Sudden strange chain problem(with video)

steve_sordy wrote:

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.