Ryan_M

I'll try and streamline this... I had a full M6000 drivetrain that was meh. Worked OK but underwhelming. I upgraded to full M8000 and holy crap what a difference! Most of those parts went to my better bike and got M5100/5120 for my more touring bike. Basically a M5100/5120 setup but with a M8000 cassette. I am totally blown away that parts as cheap as the M5100 work so well. Very clearly better than the M6000 gear. So by the number heirachy M6000 "should" be better than M5100, and comparing 10S vs 11S, the 10 should be easier to get right but it isn't.

I understand this might be an opinion only question unless someone here has swapped a lot of parts. I can't really directly compare even if I did go buy some more parts, but why do you think the M5100 is so much better than M6000? I know its one generation above so is that it? Or is it the M8000 cassette that's providing the magic?

Andrew R Stewart 

I would suggest that the set up/adjustments, cable condition, chain wear make for more differences than adjacent groups' materials do. One exception to this is the rear der's cage pivot design does affect shifting independent of what it's made of. Some ders have the upper pulley axis on the cage pivot. As the cage is played in and out the distance from the upper pulley to the cog underside is maintained. When the pulley sits off the cage pivot and the cage is rotated the gap to the cog changes. Generally, the less this gap is the "faster" the chain will shift. So if adjacent groups have a cage/pulley pivot difference I would expect a shifting difference too. Or if the set up allows for a greater gap than another set up (chain length, hanger tab length) the closer/smaller gapped set up will likely shift better. Andy

ClydeClydeson

As Andrew R Stewart said, it is likely that setup plays as big a role as the identity of the actual part.
At least, when the parts are new this is true. My experience is that even the cheaper (~300 and 400 series like Alivio, Tiagra, Altus) work F'ing fantastic when new and properly set up. The real difference is after a year or more of heavy use - your M8000 derailleur is likely going to still be almost like new, your M6000 derailleur might be starting to wear a bit but not so it will make a big difference, but an Altus derailleur will be floppy and loose, and vague memories of how easy it was to keep the new derailleur adjusted will make you feel like you are taking crazy pills.

When I started riding mountain bikes in the early 90s, I never figured this out because the derailleur regularly ripped off my bike due to user error or misadventure, so I learned how easy it was to make the cheaper stuff work perfectly (until it got caught on a stump and tore off).

AeroGut

I'm going to argue the case that the number ordering isn't indicative of build quality in this case. The M6000 was the 2x10 or 3x10 spd Deore level groupset. The 5100 is a newer 1x11 spd groupset that is also Deore level. Typically, due to trickle down, groups with the same name improve in quality and function over time, so it makes sense that the newer Deore 5100 is improved over the older 6000 Deore. Usually the new groups with the same name get higher numbers, so this group would normally have gotten an M6100 designation. But they soon released a 12 spd Deore which was probably already planned when the 5100 came out. Using 6100 for the 12 spd Deore aligns nicely with the 12 spd 7100 slx and 12 spd 8100 XT. So I think they went with 5100 for the 11 spd to indicate that it's lower level than the 12 spd groups, and maybe also they liked the 5100 pairing with the first 51t cog cassette to be offered for Deore.

The main point is that all of the x100 groups are newer than the x000 groups and may have improved beyond the next higher x000 group. So within the series, the quality increases with number: 5100 < 6100 < 7100 < 8100 and 5000 < 6000 < 7000 < 8000, but you can't necessarily assume that 5100 < 6000 or 6100 < 7000 for example.

ClydeClydeson

Don't be fooled - sharing the features of the higher level groups is not the same as having equal quality to higher level groups. Modern Sora and Alivio may have 10 speeds or whatever, but a 1990s 8s Dura Ace or XTR setup will run smoother for longer with less adjustment.

Andrew R Stewart 

And a part of the older system remaining in good adjustment is the greater width of indexing tolerances that fewer cogs offer. But I do agree with better materials bringing longer and smoother functions as the miles add up. Andy

ClydeClydeson

Maybe my example of 10sp vs 8 sp is too extreme a difference - imagine a 10 speed Tiagra drivetrain from 2018 compared to a 10 speed Ultegra drivetrain from 2008 - my experience has been that 3 year old Tiagra will be starting to get sloppy and difficult to keep in adjustment, while the 13 year old Ultegra will be perfectly functional.