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I am planning on building a 3x9 cassette. I already have a 9 speed hubs and 11-42 cassette and an alivio m4050 combo shifter.

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    You're missing the one piece of information that matters most - the rear derailleur model so we can look up its specs. Could you edit that into your question?
    – Criggie
    Commented Nov 27, 2020 at 7:22
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    I doubt it makes much sense with triple chainrings. If your smallest chainring has 22 teeth your lowest speed with a 42t cassette would be ~5km/h. With a 36t cassette it would be ~5.7km/h.
    – Michael
    Commented Nov 27, 2020 at 9:52
  • @Micheal - I have 24/36 on my 3x11, and on the odd occasion would use a lower gear, but I am not a cycle fit as I could be. That said, at those speeds it often better to get off the bike and walk.
    – mattnz
    Commented Nov 27, 2020 at 20:48

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Yes it is possible.

It will be a nightmare on your chain and arguably not a good idea.

If you want to use a 9 speed rear derailleur with a max tooth of 36t, you can make it fit a 42t cassette with a derailleur hanger extender such as the Wolftooth Goatlink

enter image description here

Your biggest problem will be the front 3 rings. You will find that if you may have problems when crosschaining. Although, you could argue that you should never be doing that anyways.

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Yes, this is possible. Before the current trend of 1x drivetrains took over on MTB, Shimano promoted triple chainsets for their advantages in terms of drivetrain efficiency and choice of gears.

There is some blurb about this here in the context of an 11-speed 3x11-40t drivetrain: https://bike.shimano.com/en-EU/technologies/component/details/dynasys11.html

I would note here that a lot of bicycle drivetrains sold today are not driven by efficiency and smooth cadence, which are some of the merits noted there of Shimano's carefully designed 3 x 11-40 system with 11 gears.

E.g., if we compare this:

enter image description here

enter image description here

11-40t 11-speed cassette, to some of the huge cassettes today:

https://bike.shimano.com/en-EU/product/component/deore-m5100/CS-M5100-11.html

11-13-15-18-21-24-28-33-39-45-51T 11-speed

or

https://bike.shimano.com/en-EU/product/component/deore-m4100/CS-M4100-10.html

11-13-15-18-21-24-28-32-37-46T 10-speed

Then e.g. the Microshift 11-42t 9-speed cassette

https://www.microshift.com/en/product/cs-h093a_11-42t/

11-13-15-18-21-24-28-34-42

or Sunrace make one with the same ratios.

have pretty similar gaps to the current 1x systems sold on bikes costing thousands of dollars, even if they are not as smooth as Shimano's 3x11 kit that was rejected by the market which wanted the simpler but less efficient 1x drivetrain.

If you had a 'touring' type triple such as 48/36/26, it's easy to see how a 42t cog could be useful

http://www.gear-calculator.com/?GR=DERS&KB=26,36,48&RZ=11,13,15,18,21,24,28,34,42&UF=2280&TF=90&SL=2.8&UN=MPH&DV=teeth

The chainline is 1.7° in small/small or big/big. This is much better than the 2.6° you'd get with a single ring setup.

The 8t jump from 34t to 42t is probably bigger than ideal, but it's nothing Shimano haven't exceeded on their 10-speed stuff. I'd probably prefer the 11-40t, or even 12-40t though, which is the same cassette with the top and bottom gears swapped out, so smaller jumps.

The biggest flaw with this stuff is the 15-18 jump, in that you might well spend a long time in the middle gears, and that 20% gap for people who like to maintain constant cadence is a bit of a drawback..... But the same issue appears in some respect in most cassettes. E.g., the 11-36t 9-speed cassette has a 17-20 gap, not to mention the 11-13 gap which is pretty big as well.

So from an efficiency perspective there's nothing really wrong with the idea, in that larger chainrings tend to mean slightly greater efficiency and durability and a 3x9 setup of this kind is likely to be better from a technical efficiency perspective than most 1x setups that people are very happy with. It probably does give you more total range than you need, but that's not a big problem exactly.

