XT Di2 RD (GS) capacity

[Bike Whisperer]

I have a confusing question... I'm looking at buying the new XT Di2, but...

 

The new XT Di2 RD is only available as a GS (Medium Cage) and claims to have a max capacity of 35T, which seems normal for a GS RD. However, Shimano markets the XT Di2 as being both 1x11 and 2x11 capable. They say you can use the 11-46 cassette for the 1x11 and either the 11-42 or 11-40 for a 2x11 setup.

 

From Shimano product page for RD-M8050-GS:

• Series : SHIMANO DEORE XT
• Maximum Sprocket : "42T(2x11-speed) 46T(1x11-speed)"
• Minimum Sprocket : 11T
• Front Difference : 10T
• Total Capacity : 35T

Here's my dilemma. Even if one uses the new shimano 10 tooth difference crank (say 36/26) both the claimed 2x11 setups exceed the maximum capacity of the RD.

 

Using the 11-40 cassette and the 36/26 chainrings (which needs less capacity than with the 42T cassette) this gives us a 10+29 needed total capacity of 39T. The school I went to thought me that  39>35. With a 42T cassette, the needed capacity climbs to 41T. So theoretically the XT Di2 RD won't handle the stated 2x11 setups based on the stated capacity.

 

What am I missing? Do I need to get the SGS XTR Di2 RD for this to work as 2x11? Does this only apply to some programmed SynchroShift which manages the total capacity?

 

So before you ask why not just do 1x11, my other bike is a 1x11, ... I like my short gear spacings for long rides (ex roadie)

 

Any ideas? Anyone from Shimano care to shed some light on this?

[Captain Fastbastard Mayhem]

Here you go..

 

EDIT: Sorry, nope. Not what I thought it was.

 

 

To be safe, I'd suggest following the normal capacity calculations when doing this. Even though the syncroshift does allow for the top and bottom of the cassette to be used only in certain instances, I'd not chance it on a 2x setup with the 11-40 / 11-42.

 

the max capacities are, afaik, only a theoretical and not meant to be used together. 

 

IE they are maximums and minumums, not 11t & 42t & 10t difference together. If that makes sense. 

[s14phoenix]

I am no expert but I think when you are on the big ring in front you don't really want to use the big one at the back... so 42T-11T = 31T +10T= 41T and is only applicable when you go big ring and big ring....

 

I think you can set up DI2 with the correct shift points so you don't do the crossing of the drive train.

 

I believe the 35T refers to the 1x use where 46T-11T = 35T (within spec)

 

2c in the bucket.

[Meezo]

website error i think should be 45T

[Bike Whisperer]

website error i think should be 45T

I think the 35T capacity is correct as this is pretty normal for GS RDs. SGS have 45T capacity, but not available in XT Di2

[Meezo]

I think the 35T capacity is correct as this is pretty normal for GS RDs. SGS have 45T capacity, but not available in XT Di2

 

hasn't the XT always been a MTB groupset, and i highly doubt shimano actually have CX specific groupsets, so yes by concept definitely correct, but logically doesn't make sense

[Bike Whisperer]

Here you go..

 

EDIT: Sorry, nope. Not what I thought it was.

 

 

To be safe, I'd suggest following the normal capacity calculations when doing this. Even though the syncroshift does allow for the top and bottom of the cassette to be used only in certain instances, I'd not chance it on a 2x setup with the 11-40 / 11-42.

 

the max capacities are, afaik, only a theoretical and not meant to be used together. 

 

IE they are maximums and minumums, not 11t & 42t & 10t difference together. If that makes sense. 

Total RD Capacity Calc...

 

(Difference between chainrings) + (Difference between Cassette sprockets) = Total RD Capacity

(36-26) + (40-11) = 39

 

I hear you, but surely their engineers would make sure that the system is designed for the worst-case scenarios (ie, using the entire gear range with any chainring choice). 

[Bike Whisperer]

I am no expert but I think when you are on the big ring in front you don't really want to use the big one at the back... so 42T-11T = 31T +10T= 41T and is only applicable when you go big ring and big ring....

