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Chain sag after installing new chain

Old 12-05-15, 05:27 PM
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raceboy
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Chain sag after installing new chain

Hi all. Advanced beginner bicycle wrench here. I've successfully changed worn out chains and cassettes a number of times so this has me scratching my head.

My old KMC 10-speed chain was worn out so I got a new KMC 10-speed chain. I used the old chain to cut the new one to length and installed it as per usual. I must have done something wrong because the chain was kinda noisy in the top speed combo--large chain ring and 12 cassette. I hopped on the bike for a test ride and it felt pretty good until I hit a steep hill in the low chain ring and 24-25ish cassette. It started slipping like crazy and when I got off the bike I noticed the chain was drooping like it was way too long or something. It hasn't popped off the chain ring...wha happened???

Because of this, I think I screwed up somehow and that this isn't a case of the cassette being worn out, too. I typically run through three chains (only about 1000 miles per chain near the beach) before I need a new cassette. This is the third so I would expect to replace the cassette next time the chain is shot.

I can always pop the bike onto my bike rack and have the LBS sort it out. I wanted to ask the brain trust here to see if I made some kind of rookie mistake first.

Thanks in advance for any insight/educated guesses, etc.
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Old 12-05-15, 05:40 PM
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Are you sure you ran the chain correctly through the pulleys on the derailleur? Put the chain on the inner chainring and middle cassette cog. Now stand on the righthand drive side of the bike. Follow the chain back from the bottom of the chainring. It should wrap around the bottom or furthest back (or furthest from the pivot) pulley clockwise, then forward and up and around the upper or closer (or closest to the pivot) pulley counter-clockwise then back to the bottom of the cassette cog. Get back to us if this is all correct.

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Old 12-05-15, 05:41 PM
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I wonder if you routed it through the derailleur correctly?

If the derailleur has a tab that spans the cage in the middle, it's an easy mistake to thread the chain to the wrong side of that tab. When you do so, it'll run noisy and sometimes bind so the RD can't take up slack normally.

Before anything else, take a look for that and report back.

EDIT --- Dang, Mooney gt it in while I was still typing. But to add to his instructions for threading. It's actually very obvious, the chain should clear the tab while running in a straight line between the pulleys. If it's touching the tab, it's on the wrong side.

One other note. If your chain is closed with a pin or non-reusable connector, you usually don't need to cut the chain to fix this. Loosen the upper pulley bolt slightly, then remove the lower bolt and slide the pulley out catching any plates or washers to keep the pulley together as you do so. Then you can scissor the cage apart, reposition the chain, and replace the pulley guiding it from inside the chain loop, and pushing the chain back as you bring it into place.
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Old 12-05-15, 06:21 PM
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Thanks FB and mooney,

I went out and double checked the chain threading and it looks good to me. I also compared to the chain threading my other bikes. The chain wraps around the bottom pulley clockwise--let's call it 6:00 to 6:00 to 12:00 and around the upper pulley clockwise--6:00 to 3:00 to 12:00. I also looked to make sure the chain wasn't rubbing against any tabs. The bike has to be in the biggest cog on the cassette and the large chain ring--big/big in order for the pulleys to be aligned under the cassette, if that tells you anything...

Thanks again for the knowledge sharing. I love the tip about how to rewrap the chain without having to cut the chain. Filing that away for future reference!
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Old 12-05-15, 06:30 PM
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Is it possible that the chain is a bit long, or the B-screw misadjusted, so that the upper pulley touches the largest sprocket (throughn the chain).

Shift to low, and take a look. If it looks close or touching, pull the chain's lower loop forward gently which will tilt the upper pulley back and lower. If that seems to help, use the B-screw to bring the entire derailleur a bit lower. HINT- pull the RD down to take the load off the screw as you adjust it, then let it spring back to position and see if things look better.
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Old 12-05-15, 07:21 PM
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i don't think i would bank on a cassette wearing out in concert with a given number of chain replacements. there have been, IME anyway, too many variables to make it a worthwhile metric in determining cog wear. usually, when putting on a new chain, i use slippage, or lack of it, on one or two cogs only as the determining factor.

really, just a mere gentle suggestion.
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Old 12-05-15, 07:37 PM
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Originally Posted by hueyhoolihan View Post
i don't think i would bank on a cassette wearing out in concert with a given number of chain replacements. there have been, IME anyway, too many variables to make it a worthwhile metric in determining cog wear. usually, when putting on a new chain, i use slippage, or lack of it, on one or two cogs only as the determining factor.

really, just a mere gentle suggestion.
While sprocket wear could cause some f the issues, ie noise and slippage, it cannot explain the sag, which points to something keeping the RD cage from taking up slack. Also, wear related slippage usually occurs on smaller or the most commonly worn sprockets. With its greater number of teeth, and muck less frequent use, the largest sprocket is probably the least likely candidate for wear slippage.

