Difference between M5100 and M5120 rear derailleur

[PJO]

Anyone know why the 5100 is marked as a 11-speed and the 5120 is for either 11 or 10-speed?
Would've thought the geometry between the two was fairly similar, but they do look a bit different.

Just trying to get some lower gearing for middle sons bike, have 10-speed shifter but the old 10-speed derailleur starts to struggle after about 40t (with longer b-screw).
The newer derailleurs seem to have better geometry for wide range cassette.

I can find the 5100 for a reasonable price but the 5120 is about $25 more.

EDIT: capacity on the 5100 is a bit bigger

 

[moorey]

Isn't the 5120 capable of taking the 11-51 comfortably, instead of the smaller capacity 5100? That's why I bought one for the spares box, to later try with bigger cassette (minus the excessive B screw), I can't comment on the 10/11sp cabability.

 

[PJO]

Isn't the 5120 capable of taking the 11-51 comfortably, instead of the smaller capacity 5100? That's why I bought one for the spares box, to later try with bigger cassette (minus the excessive B screw), I can't comment on the 10/11sp cabability.

Other way round, the 5100 has capacity to 51t while the 5120 has 46t max

SHIMANO DEORE Rear Derailleur SHIMANO SHADOW RD+ 1x11-speed | SHIMANO BIKE-AUSTRALIA

Utilizing the SHIMANO SHADOW RD+ design, the SHIMANO DEORE M5100 rear derailleur delivers consistent MTB shifting performance over rough terrain. The low profile design avoids damage from rocks whiles SHIMANO's chain stabilizing technology reduces chain drop and makes for a quiet drivetrain in...

bike.shimano.com

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

Utilizing the SHIMANO SHADOW RD+ design, the SHIMANO DEORE M5120 rear derailleur delivers consistent MTB shifting performance over rough terrain. The low profile design avoids damage from rocks whiles SHIMANO's chain stabilizing technology reduces chain drop and makes for a quiet drivetrain in...

bike.shimano.com

 

[Labcanary]

5120 can go up to 46t in a 1x10 setup or 42t in 2x10/11

5100 is stated as capable of 51t in 1x only

I'm not sure why one is marketed as 10/11 speed and the other as only 11 speed. I would have thought, in true Shimano/capitalist style, that the pull ratios would be different between 10 and 11 speed.

Edit: what you said, @PJO !

 

[Labcanary]

@PJO I bought a 5120 derailleur from George's Bike shop. I've got it running on a 1x10 drivetrain (11-46t cassette) and it's fine.

 

[PJO]

@PJO I bought a 5120 derailleur from George's Bike shop. I've got it running on a 1x10 drivetrain (11-46t cassette) and it's fine.

Awesome thanks, just looked at George's and it's still there and available for a reasonable price.

 

[moorey]

Yes, sorry peeps. As you were.

 

[Labcanary]

This thread might have some answers:

Do Shimano 10 speed and 11 speed derailleurs have the...

www.mtbr.com

 

[PJO]

This thread might have some answers:

Do Shimano 10 speed and 11 speed derailleurs have the...

www.mtbr.com

Probably does, but jeez I get sick of the back and forth arguing in mtbr threads

 

[Labcanary]

I skimmed a lot of it. Too much seriousnessness!

It sounds like, as long as the cassette range is correct for the derailleur, you can use them for either 10 speed or 11 speed.

 

[PJO]

I skimmed a lot of it. Too much seriousnessness!

It sounds like, as long as the cassette range is correct for the derailleur, you can use them for either 10 speed or 11 speed.

That was my take home as well.

 

[Tubbsy]

Probably does, but jeez I get sick of the back and forth arguing in mtbr threads

No you dont

 

[PJO]

No you dont

 

[blacksp20]

I too have the 5120 on 10 speed 11-46. It works well but is a little noisy in the 46, not that it bothers me.

 

[Labcanary]

I too have the 5120 on 10 speed 11-46. It works well but is a little noisy in the 46, not that it bothers me.

If I'm using the 46 tooth cog, my unfit gasping drowns out any noise the drivetrain might be making.

 

[The Duckmeister]

I don't know what, if any trickery Shimano pulled with the leverage, or maybe they're secretly admitting that they're the same after all.....

Along with the 12-sp family, the cage geometry is different between the xx00 & xx20 versions. If you look at the pics up the page, the 00 has the top pulley considerably more offset from the pivot; this swings it back & down as the cage is pulled forward, increasing maximum sprocket capacity.

The smaller pulley offset of the 20 version reduces max sprocket capacity, while still having enough length to accommodate a 2x.

 

[blacksp20]

If I'm using the 46 tooth cog, my unfit gasping drowns out any noise the drivetrain might be making.

I only notice it at very slow cadence so I just try to pedal faster. Fitness usually prevents this, but I try.