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Problem with new chain - Suggestions?

Old 06-24-21, 10:25 PM
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MNebiker
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Problem with new chain - Suggestions?

Bike is a 24sp commuter/hybrid with an electric conversion kit utilizing a front geared hub motor. About 6000 miles on the bike since the kit was added, no issues other than a battery replacement this spring. I ride it only in pedal assist mode, so I am always exerting force on the pedals - about like an exercise bike with moderate resistance.

Rear drive train is stock Shimano 3x8 24 speed, has performed flawlessly, shifts clean and crisp, OEM chain is Z8 series KMC. Had no issues with the chain until it broke (I was planning to replace it whenever I got around to it, suspected it had stretched a bit, but it still performed fine.)

I should not have put off the replacement, because Last week I snapped the chain climbing a long hill on the trail. It did not get tangled or jam in the derailleurs or frame - it just ran through and was trailing on the ground, caught in rear cage by the damaged link. I broke the chain, removed 2 damaged links, and rode 6 miles home with no issues, and ordered a new KMC Z8.3 chain, which KMC data states is compatible with all Shimano 8sp product.

The old chain had 114 links, including the 2 damaged ones I removed. The new chain had 116, so I removed 2 and installed the new chain. It is quiet, shifts perfectly. Both derailleurs are clean, lubed, and move smoothly They are adjusted to shift precisely and there are no shifting issues. High and Low limit screws are set correctly. There are no glitches in the shifting, both on the stand and on the road.

But - when the chain is on the smallest rear ring (11 tooth) it skips while pedaling, even under a relatively light load - no matter which front ring is used. Shift it up one cog and all is smooth as silk. I have tried the chain with 116 links, tried various indexing and limit screw settings, pulled the derailleur, cleaned and reinstalled, but no improvement. The hanger is not bent. Bike is now running with 114 links, proper limit settings and smooth indexing in all gears. I rode it 20 miles yesterday - all was fine if I avoided using the smallest rear cog.

I would suspect a worn freehub mechanism, but it does not skip in any other gear, and was fine until the chain replacement. The teeth on the small cog show some wear, but not excessive, and again, it shifted fine with the old chain.

While I had the bike on the stand I also replaced both brake cables and both shift cables - preventive maintenance while the bike was down. I stretched the cables when installing, and anticipate needing to touch up the adjustments a bit after some use, but as I indicated, everything is working fine at present - except for the skip on the small cog.

My next plan is to disassemble the Shimano freehub and see if I can find a new 11 tooth ring. Other than that, I've run out of ideas . . . . any good suggestions or something I might have overlooked?
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Old 06-24-21, 11:53 PM
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alo
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If it happens with all front chain rings, it is not a chain length issue.

My first guess would be a worn sprocket.

It is also possible the derailleur does not have enough range of movement. But if it worked before replacing the chain, that is unlikely.
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Old 06-25-21, 12:54 AM
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CliffordK
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6000 miles is a lot for a chain. Use a tape measure or ruler and measure the distance between pins at 12" (slightly more).

KMC makes E-Bike specific chains.

E8 Sport & E8 Sport EPT.

You probably need to either replace the cassette, or the 11T sprocket. If that is the only sprocket causing you problems, I'd try it first.
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Old 06-25-21, 07:29 AM
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I wouldn't mess around with the freehub until you've exhausted other possibilities. I'm with CliffordK, suspect the cassette is worn. Your LBS or a friend may have an 11 tooth sprocket that's lightly used, otherwise you'll probably have to replace the whole cassette.
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Old 06-25-21, 07:58 AM
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A new chain on a used (i.e. worn) cassette is a standard recipe for skipping under load. An 11T cog, if used frequently, is going to wear faster than a larger cog so that is likely the reason for your problem. As noted, find a new replacement 11T cog or replace the entire cassette.
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Old 06-25-21, 09:22 AM
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Bike doesn't need an ebike chain.. it's a hub motor.
My guess is OP rode the bike all the time in the highest gear (smallest cog).. which is typical for ebike riders to do.
6,000 miles = worn out 11t cog.
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Old 06-25-21, 12:35 PM
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Thanks guys . . . your comments confirm my suspicion of the worn cog, even though it worked fine on the old chain. I just wanted to cover any problems I may have somehow created and overlooked with the new chain and cables. Always good to get some other opinions.
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Old 06-25-21, 01:37 PM
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Small cog wears out the fastest, so you might try to not use it so much, if feasible, by using the bigger chainrings more.
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Old 06-26-21, 04:41 PM
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The worn cog worked on the old chain because the old chain was worn (elongated) as well; IOW the wear on the chain matched the wear on the cog.

Check your chain length every several hundred miles. As said, 6000 miles is an eternity for a chain; it elongates to the point that it wears the other drive components long before that.
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Old 06-26-21, 05:35 PM
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Originally Posted by trailangel
Bike doesn't need an ebike chain.. it's a hub motor.
My guess is OP rode the bike all the time in the highest gear (smallest cog).. which is typical for ebike riders to do.
6,000 miles = worn out 11t cog.
This has always made me wonder why mid drive is considered to be preferable.
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Old 06-26-21, 05:55 PM
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Mid drive ride more like a bike... than a moped. Mid drives much, much better climbers because you can gear the motor down with rear cassette.
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Old 06-26-21, 06:10 PM
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You can find individual sprockets on E-Bay. I've seen Shimano on the past, but the company has been working hard to give poor service, so all I can find at this time are off-brand sprockets from China. Your LBS may be able to help.

https://www.ebay.com/itm/402845699389 (8s is in the middle of the listing).

Nonetheless, you could buy a few of the 11T sprockets, and perhaps the next larger size for your cassette.

It is quite possible that in a few thousand miles when it is time to replace the next chain, more sprockets will skip, and perhaps it will be time for a new cassette.

Or, buy a cassette now. Grab the sprockets you need, and then order the replacement parts from China (which could take a month).

Last edited by CliffordK; 06-26-21 at 06:14 PM.
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Old 06-27-21, 02:03 PM
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CliffordK - I took the easy way - ordered a new cassette for a quick fix. Then I can take the time to source the individual Shimano rings and rebuild the old one as a spare. But it might not be worth the trouble as complete new units are relatively cheap.

trailangel - I don't like the "moped" feel either - I even removed the throttle assembly to make room for some ergo grips, so it won't move unless I pedal. I converted the bike primarily to assist my old knees on hills, and quite often turn off the electric on flat terrain. I still want the exercise, not just an "electric ride" and use only the lowest pedal assist setting.
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