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Sudden strange chain problem(with video)

cppercpper Posts: 48
edited July 2018 in MTB workshop & tech
Today as I was shifting between the 9th and 10th(last) cog, applying just slight pressure on the pedals, something very strange happened. The chain suddenly started slipping skipping(whatever is the correct term) on the 10 cog. It now happens all the time, even when the pedals are just slightly pressed. All 9 other cogs work perfectly. I constantly clean and grease my drivetrain, and the chain+cogs are not worn.
Here is a video which explains better:
https://www.youtube.com/watch?v=VxvhziBC5j4

I thought the H limit screw could've somehow moved so I tried to adjust it with no result. The chain doesn't "climb" on the 9th cog.
I also experimented with the B screw with no result. I also took the wheel off and cleaned the 10th cog and pulleys.

Any idea of what could have happened ? I can clearly see in the video what's happening but can't think of what could be the cause.
Thanks a lot :)

Posts

  • cppercpper Posts: 48
    Chain+cassete+chainring+pulleys were replaced in January .I have a chain gauge, the chain is not 0.75% worn. The 10th work perfectly till now, not a single skip when pedaling as hard as I can. Now it skips even when pedaling slow.
  • steve_sordysteve_sordy Posts: 2,203
    Before I had even seen the video, I was expecting it to be a mech-alignment issue. But I don't believe that it is (though it is always worth checking).

    What I think it is is this:
    Chains don't always wear evenly. I have measured a chain in 8 different places evenly spaced around the length of the chain and I can get widely different results. At first I thought it was my measuring technique, so I remeasured in the same place and got the same result. So it's the chain then!

    I noticed from the video that the teeth of the small gear slot into the chain quite nicely until suddenly it doesn't! The tooth hits the roller square on, which is quite a difference. What I recommend you do is to get yourself a marker pen and spot the chain where it slips and see if it repeats itself around the chain. If it does, then it is deffo the chain, scrap it and get another, they are cheap enough. Even if it doesn't repeat itself, I still believe it is probably the chain. I replace mine at about 0.6% Most wear on the cogs takes place when the chain is worn and it accelerates.

    I've just had another look in as slow a motion as I can make it and the side plate of the chain appears to be catching on the side of next larger gear. Which could suggest that a small adjustment to the limit screw might help. I can't see any in the video, but are there any burrs on the teeth? This is metal pushed from the contact face of the tooth out to the sides. These burrs can cause all sorts of mystery problems.It is a sign that the cassette needs replacing, but a quick fix would be a file down the flat side of the tooth, if you have one thin enough.

    PS: If it is the small gear, then it would suggest that the problem should be at least every rotation, or more if more than one tooth is affected.
  • FRAJFRAJ Posts: 5
    Looks strange. I went through a skipping problem, checking for stiff links, new cassette, new chain etc. Turned out to be the front big ring for me.
  • cppercpper Posts: 48
    Before I had even seen the video, I was expecting it to be a mech-alignment issue. But I don't believe that it is (though it is always worth checking).
    It's not, as said all other gears work fine. Note that the issue suddenly happened when I changed in the 10th cog(in a decent manner).

    What I think it is is this:
    Chains don't always wear evenly. I have measured a chain in 8 different places evenly spaced around the length of the chain and I can get widely different results. At first I thought it was my measuring technique, so I remeasured in the same place and got the same result. So it's the chain then!

    That's interesting. However, I've had chain slip caused by worn chain before, and the slipping firstly occurs when pushing hard on the pedals, then as the wear intensifies with time, the slipping also starts to occur when pedaling with less force. But as said previously, the slipping started in a matter of a second. Now it slips all the time, even with pedaling with just a bit of force. Until today, no single slip occurred even when pedaling as hard as possible. In conclusion, I believe the cause can't be any type of wear, be it on the chain or cogs.

    I noticed from the video that the teeth of the small gear slot into the chain quite nicely until suddenly it doesn't! The tooth hits the roller square on, which is quite a difference. What I recommend you do is to get yourself a marker pen and spot the chain where it slips and see if it repeats itself around the chain. If it does, then it is deffo the chain, scrap it and get another, they are cheap enough.

    Done that. I didn't use a marker pen, I used the quick link as a marker. It doesn't happen in the same place on the chain. When pedaling with my feet(not with my hands, as in the video) it slips continuously, maybe 3-4 times in a 360° pedal movement.

    Even if it doesn't repeat itself, I still believe it is probably the chain. I replace mine at about 0.6% Most wear on the cogs takes place when the chain is worn and it accelerates.

    I've just had another look in as slow a motion as I can make it and the side plate of the chain appears to be catching on the side of next larger gear. Which could suggest that a small adjustment to the limit screw might help.
    Done that too, definitely not the problem. The chain doesn't climb on or touch the 9th cog.

    I can't see any in the video, but are there any burrs on the teeth? This is metal pushed from the contact face of the tooth out to the sides. These burrs can cause all sorts of mystery problems.It is a sign that the cassette needs replacing, but a quick fix would be a file down the flat side of the tooth, if you have one thin enough.
    Interesting, never heard of this one before. Tomorrow I'll remove the cassette and 11th cog, clean it and inspect it thoroughly.

    PS: If it is the small gear, then it would suggest that the problem should be at least every rotation, or more if more than one tooth is affected.

    I don't think two teeth can be damaged when simply changing from 9th to 10th gear...If you look in the video, the chain starts to "climb" in different places.

    Thanks for your time :D
  • steve_sordysteve_sordy Posts: 2,203
    Fit a new chain and I'll bet the problem goes away.

    If it doesn't, then you have a spare! What's to lose? :D
  • 02GF7402GF74 Posts: 1,294
    It can be chain, mech or cassette or cable. Are you sure the limit screw is adjusted correctly and that the cable has enough slack.


    If yes, then try a new chain, BTW how worn is the chain and cassette.

    If still no joy, replace shifter cable.

    Then it starts to get expensive.

    It could be the parallelogram spring in the mech going weak.
  • cppercpper Posts: 48
    As I took everything apart yesterday, I found the issue. It was a straw carefully rotated around the groove on the 10th cog, covered with hardened black stuff, making it very hard to notice. I didn't even knew there should be a groove on the cog.
    In my first post I said I cleaned the area, but without removing the cogs from the cassette, the stuff there wasn't visible. I cleaned the are between the cogs with a cassette brush but it seems it didn't remove the grime.

    Here is the groove which was filled with that straw and black stuff:
    xaJeqbc.jpg



    And here is the culprit:
    https://youtu.be/p2mrV2TRODw


    I cleaned the area and entire drivetrain, and now everything works super smooth. Thanks to everyone for helping out !
  • 02GF7402GF74 Posts: 1,294
    Haha, the joys of diagnosing from a distance, sometimes you just need to be there...

    Simple fix and another lesson learnt.
  • steve_sordysteve_sordy Posts: 2,203
    The Joy of Fix!
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