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Ok so another session of Joy of Baik Tech with Bob Thod ;)
Disclaimer: It's been a couple years since I went through so please accept some inaccuracies in what I'm sharing. I dont have time to redo maths and re-research before typing.
If I remember right the more distance the mech pully is away from the cassette, the slower the shifting and the wear is accelerated on the cassette + chain. This then wears the front rings faster.
Shimano doesn't "support" a bigger cassette than a 34T for the 11spd Ultegra. When doing the math explained at the Wolftooth RLDM website on rear mech capacity, the numbers also indicate this doesn't/shouldn't work. But yet it works. It shifts fine. Why? Well, one example could be when running a 46/34 at the front, there is a difference of 12 teeth instead of 16. This makes the rear mech happier. Maybe that is the key?
Back when I went through all this I reached out to Wolftooth support, which was fantastic btw. They helped me understand what was going on, why and what I needed for the solution I was looking for. Because I wanted to run an 11-42 cassette on the back and front chainrings with a 16 tooth difference, such as 46/30 or 50/34, the only option is to use an 11 spd XT GS rear mech because it's the only one with enough capacity to handle that amount of chain slack. The WT Tanpan is then necessary to change the pull ratio of the road shifters to make the XT mech move the correct amount.
The body of the Ultegra and XT rear mech are pretty much the same dimensions. This allows the Ultegra to step up to the larger rings such as up to 40 without the pully colliding with or riding on the biggest ring.
So why does the WT RLDM exist? I don't know. Because WT need to make money? Or maybe there is a good technical reason, but as we see from the rig that got @Skinny through Italy with no issues, on there was an 11-40 rear, 46/34 front and no RLDM. So to sort of repeat his words on the Terravail tires and Petervary, if that setup works for @Skinny, then it will work for the rest of us mortals. If for some reason it doesn't, then try a RLDM.
And as @Skinny said, get the RX series with the clutch.
Part numbers for reference:
RD-RX800-GS for cable
RD-RX805-GS for Di2
Bob_Thod-
This is kinda my point - my setup shifts into every gear - why do I need a WT, unless it had some magic way of making shifting better?
I would like better shifting but it's pushing the limits of the RD so I'm happy to have ok shifting and big fuckoff gears at the back. The only difference with mine is the 50T big ring that I could probably drop to 46T but would that make much difference? It might improve shifting a little since I could run a slightly shorter chain but is it going to be enough to justify another chainring?
hippy
I've had a look and I appear to be using the Direct Mount style so I think I need this one:
https://www.wolftoothcomponents.com/products/roadlink-dm
It moves the derailler way back:
I still don't know what impact it would have on shifting and I don't really want to buy another one to find out.