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Chain Length 11-34T -> 11-30T

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Chain Length 11-34T -> 11-30T

Old 05-31-19, 01:17 PM
  #1  
August West
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Chain Length 11-34T -> 11-30T

My Trek Domane came with an 11-34T cassette but for my local road rides around here I don't need that low of gearing so I've ordered an 11-30T for the tighter gear ratios. Rear mech is an Ultegra RD-R8000-GS med. cage.

Is there any reason I can't leave the chain length as it is when I run the 11-30T? When I ride steeper terrain I would like the option to go back to the 11-34T cassette but it would make things simpler if I could use the same chain instead of having dedicated chains for each cassette.

Thanks.
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Old 05-31-19, 01:26 PM
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It might work - try it. If your chain was sized for 11-34, you will be 4 links oversized which you may or may not notice.
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Old 05-31-19, 01:30 PM
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Originally Posted by August West View Post
My Trek Domane came with an 11-34T cassette but for my local road rides around here I don't need that low of gearing so I've ordered an 11-30T for the tighter gear ratios. Rear mech is an Ultegra RD-R8000-GS med. cage.

Is there any reason I can't leave the chain length as it is when I run the 11-30T? When I ride steeper terrain I would like the option to go back to the 11-34T cassette but it would make things simpler if I could use the same chain instead of having dedicated chains for each cassette.

Thanks.
Maybe when the new cassette arrives fit it to the other stuff you have and test ride, then you'll know.
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Old 05-31-19, 01:54 PM
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You almost certainly don't need a different chain for each cassette.
A 4t difference in the biggest cog does NOT mean you need a chain 4 links shorter.

Whether you can use your existing chain for the new 30t cassette depends on how closely they sized the chain for the 34t.

Here's how you tell - put your current chain in the big-big and see how far the idler pulley comes forward - if it's really maxed out, it'll look like this, and you'll be fine with the same chain on a 30t cassette:



If the chain is longer than that, the idler pulley will be hanging down lower and might not be able to wrap the extra length with the smaller cassette. If you want to make sure - resize the chain like this or put on a new chain sized like this for the 34t.
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Old 05-31-19, 02:09 PM
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If your existing setup is, as an example, a 42/26 and 11-34 (just made up numbers) then your existing max tooth count is 76t, and minimum is 37t. The chain must be long enough to accommodate up to 76 teeth, plus chainstay length and derailleur S-takeup length. That same chain must be short enough to not cause derailleur cage overlap in the 37t setting of 26/11.

Let's say your new setup is 42/26 and 11-30. Now the maximum you must accommodate is a 72t configuration. The minimum is unchanged.

But I'll bet dollars to doughnuts that your 11-34 cassette has a cog somewhere around 30t. Maybe it's 29, or 31, but close to 30. There's no reason to think that because you went to a smaller cassette where the maximum rear cog is the same size as your #3 cog used to be would require changing the chain size.

"Require", I say... your drivetrain performance will be no worse using the same chain length. However, running a longer than necessary chain length is not optimal. Given that you are going to a smaller cassette, you can get away now with optimizing your chain length in a way that could possibly improve drivetrain performance more than was possible with the old cassette and its longer-chain requirement. In theory you could reduce the chain by four links (two outer, two inner) and possibly improve performance slightly because there will be less slack for the derailleur to take up. The smaller cassette enables you to run a chain length that is less demanding on the derailleur's capacity.

Now if your minimum setting changed too; lets say you went to a 10t smallest rear cog... in that case it may be necessary to remove a link to eliminate derailleur cage overlap in the small/small setting. Or if you went to a 36t rear cassette, it may be necessary to add two links if your chain was already as short as possible before going to a larger cassette.

It's also worth mentioning that if your chain has more than a thousand miles on it, cassette swap time is chain replacement time too. True, chains last a lot longer than 1000 miles if properly cared for. But a brand new cassette is less prone to early wear when it's not mated to a chain that has any degree of stretch (wear). My suggestion is this: Buy your new chain. Then resize your old one and ride around a bit to test the size. If it is too short or too long, it's better to discover this with the chain you're about to throw away than with the new one.

Last edited by daoswald; 05-31-19 at 02:14 PM.
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Old 05-31-19, 02:54 PM
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If you're not decreasing the size of the smallest chainring or the smallest cog, you don't need to shorten the chain.
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Old 05-31-19, 03:12 PM
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Thanks guys. The second cog on the 11-34T is a 30. So it would be the same as that only in the inner most position instead of the second. May have to wind the B screw in a bit get the proper spacing to the new smaller big cog. I'll put it on give it a try but I'm guessing it will be fine. If not, I'll shorten one of my chains as required.
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Old 05-31-19, 04:34 PM
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Originally Posted by daoswald View Post

"Require", I say... your drivetrain performance will be no worse using the same chain length. However, running a longer than necessary chain length is not optimal. Given that you are going to a smaller cassette, you can get away now with optimizing your chain length in a way that could possibly improve drivetrain performance more than was possible with the old cassette and its longer-chain requirement. In theory you could reduce the chain by four links (two outer, two inner) and possibly improve performance slightly because there will be less slack for the derailleur to take up. The smaller cassette enables you to run a chain length that is less demanding on the derailleur's capacity.
Define "optimal". Chain sizing using the longest chain the RD can take up by removing links until the small/small combo is a very common method. More chain=more spread out wear and less tension on the RD cage spring for any given gear=less drivetrain friction. Sizing using the shortest chain length useable results in a severely stretched cage in the big+big combo as well as an unoptimal cage position in any of the larger cogs when on the large chainring.
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Old 05-31-19, 08:37 PM
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You'll be fine!
You'll have an extra link set to spread the chain wear. AND the ability to switch back to the 34T without having to lengthen it.
I doubt you'll need to touch the B screw.
Just ride it.

Last edited by Bill Kapaun; 05-31-19 at 08:42 PM.
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Old 05-31-19, 08:55 PM
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Unless the chain droops on the little-little combination, you will be fine.
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Old 06-01-19, 12:34 AM
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Originally Posted by wgscott View Post
Unless the chain droops on the little-little combination, you will be fine.
It wouldn't droop any differently than now.
An 11T small cog is an 11T small cog.
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Old 06-01-19, 06:51 AM
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It's a mistake to think that properly sized means the shortest possible chain length. The longest works just as well. As long as you don't exceed the RD's wrap capacity, the little/little method works perfectly. It's what campagnolo recommends.
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Old 06-01-19, 04:49 PM
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4 links equals 2 inches. Normally not a problem.
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