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I have a Rockrider 5.2 with a Shimano Altus 8 speed rear derailleur. After an accident it started to malfunction. I planned on replacing it with a Shimano Acera M3000 but then it is marketed as 9 speed.

Is it fine if I use a 9 speed rear derailleur on a 8 speed bike?

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With Shimano 8 and 9 speed systems you can interchange derailleurs as they use the same actuation or pull ratio (the ratio of length of cable pulled to lateral derailleur cage movement). The number of sprockets in the cassette and shifter must match obviously.

If you are experiencing poor shifting after a crash you should diagnose the cause before just replacing the derailleur. Take the rear wheel out of the bike and check the derailleur movement with the shifter and by manually manipulating it. If the B-pivot, parallelogram and cage pivot all move smoothly without lots of play, the cage isn't bent and the idler wheels turn freely the derailleur itself is probably fine.

A prime suspect for cause of bad shifting after a crash is a bent derailleur hanger. If you hold the rear of the bike up look down the line of the chain the derailleur cage should look parallel with the chainrings. Good bike repair shops have a special tool to straighten and align hangers.

Also check the cable run, make sure the housing is properly seated in the shifter body, frame stops and the derailleur, and the cable is clamped on the derailleur properly.

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  • Thank you so much @ArgentiApparatus for your useful and timely suggestions! My old RD had completely splited into two different parts so I don’t think I can ever mend it again. Anyways, Thanks a lot again!
    – kartlad
    Oct 3, 2019 at 3:04
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The pull ratio, which is the important bit, is the same for nearly all Shimano 6,7,8 and 9 speed derailleurs.

Other things to look for when changing derailleurs are the capacity, minimum and maximum cog size. These will be fine for the swap you are proposing.

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No, now 9 speed derailleurs are shadow plus and will not shift reliably.

I had a customer that tried 1x8 with an acera m3000 and 11-42 cassette. Not only the guide pulley was far from the smaller cogs making down shifting slow, it never shifted well-enough. It is better to buy a dedicated drivetrain like the Box prime 8 speed.

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    AFAIK there is no 9 speed Shadow PLUS derailleur. And even old 9-speed ones (10 years ago) were Shadow. There must something more to the problem of your customer. Do you work in a bike shop? Do you actually service bicycles? Jun 15, 2021 at 16:31
  • I agree, the 8 and 9 speed derailleurs were not designed for 42t cassettes. In the old days we had triple cranks.
    – ojs
    Jun 17, 2021 at 13:54
  • I use a rd-m3100 on a 8 speed 11-32 with no issues besides a bit poor clearance between chain and cage in some gears. Shifts like a charm.
    – WornChain
    Feb 19 at 23:48
  • I tried a 9-speed 11-42 cassette on a M3000 (rated for 36T max). It shifted OK on the stand and when the chain was clean, not when the bike was being used and the chain got dirty. So the problem is likely to be the 11-42, not the 8-speed.
    – Rеnаud
    Feb 20 at 8:07
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I originally thought 'no'. But given that the derailleur shift distance is governed by the the shifter, (taking this as said above to be true - which it probably is), then effectively there is no such thing as a specific 8 or 9 speed derailleur. It's just 'a derailleur', i.e. a slaved arm. I'm writing this because I'm having shifting trouble, though have found all sorts reasons over the years. I have an 8 speed (older) Sora bike, but Sora seems to be 9 speed now. Why the heck didn't they call it something else? :))

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