I've got a 105 Di-2 12 speed setup. The bike I have has really good tire clearance and looking at setting up a gravelly setup but current official options don't stretch beyond a 11-34 cassette.
What's the deal with compatability with 12 speed MTB cassettes? There's not a ton of info online of people experimenting yet.
Any other options I've not considered?
Posted Jun 5, 2023, 12:11 PM UTC
I just watched This Video where it looks like 11-36 is the advertised max for 105 Di2, but there is tons of space for even larger than that.
Something crazy like a 2x 11-42 may very well work.
It feels like something that I might just try and see what happens 😅
That's how it is when you go past the recommended max, but Shimano is well known to be very conservative with their max capacity. It's worth trying because it most likely will work very well.
My road bike has R8000. Gearing is 11-36(sram cassette) and 52-34 chainrings. This is beyond capacity for both the rd and fd. Shifts very nicely still. I have never had a chain drop with this wacky combo. There is still lots of b-screw, and no hanger modifications required on either end.
My winter bike is an old 10sp 105-5700. 11-36 cassette, 46/30 chainrings from 'bikingreen'. I just had to use a $5 rd hanger extension to make this work.
Shimano road and mtb 12 speed parts are not interchangeable. You use to be able to run the mtb di-2 derailleur with road shifters in a 1x set with 11 speed, but since there is no di-2 mtb 12 speed parts that option no longer exits. which means you can not run a lower range cassette with out running some third party adapter.
However. With the new grx 12 di-2 groups coming out that changes. We will need to see how much cross compatibility there is, but there is already shots of it be run with something that looks like a 11-40 cassette on road compact cranks.
Basically your best bet for lower officially supported gearing is to look at the new grx rear derailleur, and check compatibility with the rest of your group. I would know more when I get my hands on it, but that may be a while.
Yeah, this is what inspired the timing of the post based off the sneak peak at Unbound 😅
Is it the spacing of the cassettes that is the limiting factor? I've built a mullet bike on 11 speed that works well but importantly without Di2.
The main limiting factor, is derailleur chain warp capacity. Rear derailleurs are designed to work with a specific range of cassettes and on 2x systems the cassette range is lower than on 1x systems but the total chain wrap capacity is the same.
This is why on a grx 11speed sgs rear derailleur in a 1x setup the official max rear cog is 42, but that same derailleur cage length in a 2x setup max low 34. Also note that modern rear derailleurs will also have a min low. This is because of the geometry of the cage and parallelogram are designed to move in a certain way along the derailleur path of the cassette range, and going below the range does not allow you to adjust the guide wheel to be in the proper Hight from the cassette. Shimano also makes the grx derailleur in a 1x and 2x specific geometry for this reason.
With all that said, outside officially supported gear ranges, I have done mullets builds with the 11speed grx very often and I am hoping the 12speed will allow this as well.
For the 11speed mullets. I have the best success using a grx-810-sgs rear derailleur, and a 11-40 cassette, then I size the chain about 2 links long. This means that you can not go to the 11 or 12 tooth cogs while in the small chainring as the chain will be slack, but you shouldn't be in those gears anyway. This set give enough chain to run big big on a compact crank, but again not a gear you want to be in for long time.
Hope this helps.
Very informative, thanks, especially around the 2* setup element.
The mullet build I have was 40t GRX 1x chainset with a 105 rear derailleur with a road link with a 42t cassette, so interesting to know that the 2x element changes the ability to do that kind of stuff.
Maybe this is just the excuse for a new bike...