


in this instance, please look at the first 15 posts again...there are plenty of great topics in this section of the forum where members can help each other out a ton. but...there are also plenty of times when just following the damn directions will get the job done perfectly and as an added bonus c40 and rx won't be at each others throats all day...
although that is pretty entertaining


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## apoint (Nov 22, 2010)

I read the Damn directions as you say. but it did not work out the way the instructions said. So now you want to beat me up for asking?


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## C-40 (Feb 4, 2004)

rx-79g said:


> Customers?


My impression from previous discussions, like the one where you ranted about stack and reach, was that you worked at a bike shop. I'm glad that's not the case. 

The fact is that any brand of RD will work just fine with the chain set to the minimum length, using the big/big method or rigorous formula. I've read dozens of postings from users who set their chain length that way, then complain when they need to install a longer chain to use a cassette with a larger cog. I always use the little/little method to get the longest chain length. I don't judge the proper length by eyeballing the RD cage angle in the big/little combo. It's vague and irrelevant.


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## cxwrench (Nov 9, 2004)

apoint said:


> I read the Damn directions as you say. but it did not work out the way the instructions said. So now you want to beat me up for asking?


well...what happened? why didn't it work correctly? chain too long? too short? give us some details


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## rx-79g (Sep 14, 2010)

C-40 said:


> My impression from previous discussions, like the one where you ranted about stack and reach, was that you worked at a bike shop. I'm glad that's not the case.
> 
> The fact is that any brand of RD will work just fine with the chain set to the minimum length, using the big/big method or rigorous formula. I've read dozens of postings from users who set their chain length that way, then complain when they need to install a longer chain to use a cassette with a larger cog. I always use the little/little method to get the longest chain length. I don't judge the proper length by eyeballing the RD cage angle in the big/little combo. It's vague and irrelevant.


I have 5 years experience as a wrench and manager in bike shops. 

The reason the method seem "vague and irrelevant" is because you don't understand the difference between the different derailleurs. But, as I already said, if the derailleur is near capacity you'll get the same results with Big/big as any other method.


Apoint, shift into your 53/12 or whatever is you highest gearing combination. Look at the two pulleys on level ground. They should be somewhat close to vertical. If adding or removing 1 or more link pair would make the pulleys more vertical, do it. But it won't be perfect - maybe 10-15 degrees from vertical.


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## DrSmile (Jul 22, 2006)

Could we agree that the large/large method is safer than the small/small? If you exceed the RD limit the small/small method may cause you to break your RD, while the large/large method won't, you'll just wind up with a hanging chain. Yes you may need a new chain if you get a larger chain ring or cassette, but usually when replacing these components a new chain isn't such a bad idea. If you regularly swap out drivetrain components you should already know how to size your chain.

In any case the OP's question is clearly best answered with the large/large solution, as he upgraded from a compact to a regular crank. In this case there is NO WAY the chain would be too long, so the small/small approach is moot.


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## rx-79g (Sep 14, 2010)

DrSmile said:


> Could we agree that the large/large method is safer than the small/small? If you exceed the RD limit the small/small method may cause you to break your RD, while the large/large method won't, you'll just wind up with a hanging chain. Yes you may need a new chain if you get a larger chain ring or cassette, but usually when replacing these components a new chain isn't such a bad idea. If you regularly swap out drivetrain components you should already know how to size your chain.
> 
> In any case the OP's question is clearly best answered with the large/large solution, as he upgraded from a compact to a regular crank. In this case there is NO WAY the chain would be too long, so the small/small approach is moot.


Sure, but no one on this thread is talking about small/small.

The Shimano method doesn't check big/big directly, but if you are within the derailleur's stated capcities the method will not short the chain. And in my experience, even if you are beyond capacity (like the Ultegra derailleur on my mountain bike with a triple) this method still produces the best chain length (small/small is a little slack).


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## danl1 (Jul 23, 2005)

DrSmile said:


> Could we agree that the large/large method is safer than the small/small? If you exceed the RD limit the small/small method may cause you to break your RD, while the large/large method won't, you'll just wind up with a hanging chain. Yes you may need a new chain if you get a larger chain ring or cassette, but usually when replacing these components a new chain isn't such a bad idea. If you regularly swap out drivetrain components you should already know how to size your chain.
> 
> In any case the OP's question is clearly best answered with the large/large solution, as he upgraded from a compact to a regular crank. In this case there is NO WAY the chain would be too long, so the small/small approach is moot.


I'd agree with you in context of this specific question, but as for the large-large being 'safest', I'm not sure I agree. Small-small method gives the longest safe chain; if it's just a bit longer, the return chain can interfere with the top pulley. It's only when it's much too long that you end up with droop, but even that has it's own dangers re: chain suck.

From that maximum length, there will never be a problem with having a too-short chain (assuming no chainring swap) Any cassette that can fit under the derrailleur will be inside the capacity. 

Of course, if you are going way outside of RD capacity specs, all bets are off.


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## olr1 (Apr 2, 2005)

I'm pretty sure you guys could cause an argument in an empty house....

My two cents; I swapped a 53-39 to a compact 50-34 and just couldn't be bothered shortening the chain. It hangs a little loose in the 34 to the smaller sprockets, but shifts perfectly.

Therefore, from a sample of one, I suspect that chain length is probably okay being a bit long...


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