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I am considering upgrading the rear derailleur in my commuter bike since it has some considerable scratch damage and since I have to change the rear freewheel / cassette for some functional damage.

While looking at the entry level Shimano components, I started considering the Altus RD-M2000 / Acera RD-M3000 / Alivio RD-M4000 since they are still cheapish (at 20 something, 30 somthing, and 40 something USD respectively) and they are "shadow" (narrower profile and stronger tensor spring to reduce chain whips/bounce and have a lower chance of them "shifting in a bump" or coming off altogether)

Are the shadow vs non-shadow rear derailleurs better for bumpy commutes? (rather than just for downhill/way bumpier rides) Are the Altus / Acera / Alivio comparably durable? Should I go for the cheaper Altus one or the Alivio one if I can spare the extra bucks?

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    Since you've described your rear derailleur that you wish to replace as having, "considerable scratch damage," perhaps you'd benefit by having the inboard, protected design of a Shadow derailleur. – Jeff Mar 3 '19 at 12:50
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Shadow describes the design that brings the derailleur body inboard making it less prone to knocks and damage, it does not help with chain slap. For that you need a clutch or in Shimano phraseology "Plus" (SRam "type 2")

You can get Shadow derailleurs without the clutch (e.g. M592), so if you want the clutch, look for a "plus" - I believe the lowest in the range with a clutch is Deore.

Without getting into buying advice (Off topic here), the difference between components one level apart is small enough most people will not notice. My choice would be based on internet shopping for the best I could get within my budget as often you can pick up great discounts.

Shadow is not a big deal unless you regularly wipe out you derailleur in rock gardens, but I would pay a few dollars more for a shadow Plus if you are noticing an annoying amount of chain slap, but from the sound of it a Deore will be more than you want to spend.

If you are on a tight budget, also have a look at Microshift derailleur, don't think you have to stick with the $himano tax.

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    $RAM is more worthy of the $ in their name, though I agree that most bike stuff is over priced. – Purr Mar 4 '19 at 3:45

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