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· Cannot bench own weight
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I just switched a 50/34 crank for a 53/39 (switched FDs too). Nothing else changed. I suspect the chain is too short now. The drive train just doesn't sound as smooth as it used to (and nothing is rubbing as far as I can tell.)

Campy Record group if it matters.
 

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If you can't shift into the big ring/big cog combo, the chain is too short. If it makes it, then it is ok. I have a couple bikes that just make it into the big/big combo and they don't make any more drivetrain noise than the others. Could your chain and new chainrings amount of wear be mismatched? i.e. worn chain, new rings, or vice versa.
 

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Different, but related

How do you tell if the chain is too long? I feel mine might be but really have no idea. When I remove the rear wheel (with the chain on the 39 and the 12, 13 or 14), I have to manually reach down and coax the RD pulley around the cassette--in other words, the wheel doesn't just "fall" out when I remove the quick release.
 

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Someone correct me if I'm wrong, but I either read in the Park repair book or Zinn's letters in velonews that to check if you have the correct chain length, shift to the big crank and smallest cog...the jockey wheels on the RD should be directly on top and below each other, making a perpendicular line to the ground.
 

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That is the correct method that I've used my entire career as a bicycle mechanic. If the bike in question has rear suspension, you should typically add a link to make sure that full articulation in the suspension doesn't demolish your hanger if you're cross-chained.
 

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tourdreamer said:
Someone correct me if I'm wrong, but I either read in the Park repair book or Zinn's letters in velonews that to check if you have the correct chain length, shift to the big crank and smallest cog...the jockey wheels on the RD should be directly on top and below each other, making a perpendicular line to the ground.

that is Zinn's method for a double chainring. For a triple, he uses the large-large combo (and not running thru the RD) with an additional two links added to where the chain ends touch (i.e. one inch extra added).
 

· Cannot bench own weight
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Discussion Starter · #7 ·
tourdreamer said:
Someone correct me if I'm wrong, but I either read in the Park repair book or Zinn's letters in velonews that to check if you have the correct chain length, shift to the big crank and smallest cog...the jockey wheels on the RD should be directly on top and below each other, making a perpendicular line to the ground.
Sounds good. I'll check.
 

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the whole story...

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 or the chain guide tab 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).
 

· Get me to In&Out
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You can tell it is too short when you shift into the big-big combo on the road by accident and your rear deraileur explodes! Seriously, if it is a little short, like I run mine 1 link shorter than needed, just be very sure what gear you are shifting into. Like I said, it isn't a problem on my bike because I am used to it and know not to go big-big or bad things will happen. I think the shorter chain helps more with cage tension on the small front chainring. I spend most of my time in the small chainring when I am alone anyways, so it works for me.
 

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the whole story...

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 or the chain guide tab 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).
Very good explanation. My only question is how do I know my rear derailleur is over extended? Do you have any pic. I'd would be nice to know when the tension is on its permissible limit.
 

· Banned Sock Puppet
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Very good explanation. My only question is how do I know my rear derailleur is over extended? Do you have any pic. I'd would be nice to know when the tension is on its permissible limit.
If the chain binds in the large/large combo or you have a hard time shifting there, the chain is too short. Otherwise it isn't a problem.
 
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