Of course it could be wear, but why immediately chalk it up to something that can't be fixed when we haven't exhausted other repairable possibilities..

To the OP -- it could also be some sort of trim/adjustment issue, so check that, but I'm leaning toward the B-screw adjustment first.
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Old 12-05-15, 08:18 PM
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the intent of my post was to gently suggest that cog replacement might be determined by something in addition to a cog vs chain replacement ratio, which was mentioned in the OP. i felt that guessing what the OP might have done to cause the chain to droop had been covered adequately by previous posts.
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Old 12-05-15, 08:30 PM
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Originally Posted by hueyhoolihan View Post
the intent of my post was to gently suggest that cog replacement might be determined by something in addition to a cog vs chain replacement ratio, which was mentioned in the OP. i felt that guessing what the OP might have done to cause the chain to droop had been covered adequately by previous posts.
But that's easy, replace the chain and ride. You'll know soon enough if you also need a cassette. I don't waste time trying to assess sprocket wear, since the chain will tell me soon enough.

BTW- part of my approach is to save a newly removed chain that's in decent shape, and if the new one skips, put the old one back on, and ride until I can't.
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Old 12-06-15, 11:42 PM
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Hi all,

So here's the dumb bell thing I think I did! After I cut the chain I didn't separate the pieces. My attention must have then been diverted by something else. Because this morning I remembered that when I picked the chain up to put it back on the bike, a short length of 5 or 6 links fell off the end. I thought a pin must have slipped for some reason and reattached the piece that had fallen off. What a dope! I bet that I put the full-length chain back on the bike and that is why it is sagging! Truly embarrassing...

I haven't had a chance today to get back out to the garage and test my theory but I'd bet a dollar that's my mistake.

The good news, huey, is that I do follow your recommended procedure of replacing a chain and then test riding to see if the chain slips to determine if the cassette also needs replacing.

Thanks again for your help. After I get out to the garage and check out the bike, I'll report back to close the loop.
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Old 12-07-15, 12:26 AM
  #11  
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Originally Posted by raceboy View Post
Hi all,

So here's the dumb bell thing I think I did! After I cut the chain I didn't separate the pieces. My attention must have then been diverted by something else. Because this morning I remembered that when I picked the chain up to put it back on the bike, a short length of 5 or 6 links fell off the end. I thought a pin must have slipped for some reason and reattached the piece that had fallen off. What a dope! I bet that I put the full-length chain back on the bike and that is why it is sagging! Truly embarrassing...

I haven't had a chance today to get back out to the garage and test my theory but I'd bet a dollar that's my mistake.

The good news, huey, is that I do follow your recommended procedure of replacing a chain and then test riding to see if the chain slips to determine if the cassette also needs replacing.

Thanks again for your help. After I get out to the garage and check out the bike, I'll report back to close the loop.
Determining correct length:

Chain length sizing for bicycles with derailleurs
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Old 12-07-15, 03:57 AM
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Originally Posted by raceboy View Post
Thanks FB and mooney,

I went out and double checked the chain threading and it looks good to me. I also compared to the chain threading my other bikes. The chain wraps around the bottom pulley clockwise--let's call it 6:00 to 6:00 to 12:00 and around the upper pulley clockwise--6:00 to 3:00 to 12:00. I also looked to make sure the chain wasn't rubbing against any tabs. The bike has to be in the biggest cog on the cassette and the large chain ring--big/big in order for the pulleys to be aligned under the cassette, if that tells you anything...

Thanks again for the knowledge sharing. I love the tip about how to rewrap the chain without having to cut the chain. Filing that away for future reference!

You wrote that you have the chain wrapped around the derailleur pulleys clockwise on the lower pulley and clockwise on the upper pulley. Either you wrote that down wrong, or you have the chain wrapped incorrectly.

