Shimano ultregra 12s derailleur with 40T cassette

I’m considering changing my 11-34 cassette to a 11-40, and use an ultegra 12s derailleur with a “1X” setup (classified powershift) (I effectively have another chainring (0.7 times the number of teeth)

While capacity would not be an issue (since using a single chainring), I see that the ultegra derailleur can only take a 34T cassette. I’ve seen some reports (mostly on older tech) reporting that this is OK, but are there first hand experiences here?

I’m heading to the alps (alpe d’huez) for a few weeks, and would like to use my gravel bike. It’s currently on a 11-34 / 42T, but I pedal out around 45kph. Changing the cassette to a 11-40 would allow me to run a 48 or 50T chainring, extending the top range by a decent amount, while keeping the ability to rev on the 11% uphills.

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Have you been to the alps before?

It feels like 48T/50T with an 11-40t in the back is… a bit too optimistic? Unless you’re 65kg and average 250w up 1-2 hour mountains.

I just did 3 weeks in the alps this summer, and running 11-34 rear and 52/36 in the front was border line being able to keep it in Z2 at all times during climbing (76kg and 250w LT1).

Just so you’re not being too optimistic, making those few weeks a pain.

I would at least aim for 1:1 ratio on easiest gear.

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I have a classified hub, so the 48T effectively becomes a 33T in the reduction gear.
My current setup is a 42T, which effectively gives me a 29T extra chainring.

Given the way the classified hub reduction works, you get more gear range the bigger chainring size you use (and vice-versa)

I’ve ridden Alpe d’huez a few times (have a place there). I don’t expect to to Z2 on the AH climb itself, and keep those part for the valleys. In any case, even with my 42T chainring, it would be nice to have more range (maybe to ride Z2 up AH, but not having to coast the whole way down either)

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Excellent, then you’re good to go! :smiley:

It’s a shame that they haven’t released a new GRX Di2, or if you had the old Di2, since you could’ve just gotten a GRX 1x rear derailleur.

I am eagerly waiting for the GRX Di2 :slight_smile:

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What about 48T with a 11-36T cassette?
How fast do you like to go downhill? 48x11 is 65+kph at 120rpm.
36T will likely work with B screw adjustment.
Powershift would give you 34T ring equivalent so below 1:1 for lowest gear.
If that’s not enough, consider 40T cassette with Wolf Tooth RoadLink Derailleur Hanger Extension (

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Thanks. Their website seems to indicate it’s only for 11s? Struggling to understand why it wouldn’t work with the 12s, so maybe just the website is out of date?

I am not sure why they are hesitant to mention 12spd.
Mechanically the Roadlink simply lowers the derailleur to provide extra clearance.
These older videos of 11spd convinced me to try it.
Roadlink model:
DM model:
Check out the comments.
I’m running the DM model with DA Di2 12spd and a 1x crank. Runs fine although sometimes hesitates to drop from 12 to 11T which makes sense since the pulley wheel is farthest away from the cog in that gear.

They claim they don’t offer a compatible 12d model:

Thanks for reaching out. We do not offer a derailleur link that is compatible with 12speed drivetrains. Let us know if you have any other questions.

For the alps the last thing I would worry about is harder gears. With your current setup, your easiest gear is 0.7 x 42:34 = 0,86, which should be plenty. 42:11 = 3.82 is hard enough to pick quite a bit of speed. On hard descents you will likely stop pedaling on the fast bits anyway.

I’ve already ran out, on this bike (did 3 or so post-ski ascents this winter).

This was the only day I didn’t run out, as very careful on the way down :grin::


On descents you will run out of gears no matter your gearing. The question is whether it matters that you spin out at 59 km/h or 65km/h or at 70 km/h? On public roads I assume you don’t know very well.

With my setup it’s currently closer to 45-50. Good for everyday, but I really enjoy sprinting out of the hairpins then braking hard before next. I usually get my max HR on the descent.

Also, makes it easier to overtake cars.

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I took your gearing as the basis, but I reckon your cadence is lower than mine. I spin out at 65ish km/h at 120ish rpm in 42:10.