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I am having a problem with my Decathlon ELOPS 500 electric bike. On the rear cassette the chain skips it doesn't specifically change up or down gear. It just skips on the same cog. I thought it might be the free wheel mechanism, but it's not because I can see the chain skipping visually. It makes no effort to skip up or down, it just jumps and reseats.

The gears are indexed properly and it got a service at the bike shop but they said they checked all cogs and couldn't reproduce the problem which is almost certainly a porky.

It happens intermittently sometimes I can cycle for a few days to and from work and not experience it and other times it'll happen repeatedly on the same journey. It happens both at high load and a low load and it is possible it's on the two smallest rear cogs.

The bike is a year old and lightly used. The chain was replaced with the correct manufacturer chain to address this but it didn’t make a huge difference. I’m reasonably sure the derailleur isn’t bent.

The only final guess I have is that maybe the chain is loose. On the smallest rear cog the btwin derailleur is at 7 o’clock, which seems okay to me (5 o'clock on largest rear cog) but the chain is a bit slack to the touch. I could remove links to shorten it or replace the derailleur if it’s not providing enough tension (But it feels okay), but I’d like some opinions first to avoid replacing the whole drivetrain one piece at a time.

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    Worn sprockets (if it only happens on a few select especially) causes slippage. A new chain can perform even worse if worn enough. Select suspected gear, apply brake firmly, backpedal slightly, then look closely at what happens at the cassette when you apply torque to the pedal. If the chain visibly rises up/slides on the teeth the meshing is poor and rings/chain worn. If nothing happens, then this is less likely the issue. Repeat and compare to a good gear/cog if inconclusive. Commented Apr 4 at 19:04

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"The only final guess I have is that maybe the chain is loose. On the smallest rear cog the btwin derailleur is at 7 o’clock, which seems okay to me (5 o'clock on largest rear cog) but the chain is a bit slack to the touch."

By this clue, and the information you are giving that the skipping occurs on just the smallest rear cogs, the chain tension is the suspect here.

The rear derailleur is not putting enough tension on the chain when on the smaller cogs, and that will induce the skipping you are experiencing, as the chain wrap around the rear cogs will be reduced, and without the tension it is prone to skip.

It is possible that your rear derailleur is a bit "gummed up" such that it is not reliably putting tension in the small cog position. One test you can try to verify this is to see if the rear derailleur can be rotated more by hand to tension the chain when on the smallest cog. If so, then ride the bike to test it for skipping. If it does not skip, you have likely found the culprit. Note that if the derailleur moves back to its previous position (from a bump or vibration while doing the test ride), it may start skipping again.

If it is a sticky rear derailleur, you can try to clean it up to restore the "snap" in the rear cage to properly tension the chain in the smaller cogs. This would involve using a bit of solvent to loosen up the gummy grease. I had the same problem with a Shimano Ultegra 6800 rear derailleur (the chain would literally sag noticeably in the smaller cogs), and for me, I removed the rear derailleur and partially disassembled it to clean out the stickiness and re-greased it with a light grease. It still works great today.

If you do try to disassemble the rear derailleur, be careful, it is not easy for a novice. It may be something you would get a trusted bike shop mechanic to do. One other option would be to replace the rear derailleur. You would need to take cost and availability into consideration.

In the interim, I would avoid the smaller cogs until you get this addressed, so that you avoid undue wear on those cogs.