 

I think you can set up DI2 with the correct shift points so you don't do the crossing of the drive train.

 

I believe the 35T refers to the 1x use where 46T-11T = 35T (within spec)

 

2c in the bucket.

Thanks

 

I though that, but total capacity is the RD's ability to absorb chain slack and is stated a single capacity irrespective of the front setup. So, according to Shimano, the RD can take up 35T of difference, irrespective if this comes from the front gear change, back gear change or a combination of the two.

[Captain Fastbastard Mayhem]

Total RD Capacity Calc...

 

(Difference between chainrings) + (Difference between Cassette sprockets) = Total RD Capacity

(36-26) + (40-11) = 39

 

I hear you, but surely their engineers would make sure that the system is designed for the worst-case scenarios (ie, using the entire gear range with any chainring choice). 

Correct. Oh - and I meant the max capacities they showed on the spec sheet. Not that that was what the system as a whole could handle, more that that's the max size in any one given direction. 

 

IE: GS can handle

Max 42t 

OR

Min 11t

OR

Max front change 10t

 

At least, that's what I'm thinking. Should be clearer though. 

[s14phoenix]

I don't know DI2 but sense tells me that long before you get to the 36/40 or 36/42 scenario you should have dropped to the 26...

 

I distinctly remember getting klapped against the head when I did that with the first bike I had with gears.

[Captain Fastbastard Mayhem]

I don't know DI2 but sense tells me that long before you get to the 36/40 or 36/42 scenario you should have dropped to the 26...

 

I distinctly remember getting klapped against the head when I did that with the first bike I had with gears.

yeah, but repeat after me....

 

People Are Stupid. 

[s14phoenix]

A recent discussion in a LBS about a particular chain......

 

Frustrated customer: "my bike gears make a grinding noise in 12th gear"

Bike Mech: "12th gear???"

Frustrated customer: " yes!!! - when I go to the second front gear and the first back gear"

Bike Mech: "oooooooh... ummmmm"

Frustrated customer: " and the chain also drops off when I ride like that... "

 

  :ph34r:  

[Bike Whisperer]

Correct. Oh - and I meant the max capacities they showed on the spec sheet. Not that that was what the system as a whole could handle, more that that's the max size in any one given direction. 

 

IE: GS can handle

Max 42t 

OR

Min 11t

OR

Max front change 10t

 

At least, that's what I'm thinking. Should be clearer though. 

Yes, you're right, so in my example I meet all the conditions, except the capacity. 

Cassette max is 40T (within the max of 42)

Min is 11T (spot on)

Front difference is 10T (as stated)

 

That's what is confusing me... especially if you consider that a standard GS RD does have a capacity of around 35, so something just doesn't fit.

 

Lets see what Shimano says, I emailed Coolheat (SA Shimano distributor) for their comment.

[Bike Whisperer]

yeah, but repeat after me....

 

People Are Stupid. 

Yes... That's why the design normally caters for that scenario (stupidity)

[Captain Fastbastard Mayhem]

Yes, you're right, so in my example I meet all the conditions, except the capacity. 

Cassette max is 40T (within the max of 42)

Min is 11T (spot on)

Front difference is 10T (as stated)

 

That's what is confusing me... especially if you consider that a standard GS RD does have a capacity of around 35, so something just doesn't fit.

 

Lets see what Shimano says, I emailed Coolheat (SA Shimano distributor) for their comment.

I have a feeling that the max capacity is allowing for the normal 11-36 on old school 2x drivetrains. That would be in the GS's capacity. 

 

I think it's just poorly put together. 

[s14phoenix]

Sounds like they just had a copy->paste of the spec sheet.

 

I don't see how it could work with 11s unless you use a road 11s cassette - not sure if that will even work.

 

I use 10-42 cassette on a m9000 - GS non Di2. then swop on the same chain between 32T and 36T depending on terrain. So that's 36T difference with (what I would estimate) capacity for possibly 37T or 38T difference abeit not recommended it is possible...

 

Shimano would possibly stick to their guns and recommend using parts within the 35T safety zone.

 

be interested to hear the official reply...