The chain should be wrapped around the derailleur pulleys going in opposite directions. Coming off the cassette, the chain should be wrapped around the top pulley in the clockwise direction. Leading to the bottom pulley, where the chain wraps onto the pulley in the counter-clockwise direction.

Or, coming from the bottom on the chainrings and back to the bottom pulley, the chain should be wrapped clockwise on the lower pulley, then counter-clockwise around the upper pulley before going around the back of the cassette and up over the top of the cassette.
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Old 12-07-15, 06:59 AM
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FWIW - something still doesn't ad up. If the chain were too long it would sag on the smaller sprocket combinations, ie. small/small where it's slackest and would sag the most.

Your description didn't mention sagging in high gear, and that issue manifested only on the larger rear sprocket, which would just about rule out an overly long chain.

Of course, any help from anyone here depends of the quality of your description. As Roadguy pointed out, you described the routing backward, but I think that's probably simply a tying error, since winding through the RD backward is impossible.
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Old 12-07-15, 11:14 AM
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Originally Posted by RoadGuy View Post
You wrote that you have the chain wrapped around the derailleur pulleys clockwise on the lower pulley and clockwise on the upper pulley. Either you wrote that down wrong, or you have the chain wrapped incorrectly.

The chain should be wrapped around the derailleur pulleys going in opposite directions. Coming off the cassette, the chain should be wrapped around the top pulley in the clockwise direction. Leading to the bottom pulley, where the chain wraps onto the pulley in the counter-clockwise direction.

Or, coming from the bottom on the chainrings and back to the bottom pulley, the chain should be wrapped clockwise on the lower pulley, then counter-clockwise around the upper pulley before going around the back of the cassette and up over the top of the cassette.
Originally Posted by FBinNY View Post
FWIW - something still doesn't ad up. If the chain were too long it would sag on the smaller sprocket combinations, ie. small/small where it's slackest and would sag the most.

Your description didn't mention sagging in high gear, and that issue manifested only on the larger rear sprocket, which would just about rule out an overly long chain.

Of course, any help from anyone here depends of the quality of your description. As Roadguy pointed out, you described the routing backward, but I think that's probably simply a tying error, since winding through the RD backward is impossible.
Check out post #10 ...
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Old 12-07-15, 11:50 AM
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Originally Posted by raceboy View Post
Hi all,

So here's the dumb bell thing I think I did! After I cut the chain I didn't separate the pieces. My attention must have then been diverted by something else. Because this morning I remembered that when I picked the chain up to put it back on the bike, a short length of 5 or 6 links fell off the end. I thought a pin must have slipped for some reason and reattached the piece that had fallen off. What a dope! I bet that I put the full-length chain back on the bike and that is why it is sagging! Truly embarrassing...

I haven't had a chance today to get back out to the garage and test my theory but I'd bet a dollar that's my mistake.

The good news, huey, is that I do follow your recommended procedure of replacing a chain and then test riding to see if the chain slips to determine if the cassette also needs replacing.

Thanks again for your help. After I get out to the garage and check out the bike, I'll report back to close the loop.
I am not believing this story. If you cut chain and it fell apart you would not put it back together with the pin, you would use the master link provided, and then you would be short another master link to close the chain.
And then you would have noticed it.
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Old 12-07-15, 05:28 PM
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Originally Posted by FBinNY View Post
I wonder if you routed it through the derailleur correctly?

If the derailleur has a tab that spans the cage in the middle, it's an easy mistake to thread the chain to the wrong side of that tab. When you do so, it'll run noisy and sometimes bind so the RD can't take up slack normally.
I managed to do this once. It was a little noisy but other than that I didn't detect any problem. Didn't even check it out till I got home, then it was like "what a delta sierra".

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Old 12-10-15, 11:13 PM
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Thanks for the responses! Here is the update. I had indeed been a mechanical idiot and reconnected the chain links to make the chain too long. I took it apart, re-measured against the old chain, and immediately saw what I had done. Cut the chain to the right length again and reinstalled it on the bike. Voila! No sag.

The only problem now is that the bike doesn't want to shift into the smallest cog when I am on the large chain ring. I think I can fix this if I disconnect the rear shift cable and make sure there is no slack. I'll have a chance to work on the bike tomorrow afternoon...
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