12 speed Di2 mullet build query (11-40 cassette on 2x)

[KingswayRider]

I'm looking at building up a new gravel bike with capacity for bigger tyres (40mm currently, would like to run up to 50mm). My current thinking is a gravel bike that can, with a swap of wheels, stand in as a winter road bike. Current road bike has Ultegra Di2 12 speed (but absolutely no room for any sort of front mudguard)
So, with current road bike in mind, I want a 12spd setup. I also want to keep a double up front - more versatility & better gearing on smooth sections with less cadence change. However, I know from the difference between my road & gravel bikes when braking, that I want servo wave, so that currently only leaves Ultegra/Dura Ace Di2 for shifters/disc calipers (GRX800 series has servo wave, but currently only 11spd Di2 or mechanical 12spd)
Now comes the sticky bit...I'd like a better low range than 46/30 & 11-34. I know it's low compared to a road bike & on fireroads it's not so much an issue, but many trails that can be ridden on the gravel bike have steep climbs more suited to mtb gearing.
Has anyone fitted a wider range cassette? I'd like to fit an 11-40 cassette. I can see that Shimano details 12spd Di2 as max 34T for Ultegra & 36T for 105 rear derailleurs. However, it's common knowledge that Shimano are always conservative with advised max.

 

[Deleted member 121159]

My suggestion: fit 44/28 chainrings from TA Specialte on a GRX crankset. Use 11-36 at the back. Then you have 28-36 which is the same gear ratio as 30-40.

There are three issues with your plan: finding a 12spd 11-40 cassette, b screw adjustment and indexing.

I'm not sure if 11-40 in 12spd exists at all. Shimano & Sram definitely don't make it. Perhaps try Aliexpress. I haven't seen one online.

Assuming you can find one, you have to adjust the b screw so there's a gap between the largest sprocket and guide pulley. It is possible to just use the original b screw but this isn't a good idea as it'll grind into the derailleur. So you need to get a longer b screw or use a longer 'shadow' link like Wolftooth goatlink.

Now I've tried this. I tried using 11-40 with an 11spd GRX rear derailleur that's meant to take up to 11-34 with up to 17t gap at the front. There's plenty of videos on Youtube that show exactly this and all say it works. Perhaps I'm a shoot mechanic is why but I couldn't get it to shift smoothly. It just didn't work very well.

 

[Cycleops]

11x 40 on a double bit weird. What are you trying to achieve?

 

[KingswayRider]

I have a Rotor Aldhu carbon crankset with direct mount rings. No option to reduce ring size within Rotor's range. I'm now looking at GRX 11spd Di2, which does offer a 1x option for wide range cassette (& suitable cassette), but also offers 2x spec. Question is going to be whether the 1x spec di2 rear mech will play nicely with a 2x front setup. Can eliminate cross-chaining & low/low / big/big combo's by using auto shift for front mech & can configure exactly where on cassette that change happens, to best manage ratio's.

11x 40 on a double bit weird. What are you trying to achieve?

More go anywhere capability than current 11- 34 cassette. It's for a gravel bike, but want to do more than just fireroads. A lot of what I ride is very steep, but I don't want to compromise speed on sections between techy bits.

 

[Deleted member 121159]

I have a Rotor Aldhu carbon crankset with direct mount rings. No option to reduce ring size within Rotor's range. I'm now looking at GRX 11spd Di2, which does offer a 1x option for wide range cassette (& suitable cassette), but also offers 2x spec. Question is going to be whether the 1x spec di2 rear mech will play nicely with a 2x front setup. Can eliminate cross-chaining & low/low / big/big combo's by using auto shift for front mech & can configure exactly where on cassette that change happens, to best manage ratio's.


More go anywhere capability than current 11- 34 cassette. It's for a gravel bike, but want to do more than just fireroads. A lot of what I ride is very steep, but I don't want to compromise speed on sections between techy bits.

1x grx rear derailleur does not work with 2x. It doesn't have the capacity. If you get the 2x derailleur then it'll work fine with 11-36 11spd cassette. Then you can fit 46/30 direct mount chainring on aldhu so you get 36-30.

 

[equilibrium]

I'm looking at building up a new gravel bike with capacity for bigger tyres (40mm currently, would like to run up to 50mm). My current thinking is a gravel bike that can, with a swap of wheels, stand in as a winter road bike. Current road bike has Ultegra Di2 12 speed (but absolutely no room for any sort of front mudguard)
So, with current road bike in mind, I want a 12spd setup. I also want to keep a double up front - more versatility & better gearing on smooth sections with less cadence change. However, I know from the difference between my road & gravel bikes when braking, that I want servo wave, so that currently only leaves Ultegra/Dura Ace Di2 for shifters/disc calipers (GRX800 series has servo wave, but currently only 11spd Di2 or mechanical 12spd)
Now comes the sticky bit...I'd like a better low range than 46/30 & 11-34. I know it's low compared to a road bike & on fireroads it's not so much an issue, but many trails that can be ridden on the gravel bike have steep climbs more suited to mtb gearing.
Has anyone fitted a wider range cassette? I'd like to fit an 11-40 cassette. I can see that Shimano details 12spd Di2 as max 34T for Ultegra & 36T for 105 rear derailleurs. However, it's common knowledge that Shimano are always conservative with advised max.

I built my older Serotta ti all road with Dura Ace Di2 (11spd) and GRX 1x rd. Ran 42 on 650bb gravel and 30’s on 700cc road wheels. Now building new one for wider gravel tires, 42’s on 700cc and 50’s on 650bb. Would still consider that same as my priority option, yet would just need to run Dura Ace rd until GRX Di2 is launched and suffer a little on the steep bits.

On 11spd I had 11-34 on my road wheels and 11-42 on gravel and assuming could go upto 11-46. Chain is finiky to adjust and will not take wear as long as normally, also I ended up swapping the front rings to new ones earlier than before. Yet I think it totally has been worth it.

1x grx rear derailleur does not work with 2x. It doesn't have the capacity. If you get the 2x derailleur then it'll work fine with 11-36 11spd cassette. Then you can fit 46/30 direct mount chainring on aldhu so you get 36-30.

Yes, 1x GRX Di2 will work pretty perfectly with 2x Dura Ace 11spd, would boldly assume the same for 12spd.

 

[Deleted member 121159]

Yes, 1x GRX Di2 will work pretty perfectly with 2x Dura Ace 11spd, would boldly assume the same for 12spd.

So that's exceeding the derailleur capacity by 16t, and it still works! We've been lied to by Shimano. Normally I don't have any qualms about exceeding it by 2-3t, but never knew it would work with 16t difference at the front.

 

[rivers]

11x 40 on a double bit weird. What are you trying to achieve?

I have an 11-42 on a double (46/30). I rarely use the 42 unless I'm bikepacking or towing my toddler (or bikepacking and towing my toddler). It's rarely used, but is definitely necessary to have when I need it. As it is, I'm running an 11 speed mechanical double mullet (mix of 105 and deore, plus some aftermarket parts). Chain is a bit slack in the small-small combination, which I don't use, so it isn't a problem.