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I'm looking at buying an e-bike conversion kit with a rear geared hub motor that requires a freewheel.

On my current bicycle, I've an 8 sprocket cassette (Shimano HG CS-HG41-8an) and my rear derailleur is Acera (RD-M340).

Will this be compatible with a Shimano 8 sprocket freewheel?

How do I go about determining if these are compatible? Is there any compatibility info online?

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Derailleurs don't care if you have a freewheel or cassette. Also, you don't have to buy a Shimano freewheel; all modern freewheels will be inter-changeable in their threading/usage (most likely you will end up with a Sunrace or random Chinese one, anyway).

You have several parameters when you buy a rear derailleur (aside from things like direct mount vs non direct mount for getting the derailleur on the bike); these specs are on the info sheet for the derailleur:

  • Max/min cog (gears in back) sizes (self explanatory)

  • Derailleur capacity (how much slack the derailleur can take up in the chain); the necessary capacity depends on the cog and chainring sizes; the necessary capacity is (largest chainring - smallest chainring) + (largest cog - smallest cog)

  • Compatibility with the shifter (with an 8 speed shimano setup, you can use <=9 speed Shimano mountain derailleurs/ <=10 speed Road derailleurs)

Derailleurs are sold in short/medium/long cage versions, with higher capacity as the cage length increases.

Changing gearing may also necessitate adjusting the derailleur and replacing the chain (on length grounds, though most people coincide cassette+chain replacements anyway).


If the parameters above fit (cog size, capacity; since your derailleur is already mounted and working with the shifter), you just need to do adjustments. If not, you'll probably end up buying a long cage Shimano mountain derailleur which is 8 or 9-speed.

  • How do I find the list of supported cog sizes for a derailleur? Are you referring to the number of teeth in each cog? I've did a search in the Shimano's techinical documentation section for my derailleur and I've only found a couple of comprehensive service manuals but these don't appear to include such info. – axk Oct 28 '18 at 18:47
  • What about the distance between the teeth, is this the same between freewheels and cassettes? – axk Oct 28 '18 at 18:54
  • Yes the distance between teeth, distance between sprockets will be the same. The number of teeth is what i mean by min/max cog sizes; these are listed on the derailleur's product page. – Batman Oct 29 '18 at 23:08
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Cassettes and freewheels from different manufacturers are basically interchangeable.

The spacing of sprockets is pretty much standardized. The spacing is different for different numbers of sprockets but basically all 8 speed cassettes or freewheels have the same spacing.

An archive of Shimano's groupset can be found here. The M340 Acera was offered 2002-2007. (Shimano groupset timelines are on the Shimano Wikipedia page.) The specification docs are quite large but you can search for the derailleur component name (i.e. RD-M340) and find the max sprocket size and total capacity specs. The derailleur was offered in long and short cage versions so you'll have to figure out which you have.

  • Have found they have silver (-S) and black (-L) versions, haven't found large and small. Silver and black appear to have the same specs – axk Oct 28 '18 at 23:42
  • @axk You are right, i glanced at -l and -s and assumed long and short cage (forgetting that Shimano usually uses 'GS' and 'SGS' to denote cage length). – Argenti Apparatus Oct 29 '18 at 0:07

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