Fitting a 9-speed 11-50T cassette on MTB: which derailleur?

Derailleur will not handle that, as Criggie says in his answer. Apart from that, an 11-50 9 speed would have horrifically big jumps between gear ratios.

1x systems only really became feasible with the advent of 11 speed drive-trains, and still sacrifice ratio jumps to get a full spread of ratios to some extent.

If you really want a 1x 9 speed, get a cassette a 9 speed derailleur can handle, then get a small enough chainring to give you the low gears you want, and forgo the higher gears.

BTW, Shimano provides tech specs for all their products (as Criggie linked to). This page is a good place to start looking for specs of a particular product.

21% gear spacing is not a problem. Shimano has over stated the need for tight ratios. The shifting won't be quite as smooth, I have an 8spd 11-40, but will be well within acceptable standards. 11-36 has never been a problem with 9 speeds and I usually have to double shift my 11-36 10 spd because the ratios are too tight.

You will have to use a hanger extender with a long cage derailleur. The warnings that the extender doesn't allow the RDR to track the cassette properly is also over stated. Shimano claims they do no testing with components other than their own. If you believe that Shimano wouldn't know. Obviously that is ridiculous.

Shimano has stubbornly lagged 2 to 3 years behind the market in the area of gear ranges. They don't belive in it and only participate well behind the market because they have no choice so pick another brand of RDR. There are many options.

I have to agree with Dave - 11-36 works like a charm, gear spacing is just right. A 11-40 cassette just works OK with the B-screw screwed in all the way or using an even longer replacement screw. I also tried an 11-42 which didn't really work well.

AFAIC the issue with the wide range cassettes reg. 9-Speed is that they have the cassettes but there are no (Shimano) derailleurs to shift it. At least not out of the box. You could try a Shimano 10-speed derailleur and if it doesn't work ok (which it likely won't), try tweaking it. There are howtos online with re-routing the cable for a different pull ratio. 11-speed derailleurs might be too narrow, chain-wise. You could also use a Sram 9-speed shifter with a Shimano 10-speed derailleur.

Trouble is, all of these solutions are rather expensive if you haven't the parts in the spares box. Rather than spending that much money on an exotic 9-speed solution that doesn't get better over time, invest it in a Sunrace 11-speed cassette and Shimano SLX derailleur plus shifter.

What you don't want is a solution based on hanging your derailleur lower and the Wolf Tooth link extensions don't work with 9-speed derailleurs. I tried it (and it's rather expensive too).

Best to stick to inexpensive 11/12-36 and a 30 or 32 NW chainring. Or try the 11-40, but that's the max you will be able to get working properly.

I looked at some offerings of such cassettes and what a vendor suggests as a derailleur for them.

An example of a cassette:

Compatibility chart from the same page:

It looks that no 9- or 10-speed dérailleurs are compatible with that cassette, but they claim that some 11-speed ans 12-speed parts from both Shimano and SRAM may be used instead.

I run a shimano m4000 with a 9spd goat link on a 9spd 11-50t cassette with no problems b screw doesn’t need to be fully wound in.

More recently, I’ve reverted to a 10spd SLX shadow+ mech sgs cage which I run with a sram x7 9spd shifter, giving even better shifting.

I have used a RD-M2000 and 11-50 on 9 speed with hanger, but the very connection of the gear wire on the inside just touches the bigger cog when in 1 gear, otherwise ok (almost :-) Would probably try the RD-M370 instead next time

Box Components have released a 9-speed 11-50 drivetrain (cassette, RD, and chain) for EMTBs (thicker chain and cassette to cope with the torque), and I think the gear jump ratios don't matter when it comes to mountain biking. Even Seth's Bike Hacks uses it, and he's a very nitpicky kind of guy.

What I don't know is if it's compatible with a standard 9-speed system. Box Components says it isn't compatible with Shimano and SRAM 9-speed systems, but I have a feeling they're just saying it for marketing purposes.

best way to go is to use sram 9v trigger (only sram) coupled with new generation clutch type derailleur - i use shimano slx 11v but also sram should go. Work like a charme with 9v cassette and chain, no adaptation needed.

I been using a Shimano Alivio M4000 RD paired to a 11-46T cassette for almost 3 years now and I didn't Have the issue that was being pointed here. Alivio m4000 has max 45T so just used hanger extension it works great. But going to 11-50 I don't think the Alivio RD can handle that. I don't know about the 2021 edition of the Alivio perhaps you can try that if you want a 11-50T Cassette. For me I am fine with the 11-46T cassette picture shows a 11-40T cassette which I used before upgrading to an 11-46T. No Hanger extension works well with no issues

Do you have some derailleur's for 9 speed that can handle 50T cassete's. Search for "one box components". They have recently launched a new group with 9 speed and 50T.

My Altus RD-M2000 9 speed can handle 10s cogs max 42T as I tested I can still go further to 46 i believe as long as The M2000 can still go further inside and still can adjust the B screw it can take max to 50T if the roller closer in 46-50T then it is time use goat link possible

I want to max as 12s cogs because I have DEORE M6100 12S RD Shifter but I think U have to change my hub SRAM X9 not sure if fit the 12s cogs to my SRAM X9 Hubs

Planing for 32-34T Chain ring 1x crank to My 10-12Speed cogs using Altus RD-M2000