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Measuring chain length

Old 08-03-08, 08:26 AM
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Measuring chain length

I'm doing my first chain replacement and I'm not certain how to measure the chain length.

The guy at the LBS advised threading the chain and finding a length where
1) when on the largest front&back gears, the derailleur isn't too tight (hitting cogs), and
2) when on the smallest front&back gears, the derailleur isn't too loose.

However, the Sheldon Brown site simply suggests running the chain around the largest gears and using that length. I'm worried that might not be loose enough when threaded around the derailleur, though.

What to do?

Thanks
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Old 08-03-08, 09:22 AM
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Two easy methods:

1. Assuming your current set up works and you still have the chain, lay them both out on the floor and make the new one the same length as the old.

2. If you don't have the old chain or if it was the wrong length, wrap the chain around the largest chainring and largest rear cog then make the chain 2 links (1") longer than the length required for that distance.

In that big/big gear combination, the derailler will be pulled almost straight (remember you added an inch) and will not add any distance to length required for the big/big combo. Just don't go changing out your cassette for something bigger and not adding in extra links

One more thing, to keep your chain from being too loose in the little/little combo, you just need to make sure your derailler has enough wrap capacity to handle it.
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Old 08-03-08, 10:13 AM
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Originally Posted by ISaacG
However, the Sheldon Brown site simply suggests running the chain around the largest gears and using that length. I'm worried that might not be loose enough when threaded around the derailleur, though.

What to do?

Thanks
Use Sheldon's method. Run the chain around the largest chainring and the largest cog WITHOUT going through the rear derailleur, add one full link (i.e. two pins). Round up as needed to get the mating ends to match properly i.e. one inner and one outer link if using a pin connector or two inner links if using a master link. Cut the chain to the length you have determined, then thread the chain through the rear derailleur and fasten the ends together.

The rear derailleur has no influence on chain length when the chain is stretched tight as the cage follows in a straight line along the lower run.
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Old 08-04-08, 12:20 PM
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The old chain was about 1.5 to 2 inches longer than Sheldon's method indicated. I used Sheldon's approach and made my chain a link or two shorter than the old.

The derailer on that bike was... odd. Looking at the other derailers, they all appear to have the spring fixed. On this derailer, the spring arm is exposed. There are two hooks on the derailer that can catch the spring. I rotated the derailer 270* CCW and it then had proper tension. Well, I unrotated, put the chain on and then slipped the spring arm into the hook. Maybe pictures are in order.
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Old 08-04-08, 01:42 PM
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It's simpler than that. "Assuming" your gears ratios are MATCHED to the RD cage. Small overlapped ratios and NO HUGE GAPS in gearings!

Big Chainring + smallest cog, cut chain where RD cage is more/less pointing vertical to ground, as measured from the center of the two pullers.

Last edited by jsmithepa; 08-04-08 at 01:46 PM.
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Old 08-04-08, 01:44 PM
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The only chain sizing method you need is big/big +1 link.

All others are worthless.
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Old 08-04-08, 05:34 PM
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Originally Posted by operator
The only chain sizing method you need is big/big +1 link.

All others are worthless.
This ought to be a sticky!
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