So my road bike has 105 di2 which limits cassettes to 34T with the longer cage mech.
Would there be any compatibility issues with an Ultegra rear mech to run 30T and 28T cassettes? That is, keep the 105 di2 shifters and the rest of the system, just get the shorter cage mech from the Ultegra di2 group and some tighter cassettes?
This site is a bit behind and in need of 105 updates, but AFAIK you can use an Ultegra 12s RD with your setup as mentioned, along with the proper cassette sizing.
wait… you cant just slap a 11-30 cassette on there?
I don’t think you’d gain anything swapping from one to the other, they are virtually identical except in some choice components to save a few grams. Both are mid-cage in length, there is no such short cage option anymore.
Sure, totally possible but most of the advantage in choosing a shorter cage is shortening the chain and getting snappier shifts from the tighter RD & chain setup & action.
I’d personally try a 12sp 11-30 cassette with the 105 setup (as you’ll need one anyway) before buying a UDi2 RD.
In terms of compatibility, all the 12sp stuff talks to each other. My road bike has full UDi2 except for a 105 RD.
fwiw theprosclosest has had helluva good deals on di2 lately
I can’t say from experience but based on loads of other Shimano experience I’d be shocked if you come across any compatibility issues from just straight swapping your rd-r7150 for an rd-r8150.
If it were my bike I’d probably buy, fit, and test the new cassette before bothering with the new rear mech at all though. Chances are it works functionally the same, and if you notice any degradation in shifting performance it will only be between the 2-3 biggest cogs on the block, which just might not matter enough to you to be worth the spend on a new derailleur. I can 100% guarantee that at the very minimum you definitely won’t damage any parts by trying that combo.
I can confirm that you can run the 105 di2 RD and a 11-30 ultegra cassette. I am doing it on my bike I just built up.
You will need to use the ultegra spacing guide and not the 105 one, or just eyeball it to set the b-screw amount to account for the smaller cassette.
Otherwise works perfectly.
You can also run a ultegra RD off of the 105 shifters but its not really necessary unless you want the marginal weight savings. THe shift speed is almost identical on the rear mech, the front is quicker on ultegra, but rear is near identical.
^This.
I wanted the faster front shifting and the hidden buttons to change screens on my Garmin, so I got UDi2 shifters and FD.
AFAILK the Rd is virtually identical, and we weighed the chainsets in the shop; there is less than 30g difference. So both of those are 105.
The Ultegra cassette has hyperglide+, which makes a subtle difference.
Yeah i am running the ultegra shifters as well to get the top buttons, 105 FD/RD as its a crit bike and i dont really need the 1/2 second quicker front shifting which is virtually non-existent.
Thanks, I will run it as is for a bit and see how it goes.
I’m coming from 10sp Ultegra with a 12-25 cassette and never really felt I needed anything lower. 34T is rather overkill around here. No way an Udi2 bike was in the budget, tho!
Of course the chainrings are also different (smaller) vs my current set up. I know the shifting quality is going to be night and day better with di2, but it would be nice if I could get a cassette with the 16T and 18T if I really end up missing them. I know I won’t miss the 34T. Even on a 28T cassette I’d be getting a lower gear than I currently run.
Can I humbly suggest you take a bike vacation somewhere with some climbing? 
That sounds like a good plan! I have seen stuff about the triple bypass ride in Colorado. I am sure I’d appreciate a 34T on that one.
It’s interesting you say this as I recently read somewhere that you CAN’T run 11-30 cassette (Ultegra) with a 105 Di2 derailleur. If I can find the article, i’ll post it here for reference, but cool that to know that it DOES work.
I can confirm this WAS the case… though it may not be anymore (at least at this moment)…
There was an “end of year” sale going on recently in which I picked up a 105 Di2 RD for $85 and 105 Di2 Shift/brake levers w/calipers for $125 each… I couldn’t say NO!
But I always struggle to understand why this is an issue for the “MIN” side of RD specs. Using a cassette that tops out at 30t instead of 34t still has a range of motion that works fine with the smaller 30t. Depending on the method used to size a chain, you may end up with a shorter chain when using the 30t, but I still don’t know why a smaller low gear is “a problem”.
Shimano’s spec page is CERTAINLY a great reference, but it was a 3rd party site/blog/forum somewhere I was reading… I also wonder if their specs may be conservative anyway…
I remember reading at one point with 105 mech the MAX is 34t, but i’ve read articles where folks were saying you COULD stretch it to 36t if needed.
Yeah, people often push Shimano RD to larger low cogs than spec’d. The 34t you mention was likely related to the older 11-speed setups since the new 12-speed actually lists 36t as the max above.
Yes, it WAS the 11sp 105 Mech to which I was referring! Sorry for not being clear! 