What is the smallest cassette compatible with Deore M5120??
 

What is the smallest cassette compatible with Deore M5120 11s?? I understand the M5100 has a min 11-51 cassette, but what about the M5120? Could I use a 11-36 or 40?

If I'm reading you right, there's never a minimum size cassette, beyond what's needed to not duplicate sizes. By example a 12s cassette starting at 11t would need to top out at 22t.

Other than that, using cassettes smaller than the max size isn't different than not using the larger sprockets on any cassette, which is obviously OK.

Of course, going with a smaller cassette may create other options, like shortening the chain, or using a different RD, but those are only options, not requirements

Low sprocket | Min. is 42T; you can probably get away with an 11-40 cassette assuming that you shorten the chain accordingly.

SHIMANO DEORE Rear Derailleur SHIMANO SHADOW RD+ 2/11, 1x10, 2x10-speed | SHIMANO BIKE-EU

Several MTB derailleurs have minimum cog sizes because the because the parallelogram slant is so steep that the pulley ends up too far from the low cogs. So this isn't true.

That's more about optimization than a true minimum.

Well, if it doesn't shift...

Well the important thing is that it'll do the Kessel Run in only 11 sprockets. If that's not minimum, I dunno what is.

That is what I thought, but the shimano spec says the min sprocket size is 42t. I see others seem to post that it will only work with a 42t or anomalies will occur.
https://bike.shimano.com/en-EU/produ...M5120-SGS.html

As Kontact pointed out, the pantagraph is set at a downward angle to better follow the slope of a very large cassette. That means the jockey wheel will be farther away on the inner sprockets of a smaller cassette.

In turn, you can expect progressively poorer shifting on the inner sprockets of a smaller cassette.

But it's a question of degree, so if Shimano predicts good shifting on a 42t sprocket, the drop-off in performance with a 34t shouldn't be too bad. Some fudging on the inner limit may be needed, to allow extra overshift, so make sure there's good cassette to spokes clearance.

So, it's not about whether it'll work, but what you'll accept in lost shift response.

I recently changed the 11-36 on my Ebike to a 14-30 I made up from parts.
RDER is a RD-592 which specifies an 11T smallest and 3x largest.
I didn't touch ANY adjustment screw.
I turned the barrel adjuster 1 click and ended up turning it back.
It shifts better the the original, mainly going to the largest 2 cogs.

Sometimes you get lucky.-

Given the perhaps sloppier shifts onto the lowest (largest) cog, with more lateral chain flex and thus may overshoot, I'd mount a tasteful clear plastic spoke protector (aka dork disc), I think you will be glad you did if the chain drops between the cog and spokes; it tears up spokes quick, later failure points, and sometimes can be a bear to pull back out, as those big cassettes are often on an aluminum carrier(?) with steps in the back and I had a HECK of a time pulling the chain out for someone on the road with no spoke protector, it was jammed in there real good; with no lockring tool to loosen the cassette, the bike owner resorted to *prying* with a screwdriver I had, between the inboard side of the cassette and the hub flange, I winced, but it got the chain out.

I do not know about the Deore, but this is my Raleigh International
10 speed 14-25
https://cimg5.ibsrv.net/gimg/bikefor...7ba4f1e900.jpg
https://cimg6.ibsrv.net/gimg/bikefor...9c063f3aba.jpg
https://cimg8.ibsrv.net/gimg/bikefor...95ee7ff867.jpg