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I have replaced the Shimano Deore 9 speed cassette into another. Both are 9 speed and the teeth count matches (it is 11).

The low gears with the new cassette work great at expected, but the 9th (the fastest) gear provides no any useful traction before it skips. From that I hear I am under impression that the chain simply rotates around the cog with not enough interaction.

It worked with the previous cassette. Have I assembled it wrongly? I tried to adjust the cable tension but this works on other cogs and does not help for the smallest cog.

The chain in new. The previous cassette worked with this chain, and later I replaced the last 2 cogs - the replaced cogs also worked well. Hence I assume it cannot be not enough tension or anything the like. But finally I needed to replace the whole cassette as other cogs are also worn.

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  • Have you tried adjusting the rear derailleur limit screw slightly? Did you install the cassette correctly, with enough torque and without any excess spacers?
    – Michael
    Commented Dec 16, 2021 at 14:03
  • How would not enough torque would affect this? Was not wrestling like King Kong, has been afraid to break something.
    – nightrider
    Commented Dec 16, 2021 at 16:05
  • The cassette lockring requires ~40Nm of torque. That’s a lot. I guess all this torque/force could compress the sprockets or plastic spacers (or at least any dirt) ever so slightly. In general it’s a good idea to fulfill this torque requirement to prevent the sprockets from biting into the freehub body during load. Unless you have a really long lever arm you’ll have a hard time exceeding 40Nm. Just make sure the lockring tool is properly inserted and doesn’t slip out. Oh, and use grease on the threads.
    – Michael
    Commented Dec 16, 2021 at 16:46
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    Does the chain run properly through the rear derailleur?
    – Lars Beck
    Commented Dec 19, 2021 at 7:56
  • The question makes sense but I have checked on other similar bicycles nearby.
    – nightrider
    Commented Dec 19, 2021 at 9:06

2 Answers 2

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You say the chain is new but also that you were running it with the old cassette. If the old cassette was sufficiently worn, it could accelerate wear by enough to cause skipping under load on a new cog even if the mileage seems low. Measure the chain wear to see how fresh it really is.

If it's not that, the next things to check are the high limit adjustment and the hanger alignment. You wouldn't normally expect either to be functional on the old cassette/chain but not the new one, but it's still worth looking.

If you've somehow added a spacer to the cassette where it's not supposed to be, that could do it too, but usually that won't come into play with 9 speed unless it's on an 11-speed road hub.

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The spacing between the last two cogs was less than between other cogs, and not enough.

The hint to this solution was the two cog segment (last and second to last) from another (Shimano Alivio) cassette that worked well as a temporary fix for the old cassette (making the gears 9 and 8 usable again). I simply placed this segment on my new cassette and it worked there no problem as well.

I have looked into another new Deore XT cassette. The spacing between the last two cogs is the same as for other cogs. There was something wrong with this spacing. Probably the wrong cog was swapped by chance in web shop or maybe by a customer who returned the product. With the two cogs and spacer from Alivio, now it works well.

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  • From all the images and specs I can find of the Shimano CS-M770 there shouldn’t be a spacer between the two smallest sprockets. The smallest sprocket is shaped in a way which already includes the “spacer”. There should be a thin washer between lock ring and smallest sprocket. For the 11–32 cassette there should be a 2.56mm black spacer between 16t sprocket and 14t sprocket. tnc-hamburg.com/TNC-Shop/images/product_images/info_images/…
    – Michael
    Commented Dec 17, 2021 at 8:58
  • But still you think the last gap between the cogs must be comparable to the previous (next to last gap)? How I initially assembled, it was definitely not so.
    – nightrider
    Commented Dec 17, 2021 at 9:13
  • Yes, the spacing between sprockets should be the same for all of them. Strange this was not the case for you. Is your smallest sprocket just a simple flat piece of stamped steel? Where did you get the cassette from? Is it maybe a fake or mix-up?
    – Michael
    Commented Dec 17, 2021 at 9:31
  • Bought in the web shop that is expected to be more or less normal. Yes, it is flat and not shaped to produce the distance.
    – nightrider
    Commented Dec 17, 2021 at 9:57

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