Compatability - Deore XT 11sp M8000, 10sp T8000, running a clutch derailleur on a triple..

[wafter]

Looking for a bit of advice please!

I like triples and hydro brakes so am looking at the 10sp Deore XT T8000 "Trekking" groupset, which centres around Shimano's last (and I think now discontinued) high-end triple crankset (48/36/26).

This appears to be an offshoot of the 11sp M8000 mtb groupset and many components are similar, so I think things should be broadly compatable.

There are a couple things I'd like to try but I'm not 100% sure would work...


Use with an 11sp cassette and chain with M8000 shifters

The T8000 shifters are 10sp while the very similar M8000 shifters are 11sp. I assume that all being MTB-derived the derailleur actuation ratios (cable pull : derailleur movement) should be the same, so can I use M8000 shifters to control the T8000 FD and (an as yet indeterminate - please see below) RD to run an 11sp chain and cassette?

I'm aware that this would involve running the 10sp crankset with an 11sp chain, although from what I've read it seems that many are doing this... but would welcome any thoughts on this too


Use of a clutched rear derailleur with the triple setup

The T8000 RD has a long SGS cage giving 47T capacity, 22T max front difference and 32-36T low sprocket on the cassette - but unfortunately does't have a clutch (which I'd value very much given where I ride).

The M8000 RD with SGS cage also gives 47T capacity but only 18T max front difference (the M8000 triple is 40/30/22 IIRC) and 40T low sprocket on the cassette - this model does have a clutch.

Can anyone suggest whether I might get away with using the RD-M8000 with the triple, or whether I'm likely to be able to lash up some hybrid abomination by swapping bits like cages around please?

Also, can anyone shed any light on what factors determine the amont of chainring difference / range an RD can handle, since the two above have the same capacity? Also, what determines the ideal low sprocket on the cassette - perhaps the distance the top jocket wheel sits below the cassettes, so something that could possibly be manipulated by swapping the RD mount that bolts to the hanger..?


I'm aware that some are fitting MTB cages to GRX 810 RDs to get more capacity, but obviously I can't run this RD as it's road-based so has a different actuation ratio to the MTB stuff.


Cheers!

 

[wafter]

On the offchance anyone has similar questions, many more hours spent scouring the internet suggest that:

There are questions over the compatability of 10/11sp MTB drivetrains as apparently (according to Bike Gremlin) 10sp ("Dyna-sys") has a slightly lower derailleur ratio and higher cable pull ratio than the later 11sp "Dyna-sys II) offerings; suggesting that a system would have to 100% one or the other to work correctly. Conversely however, Shimano list some of their RDs as both 10 and 11sp compatable..


Also, I've read that the "max front difference" expressed for RDs is just a function of their total capacity and the range of the cassette they're expected to be used with - ostensibly they're described this way to make it easier for consumers to understand... but also seems to muddy the waters..

 

[bikeworkshop]

I think you'd best avoid it - here's my take:

Clutched derailleur was really to improve chain retention with a single front ring, and although it does have some benefit on a double, it's often a bit nasty.

i would think the extra brake on cage rotation might throw the chain off on a downshift to the small ring, or cause skating either way.

There is also some very complex shaping to the rear derailleur, which changes alignment as the cage rotates - I assume this is to deal with the poor chainlines 1x demands at either end of the gear range.

This might not be at all good with a triple -
the larger overall tooth difference, and wider varying chainline are outside its designed parameters, which are pretty strict.

Rear cassettes have such wide ratios now that you can get a huge overall range with a double.

Mixing shimano groupsets up gets worse results with the bigger numbers of cassette sprockets - even when they ought to be compatible, like mtb 10&11.

But different riders have widely varying tolerance to fussy, clunky or problematic shifting, unusable gears and baggy or too short chain.

And many love a challenge

 

[wafter]

Thanks for taking the time to go through all that - I'd not considered the presence of a clutch to potentially be problematic.

You make an interesting case about clutched RD use on doubles; however FWIW I have a clutched GRX 810 derailleur on an otherwise 105 2x drivetrain without any obvious issues and I'd hope that a triple wouldn't be any worse since the difference between the front rings is a bit less..

Shimano seem to offer plenty of clutched derailleurs on non-1x systems. The GRX range offers 10, 11 and 12sp doubles with clutched RDs, while the last of the line MTB triples (M6000, M8000) also run clutched RDs IIRC. The outlier seems to be the MTB-derived touring groupsets (T6000, T8000) which have clutch-less RDs...

One assumption would be that clutches were omitted simply because the manufacturers didn't anticipate the touring groupsets to be used on terrain that would warrant a clutch. Another, less paletable angle might be that the clutch somehow limits the max tooth capacity as the unclutched touring RDs (RD-T6000-SGS, RD-T8000-SGS) state a 47T capacity (evidently intended for use with the 48/36/26 FC-T6010 and FC-T8000 cranksets with an 11-36T cassette) while the most similar clutched derailleurs I can find from the MTB range (RD-M6000-SGS and RD-M7000-10-SGS) state 43T capacity - presumably for use with the common 40/30/22 TB triple format with an 11-36T cassette.

I do wonder if there's a bit of "elasticity" in the stated RD capacity to reflect the available components - for example I don't think you could exceed the RD-M6000-SGS's stated 43T capacity with the available products in that range - i.e. the widest cassette was 11-36T and the widest crankset 40/30/22..

Shimano have a rep for being conservative and I reckon you could probably get away with the 43T stated capacity of the RD-M6000-SGS with an DC-T8000 48/36/26 crankset and 11-34T cassette (which is 45T range)- especially considering it's a lot harder to find yourself horrendously cross-chaining / operating at the RD's limits on a triple with sensible ratios (being in the middle ring 95% of the time) rather than the excessively high doubles I'm used to using - and in doing so frequently find myself unwittingly in the big / big combination..

On the subject of which I appreciate your point about a double with a wide-range cassette; however I have a number of issues with this:

- Lack of high gearing: I find the 36T in the middle of my triple ideal for nearly all riding conditions; however occasionally it's too low and it's nice to have the option of the 48T if I'm smashing it down a big hill.. while of course the small ring's great for loaded climbing and on a triple would likely be lower than found on a double with a tolerably-sized bing ring.

- Difficulty finding suitable ratios: MTB cranksets tend to be too low (typically 36-24T or 38-26T at best perhaps), gravel too high (46-30 being the lowest I can find).

- Pushing the range higher on the cassette means larger jumps between sprockets; especially on the 10s system I'm looking to run.

- Large cassettes tend to cost more..


It does look like I'll be going to a sub-compact double on one of my bikes as I want to retain the hydro brakes and can't run a triple with the drop-bar levers... something that's not been easy to achieve and it looks like I'll be settling on a 42/28 front end courtesy of some aftermarket chainings... if the supplier ever pulls their finger out and actually gets them to me.

However on the straight-bar utlilty bike that's inspired this post I'd love a triple, which I think I can achieve with bits of Deore XT T8000 drivetrain (FC-T8000 48/36/26 crankset, FD-T8000 derailleur, SL-T8000 shifters) along with the M6000-SGS RD in an effort to get the clutch I desire (rightly or wrongly!) as the bike won't be used just on-road.

Of course the clutch can always be switched off if problematic, while if I really screw it up I can always fit the intended RD-T8000 and accept the lack of clutch; as I've seen the RD-6000-SGS at a pretty good price so the stakes wouldn't too high if it didn't work and I could always sell it on.

Thanks again for your input - I'll let you know how badly it goes wrong if / when I get it built