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I'm planning to upgrade my MBT bike for a real one, well entry-level. Talon 3 2019, which comes with these specs for shifters and derailleurs

Shifters: Shimano Altus, 3x8

Front Derailleur: Shimano Altus

Rear Derailleur: Shimano Acera

I was thinking, maybe I could upgrade the rear derailleur Acera for this one :

Shimano Deore SL RD-M591 SGS 8/9 Speed Rear. Is is possible or maybe Alivio ?

Thanks I'm new to this world, I just love biking and the derailleurs make a huge difference, I want a good one! Thanks a lot

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Replacing the Acera rear derailleur with the RD-M591 is possible. Despite being marketed as a "9-speed" rear derailleur (the 9 referring to the number gears/sprockets/cogs--all mean the same thing--that make up your rear cassette), the M591 will work. It is of a higher class/quality of components that Shimano named "Deore." So this Deore derailleur will work with your 3x8 drivetrain because all Shimano 7, 8 & 9 speed rear derailleurs share the same "actuation ratio," meaning these rear derailleurs move the same amount per mm of cable pulled by the shifter. Thus, your 8 speed shifter will pull an amount of cable to make an 8 speed spaced shift and the Deore will move the correct amount--the same amount the Acera or even a 7-speed, Tourney-class, rear derailleur would.

Conventional wisdom states that upgrading components on a new bike (& even more so, on a used bike) is more expensive than paying a little more for a bike that comes equipped with a higher class of components already. Especially if labor (your own or a bike shop's) to replace components is considered. In addition, the trend in mountain biking is going to a 1x drivetrain (expressed, "one-by") where there is a single chainring in the front and 10, 11 or 12 rear sprockets in the back. This eliminates the weight and maintenence of a front derailleur and shifter, and two chain rings. As your biking develops and you learn how u like to ride, open it up to different things.

Your derailleur upgrade is reasonable. It will work. One other thought is that you'll require the SGS caged (the longest option) M591. While it's still the rear derailleur we're discussing, the reason for needing a long cage (the lower part of a rear derailleur that houses the two small pulley wheels) is related to having a triple chainwheel up front. There's more chain to take up and keep tight between the large ring and the small ring and rear derailleur's cage must be able to handle that range.

  • Thanks a lot, really appreciated, i found this one who comes with what i want. Hope i can get it in store! giant-bicycles.com/fr-ca/talon-2-2017 – user2731506 Oct 25 at 14:26
  • Nice! That bike in your link has s nice set up. I personally run Deore or XT components on both my hardtail and full suspension MTBs and they work very well. – Jeff Oct 26 at 0:14
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Jeffs question is technically correct, you could put a M591 derailleur on that bike.

However, the Deore M591 is a 2009 design and probably does not work better than the current M4000 Alvio range. Also, shifting quality depends on the shifters and cassette as well as the derailleur so it's questionable as to wether you'd really see a benefit.

It's not cost effective to buy a bike then start replacing components. Buy the bike that has the components you want in the first place. What I would do in your position is spend a little more money and get a bike with mostly Alvio level components, or perhaps even Alvio/Deore mix.

  • Hi, thanks, really appreciated, that's what i was thinking too. It would be simplier. Actually. It's exactly this mix i found Deore shadow with Alvio shifters. I'm just asking, if it's efficient in the city too, to have Shimano HG400, 12-36, 9-Speed. I heard to have two cassettes and more rear speeds would make the same then having 3 cassettes and less speeds at the rear. Is it true ? Sorry if i don't use the rights terms, i'm kind of new in the technical names. – user2731506 Oct 25 at 14:23
  • You may be erroneously calling the front Chainwheels, "cassettes" which is the term used for the the rear sprocket cluster as a whole: the cassette is the collection of toothed sprockets at the rear wheel (in your comment the "9 speed, 12-36, and HG-400" are all descriptors of the rear cassette). In the front, the bike may have one, two or three front CHAINWHEELS. Going with one or two Chainwheels (1x or 2x "1-by or 2-by") with a wide range, 9, 10, or 11 speed cassette in back is proving popular. – Jeff Oct 26 at 0:07
  • Shimano calls the whole crank assembly a 'chainwheel', The front sprockets are generally know as 'chainrings'. – Argenti Apparatus Oct 26 at 0:34
  • @Argenti Apparatus True. I guess I've used chainwheel & chainring interchangeably to refer to a front sprocket cuz I say "crankset" for the whole. As I understand it, a strict interpretation of "crankset" includes the bottom bracket as well, though not often in my use. – Jeff Oct 27 at 0:11
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The four issues you face when swapping rear derailers are

  • number of gears
  • pull ratio -- distance moved per unit of cable movement
  • number of links that the derailer can "absorb" (and hence difference between min and max gear ratios)
  • mounting scheme
  • Ok thanks, it's better then if i buy the bike with deore derailleurs and alvio shifters then. I found what i want on the web, hope it's in store too. fingers crossed. – user2731506 Oct 25 at 14:24

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