As far as the derailleur goes modern Shimano Shadow derailleurs tend for 9-speed to be rated for 11-36t, while the 10-speed stuff and higher goes to 11-42t or more. These are based on what Shimano sells, so this doesn't mean Shimano's 9-speed RD is any different in its capacity from the 10 or 11-speed, except that Shimano sell two different 10-speed RDs for 11-36 and for 11-42t, so it's likely that the 11-36t is not particularly at its most efficient when shifting on 11-42t. (I'd note that the 10-speed 11-42t is probably all but identical to the 11-speed 11-46t model, but Shimano don't sell an 11-46t 10-speed cassette.)

For this reason, while 11-42t is likely to work with a Shimano 9-speed RD rated nominally for 11-42t, I'd be happpier with 11-40t, as it's closer to spec.

Note that you can buy products such as these

https://www.wolftoothcomponents.com/collections/components/products/roadlink

https://www.wolftoothcomponents.com/products/goatlink

to extend derailleur capacity, but all things being equal it might be better to avoid this.

Note also that rear derailleurs have what's called a 'total capacity'. This relates to the number of teeth it can take between big/big and small/small. IME you're quite unlikely to use these ratios on a triple chainset, because you'll tend to get down into at least middle ring, whereas it's more common to find yourself in these ratios on a double chainset. However the figures are there, and given a 11-42t cassette, you've used up 31t of capacity, whereas the 'total capacity' for, say, rd-t4000 is 45t, which would imply just a 14t gap in the front. This implies the angles that the RD will be forced to run across if you use big/big are likely to be bigger than it's designed for.

So like a lot of things yes, it definitely would work, but it might not be totally advisable in all respects and without considering the possible pitfalls, and by sticking to specs issues are less likely, but if your use case demands 3x9 then there's nothing fundamentally against it.

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  • Followed the first link - how old is it? Eitehr its old or MTB industry has done a 180 degree turn and told very few people.
    – mattnz
    Commented Nov 27, 2020 at 20:44
  • it's from the launch of 11sp. 2014? Shimano were still pursuing the 'pedalling efficiency' at that time. The market quickly told them 'no, we want 1x, we don't care about efficiency'. But it's still up on their site. 12x is designed from the ground-up around 1x, although thankfully they still make a 2x.
    – thelawnet
    Commented Nov 27, 2020 at 21:41
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I did just now upgrade set up 3x9 speed with Shimano RD-M772 Derailure ,11-42 Microshift sprocket combine 24-36-48 Sugino chain rings on my Surly Disc Trucker, I have try with Goatlink but then it’s two long, now it’s on without Goat link, just fine ,as far we don’t ride small to small and big to big ring,yai! now I can climb Pyrenee.

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My previous post was deleted. No, Criggie I didn't take it personally as i knew to begin with that this probably wasn't the place for this. I am however posting now as more of an answer. I got my 3 x 9 to work with a 42 tooth rear cassette and an Alivio RD and tweaking things around a bit using the 3rd adjustment screw on the RD and chain link adjustments. It took me a few tries to get the right number of chain links to make it work but it does, and quite well.

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Someone in this thread talked about running a XT RD-M772 with a Goatlink. I was keen to try this out but understood that shouldn't be possible. To quote the Goalink compatiability section from the manufaturer's website:

"Compatible Shimano Shadow+ 10-speed rear derailleurs (clutch type ONLY) have a Direct Mount link (including spacer, if so equipped) or B-pivot knuckle with a thickness of 7.9-8.1mm. Shimano changed to this sometime in 2013. Very early Shadow+ rear derailleurs have a thinner link and are not compatible."

The XT RD-M772 came out in 2008 and isn't Shadow Plus, merely Shadow, hence why I'm keen to hear that modders experience. Maybe they meant to say Roadlink (which will work) rather than Goatlink?

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