Rd m5100 vs Rd m5120

So the m5100 is compatible with 11 speed

But the 5120 is compatible with 10 AND 11 speed

How can this be? How can the 5100 not work with 10 speed also?

They are very different shapes. The m5100 is technically only compatible with an 11-51 cassette

Ask me how I know…

Pretty sure now that all shimano rear mechs 10-12 have the same pull ratio. That *should* mean you match shifter to cassette speed and then derailleur capacity to cassette size. I can’t see why a 5100 wouldn’t work on, say, an 11-48 10s with the right shifter unless the top jockey position just doesn’t track the cassette well enough.

Shimano MTB 10/11/12 speed use essentially the same cable pull ratio.

M5100 is for 11-51 cassettes

M5120 is for 11-42 cassettes in 2x 10/11 or 11-46 in 1x 10. I think the latter is just marketing as it will work with 1×11 too.

They are completely different geometry – the m5100 is based on the m6100 rear mech and is essentially identical. It has the geometry for the larger cassettes.

The m5120 is based on the older shadow plus mechs and can’t manage the big end of larger cassettes.

I mean why is one and to change gears on 10/11 speed and one is only 11 speed, surely if they have a common factor then they will both work on 10 and 11 speed

🤔

Anyway 5120 on the way for the 10 speed 11-46 cassette

I should have expanded more rather than replying half asleep.

It’s just marketing. Both mechs will work across 10, 11 and 12 speed. It’s biggest low gear and the subsequent path the mech needs to follow to stay close to the cassette teeth without crashing into them that the different mechs are designed for.

An m5100 won’t work as well as an m5120 on an 11-42 or 46 cassette because the top jockey wheel will be further from the cassette.

It’s not just about cable pull ratio – it’s about optimally following the profile of the cassette.

Strange, my M5100 is working just fine with an 11-48 10 speed cassette.

It’s almost as if you should take Shimano tec-docs with a pinch of salt.

My thoughts entirely. It shifts beautifully up the cassette into the bigger cogs but it’s slow and shit on the way back. Obviously the top Jockey is a million miles away from the cassette profile when with b tension screw all the way out. This was more of a sense check. If one 11 speed mech works with 10 and 12 also, all will!

I should have expanded more rather than replying half asleep.

It’s just marketing. Both mechs will work across 10, 11 and 12 speed. It’s biggest low gear and the subsequent path the mech needs to follow to stay close to the cassette teeth without crashing into them that the different mechs are designed for.

An m5100 won’t work as well as an m5120 on an 11-42 or 46 cassette because the top jockey wheel will be further from the cassette.

It’s not just about cable pull ratio – it’s about optimally following the profile of the cassette.

The 5100 will also have a massive tantrum if you try and use it on a 2x set up. The offset for the top jockey is not 2x friendly.

I’ve got another one to try now on my other bike.

I have a spare M5100 mech and another 10 speed Cassette, but I’ve also got an old 11 speed XX1 Grip-shifter with “X-actuation” which apparently pulls 3.48mm per click, where Dynasis 10 speed uses 3.4mm in my head 0.08mm is down in the weeds tolerance wise especially and with a baggy old grip-shifter leave one dead position it should work… Right?