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Help choosing rear derailleur

Hi everyone, I have a décathlon Rockrider 520 3x8, for my occasional use it’s been a good bike, however I would like a clutch rear derailleur, at the moment its fitted with a MICROSHIFT RD-M46-L and the cassette is a shimano 8 speed (CS-HG31-8), but for the life of me I cannot find a 8 gear clutch rear derailleur, I think I can fit a 9 or 10 speed and limit it to 8 speed, is that correct?
I think I can use a Deore M610 10 speed, can anyone advise please?
Thank you

Posts

  • JBAJBA Posts: 2,772
    I don't think you can get a clutch-equipped 8-speed mech.
    You can't use a 10-speed mech with an 8-speed shifter as the cable pull ratios are different.
    Why do you want a clutch mech? Are you regularly dropping the chain?
    “Life has been unfaithful
    And it all promised so so much”

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  • Unfortunately the Shimano Deore M610 10 speed derailleur and shifters have 'Dyna-sys' and use a different pull ratio so can't be used.

    Also the cassette spacing changes on 10 speed compared to 7,8 and 9 speed cassettes.

    You would have to buy a 10 speed shifter and cassette along with the Deore derailleur if you want a clutch derailleur.
  • steve_sordysteve_sordy Posts: 2,239
    I am no expert on this, and I have never done what you describe. But I do read a lot of bike stuff, so here goes...
    Rear mechs are not indexed, they take their cue from the shifter. So if the shifter lever moves the mech by the amount necessary to shift one gear on an 8-speed, then the mech will follow. So why aren't all mechs the same? Why have 8, 9 or 10-speed? The answer is that 9 and 10-speed cassettes are larger diameter than 8 speed, so the mech arm needs to be able to cope with that larger diameter big gear. Therefore a 9 or 10-speed mech should be able to work on an 8-speed cassette. Count up the gear difference to determine what length of mech arm you need.

    As I said, I've never done this and I claim no expertise, but at least you have something to go on.
  • reaperactualreaperactual Posts: 895
    edited October 2020

    I am no expert on this, and I have never done what you describe. But I do read a lot of bike stuff, so here goes...
    Rear mechs are not indexed, they take their cue from the shifter. So if the shifter lever moves the mech by the amount necessary to shift one gear on an 8-speed, then the mech will follow. So why aren't all mechs the same? Why have 8, 9 or 10-speed? The answer is that 9 and 10-speed cassettes are larger diameter than 8 speed, so the mech arm needs to be able to cope with that larger diameter big gear. Therefore a 9 or 10-speed mech should be able to work on an 8-speed cassette. Count up the gear difference to determine what length of mech arm you need.

    As I said, I've never done this and I claim no expertise, but at least you have something to go on.

    In basic terms 10 speed has been specifically designed by very clever people to not work with lower speed stuff to keep a distinct hierarchy between the new and old is my simplistic take on the subject.

    Rear mechs are dumb and controlled by shift inputs but imagine a slight difference in anchor bolt distance from the point of where the derailleur pivots can have a big effect on pull rations.

    I won't commit to saying it won't work as I've never tried mixing compatibility but would commend anyone who tries and is successful.

    It could turn into an expensive headache, you'd probably need an engineering degree to know what would need to be tweaked to make these things work together, way above my knowledge level and the hassle I'd be prepared to go through in order to find a solution if needed.

    I'm no expert either just throwing my theories and thoughts out there, don't know if it's accurate or not.
  • mully79mully79 Posts: 308
    I have managed to get My 9 speed xt derailleur working with Shimano 7 speed Stx rc shifters and a 7 speed cassette but why mess about when Shimano stuff is reasonable at cheap prices ?
    There’s usually always a sub £25 clutch Shimano derailleur and some £10 shifters.(zee etc) I think I paid £32 for a 11-40 deore cassette when they first came out. COVID prices are All a bit too rrp for me.

    Anyone who has SRAM don’t feel bad just pretend I missed a zero from every price I’ve listed :-p

  • Ok, thanks all, I think I’ll just stick with what I’ve got for now and look to upgrade to a 1x10/11/12 at some point in the future, the only reason I wanted to upgrade was to reduce the pedals falling way when changing to a lower gear under load.
  • reaperactualreaperactual Posts: 895
    edited October 2020
    Maybe you don't need a clutch derailleur for now?

    When you upgrade to a 1x drivetrain you will be able to feel the difference the clutch mechanism makes and avoid the distracting noise of chain slap and potential damaged caused to chainstay.

    https://amp.reddit.com/r/MTB/comments/1xulaz/clutch_derailleurs_explain_to_me_like_im_5/
  • steve_sordysteve_sordy Posts: 2,239
    Basic question now, why do you need a clutch derailleur?

    My assumption is that it is to help prevent the chain from coming off. When riding over bumpy ground at speed the lower strand if chain can whip up and down It can even form an "S" shape (laid on its side). The problem is that the position of the S can move forwards and backwards as your sped varies and the bumpy ground changes. Frequently, the lower part of the S can reach the chainring and off it comes!
    Clutch mechs work by inhibiting the back ad fort movement of the mech arm and stop the chain from building up a big S. But there is an alternative that was used on 3x, 2x and 1x drives before clutch mechs were thought of.

    It is a device generically known as a dangler. It is a small tube that the lower strand of chain passes through. The tube is suspended from the chain stay. This stops the movement of the mech arm allowing a loop of chain from reaching the ring. I have used the RSP Director (see link below) on 3x and 2x drives. I would have used it on a 1x but the bike came with a clutch mech and narrow wide teeth on the chainring, so it wasn't necessary.

    https://www.tredz.co.uk/.RSP-Chain-Director-Chain-Guide_76633.htm?sku=243457&utm

    They either fit onto your gear cable or zip tie onto your chainstay, whichever is most convenient. The design is a direct copy of the Bionicon, which is 3x the price.

    When I was using one of these, I never lost a chain. Keep your eye on the hourglass as it is a wearing component; be prepared to turn it over and swap it around to extend life. You can get replacement hourglasses, but they are mostly Chinese copies ad don't last 5 mins. The RSP Director is so cheap its not worth pratting about, just get a new one when the time comes.

    If you want to test the concept, you could make one with a piece of garden hose and some Zip ties, but don't expect it to last long at all.
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