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  • That's very interesting. I replaced the chain with an exact match 2 months ago and in doing so I didn't notice any problem with the rear derailleur, it snapped back to full retraction fine. However I can see that there is a very slight sag in the chain, it's not much but it is there. I also compared it to other bikes by pulling the chain gently, and while the difference is slight, it is perceptibly looser. In the morning I will try WD40 on the derailleur. Would shortening the chain help? The length seems correct and matches the original. Is there a tension control on the derailleur? Commented Apr 4 at 20:07
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    Do not shorten the chain unless you measure that it's too long. Too short chain risks breaking the rear derailleur (hanger) when shifting to big-big. You can measure if your chain is of correct length easily (e.g. this guide from Shimano or this guide from Park Tool). Commented Apr 5 at 12:55
  • @spl in my one experience with this, my RD snapped back fine for most chain positions. Only in the smallest cogs (while on the small chainring in front) did the stickiness overcome the RD cage spring tension. It was a little tricky to diagnose at first because in most cases the tension was fine. I also forgot to mention that the chain length could be an issue (if too long). It usually is correct from the factory, but… even that is worthy of inspection/investigation to eliminate causes. Commented Apr 5 at 16:01
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I had something similar and it was the quick link in the chain. The quick link might have an arrow on it and that arrow should face forwards (the direction of travel) when looking at the bike.

The quick link has one curved and one flat edge and if it is installed backwards then it will cause the chain to slip (especially on the lowest/hardest gears).

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Two words: Sprocket wear.

Cassette teeth are rather short for their purpose because they have to accomodate the chain across them. As such, they barely reach over the center line of the chain. And when the chain wears them down, it tends to eat away at exactly the one corner of the teeth that retains the chain on the sprocket. The inevitable consequence is that the chain gets pushed up on the teeth as it tries to rotate the sprocket, eventually lifting itself fully above the teeth, jumping a tooth forward and reseating itself temporarily. The chain skips.

As such, there is only one remedy: You need to replace the worn cassette. You also need to check the wear of your chain to decide whether to replace that as well as a lengthened chain will wear down the new cassette very quickly.

This failure mode is inherent to chain shifted bikes. Single speed bikes as well as internal gear hubs use sprockets with significantly longer teeth that are immune to this failure mode.

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  • And this could happen after 1 year and 1200 km? You see I had it in a bicycle shop and they said they checked that ... however they were pretty useless overall and may have just not been interested in finding the problem. @cmaster - reinstate monica Commented Apr 8 at 11:10
  • @spl Depending on conditions (chain wear status, cassette material, forces, dirt, lubrication) it could happen within 100km, I'm sure. I totally ruined a new IGH sprocket within a week or two simply by not replacing the worn chain along with it. Since it was an IGH sprocket, the chain couldn't skip, but it quickly ate its way into the flanks of the teeth again, and I was back to square one with worn chain and sprocket in no time. Derailleur setups are a lot less forgiving as they don't have as much material to abrade away before the chain starts skipping. Commented Apr 8 at 13:33
  • @spl why not just check it since you seem to have little faith in the shop and it takes like 1 minute to do? My suggested test also works on chainrings. It isn't clear from the Q if the issue was always there or developed after 1 year of use. The latter indicates wear or damage. Commented yesterday
  • It's probably not sprocket wear if it only happens infrequently, like every couple of days. Sprocket wear tends to remind you of its existance with a skip several times a minute. Commented yesterday
  • @Kaz That depends entirely on how frequently you excert your top force on the pedals. Some people spin their legs very evenly, they will have the experience you describe. Others use their top force only during that one ascent on their route, or when they start from that traffic light where they know they'll only catch the next one green if they start like a bullet from a gun. For these people, skipping will only happen at these points, and when they are in that one bad gear, so rather infrequently until the wear worsens a bit. I know exactly where my chain would skip if it could skip. Commented 19 hours ago
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Sounds like a worn cassette/freehub body, new chain on old cogs can cause skipping, especially on smaller gears. I’d check cassette wear and derailleur hanger alignment before replacing more parts.

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This happened to me. The spacer that is behind the biggest gear is a different thickness to the spacers between gears. If that end spacer ends up somewhere inside the cassette rather than at the end end, all gears beyond that wrong spacer will be out.

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    The original question did not state that the cassette was removed, just the chain replaced. Additionally, that would be an indexing issue or a shifting range issue. This is just a chain skipping-drivetrain issue. The original question even stated that the indexing is good. So cassette wear, chain wear, chain tension are the suspects. Commented Apr 5 at 18:51

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