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Determining proper chain length(7 posts)

Determining proper chain lengthTNSquared
Oct 6, 2003 12:24 PM
According to the Zinn book, the proper way to determine chain length for a double chain ring setup is that the rear derailleur jockeys should be perpendicular with the ground when the chain is on the big chain ring and smallest cog.

Does this work regardless of chainring size, cassette and rear derailleur combinations, or is it just a starting point?

I'm swapping out an Ultegra 12-25 cassette and Ultegra derailleur and installing an XT 11-32 and XT derailleur. Any extra steps I need to take, or will the derailleur test get me the best chain length?
re: Determining proper chain lengthsaltytri
Oct 6, 2003 12:46 PM
The Sheldon Brown method has always worked fine for me on road or MTB drive trains (and mixed road/MTB, for that matter):

1. wrap the chain on the big/big, bypassing the rear der

2. the chain ends should overlap by one inch (two link segments or one segment and a quick connector)

3. put in a rivet or a quick connector (after running chain thru the rear der, of course) and go riding

If the overlap isn't exactly an inch, I err on the long side without ill effects.
re: Determining proper chain lengthCrankist
Oct 6, 2003 1:07 PM
The method surprised me too, but Zinn and Shimano agree. If the der. is designed for wrapping the 32 then you're prob. OK. Nothing wrong with running it through the gears a few times on the stand to check.
re: Determining proper chain lengthlyleseven
Oct 6, 2003 2:47 PM
The rear der jockey must also be in vertical alignment with the axle, but I assume that is what was meant above.
Zinn=double Sheldon/Zinn= triple re proper chain lengthcharlieboy
Oct 7, 2003 12:12 AM
The Zinn method is spot on for the double chainring setup. The book also describes a method for a triple (with long cage derailleur), which funnily enough is exactly the same as the Sheldon Brown method.
don't bypass the derailleur...C-40
Oct 7, 2003 4:44 AM
Quicky methods may work, but the best way that guarantees the correct length is describe below.

Two simple tests will determine if the chain is the correct length. First, it must not hang loose in the little ring, little cog combination. If there is no tension on the chain in the little ring, little cog combination, remove two links (one inch) at a time, until there is. When the ends of the chain are brought together, some movement of the lower pulley should occur, indicating tension is being applied. Two more links (another inch) may need to be removed, beyond the point of absolute minimum tension, to keep the chain from rubbing on itself as it passes under the upper derailleur pulley. If you want to see how much lower pulley movement will occur, without removing the extra inch of chain, shift up four teeth (11 to 15 or 12 to 16). This has the same effect as removing two links. Once this is done, the chain is set to the maximum useable length. Removing additional links will do nothing but reduce the derailleur's capacity.

Second, the chain must be long enough to avoid over-extending the rear derailleur when shifted to the big ring and biggest cog combination. If the chain is set to the maximum length as described, it should always pass this test, unless your setup exceeds the derailleur's stated wrap capacity. If you deliberately exceed the derailleur's capacity and the derailleur is over-extended in the big ring/largest cog combo, then you must either avoid that combo or add another inch and avoid using the little chainring and the smallest 3 or 4 cogs (since the chain will hang loose).
Too-simple technique that's never failed for me...Silverback
Oct 7, 2003 7:57 AM
A bike shop mechanic told me this when I was about 15--I've used it for every bike I've ever owned and never had a problem: Put the chain through the derailleur, then around the big ring and the large cog, pull it tight and add one link.
His explanation was that you don't want it any tighter than that because you'll break something if you inadvertently shift into big/big, and you don't need it any looser because that's the biggest combo you have.