JamieDE

Is it possible? If yes, anything I should know before I dive in?
Thanks

Polaris OBark

Ultimately, it depends on your rear derailleur.

This is a useful guide: https://bettershifting.com/di2-synch...omplete-guide/

Steve B.

Shimano is picky in the e-tube software as to what components you can tell it are on the bike. As example, they no longer sell the 11 speed 11-23 cassette, so you cannot use that cassette in the setup. It probably will not recognize and allow a 30/46 as thats not the GRX crank in the Di2 system, which shows a 31/46 ( It might though as the mechanical GRX has a 30/46), You can cheat and just tell the system you have a 31/46, wont make much difference, only one tooth. But I think e-tube needs to know what components are on the bike in order for Synchro to make intelligent shifting choices,

Koyote

Two things:
1) The GRX crank is 48-31, not 46-31.
2) I'm not a Di2 expert, but I think the Syncro shift doesn't care what chainrings you've got -- it just shifts the FD when the chain gets to a specified position on the cassette.

The person to answer this definitively is probably TerryDi2C .

Steve B.

Well, as far as I've been able to figure out, in order to know where to shift, it needs to know rings and cog sizes. Thats how it calculates the shift. It needs to know so when you are in a 13 cog on an 11-34 and in the 31 ring and want to shift up it can determine the 46/19 is next (using Mike Shermans gear chart), but it should need the ring size to determine what combo. I cannot recall if you had control over where the shift occurs, I remember being able to set something, I mostly set it up and have left it alone and I never really use Synchro.

And thanks, I had a typo on the Di2 GRX crank

Kontact 

It really isn't that technical, and the rear derailleur doesn't really matter as long as it has capacity. And 46/30 has the same 16t differential as any compact crank. Di2 isn't making complex choices to shift - it just needs to know where the cogs are laterally.

I don't think there will be a problem.

Koyote

Hmm. Like you, I set it up once after getting the bike, but I haven't really used it much. Yes, you do plug in the cassette and chainring info, but I think it really just shifts based on position: e.g., in your example, it's just shifting to the larger chainring when you shift the rear from the third smallest cog 13t) to the second smallest cog (12t). At least that's what I think -- I could be wrong.

Kontact 

After working with Di2 and eTube for years, I don't think inputting the cog numbers is for anything but selecting shift patterns in eTube if you choose to use synchro. It has no effect on how the derailleurs actually move from cog to cog or trims the FD.

It is a grossly complicated system with simple results.

JamieDE

[QUOTE=Kontact;22755413

It is a grossly complicated system with simple results.[/QUOTE]

Thanks all, sounds like it should work. Hopefully I can position the FD low enough on the braze on!

Iride01 

Assuming the rear DR can handle the front 16 tooth difference in chain ring size the 46/30 is along with the total capacity, then the Di2 should have no problem. You might not have the correct ratios and stuff represented in eTube or on your GPS devices, but it will shift when told to shift.

AFAIK, full synchro doesn't need to know the gear sizes. All you tell it to do is how many cogs to auto shift the rear up or down when when you shift the front or when to shift the front when a certain cog is reached on the back. Though please read what I mean and maybe not so much what I wrote. I don't think I explained it very well. <grin>

JamieDE

I’ll give it a go, thanks 😊

Iride01 

I'd only give it a go after checking the specs for your rear DR. Not all Di2 rear DR's will handle that much front difference.

If you don't know where to find the spec's or how to interpret them then just give us the model numbers and someone might check for you. If you tell us the min and max cog on your cassette, then the total chain capacity can also be checked out.

Spec's of current products can be found for that item on Shimano's product site.... https://bike.shimano.com/en-US/home.html might need to change the country up in the top right of the page.

You can also find specs for all product here.... https://productinfo.shimano.com/#/ but for older products you might need to go into the archives which also is a link on that page.

Steve B.

Pretty sure a non long cage rear derailer will handle a 16 tooth jump, thats what a 34/50 crank has.

JamieDE

rd9070 11/30 cassette 46/30 chain rings

Steve B.

Been a while, but I think this is correct. I recall while using the e-tube software, the ability to define to what cog the system will shift. Thats how you set the Synchro move, but you need to look at a gear chart to determine to which cog to move to,

Iride01 

Well we don't really know what the OP has do we? Many times people say long cage when they have a GS version which is medium or mid cage. Some do handle a 16 tooth difference or better and some don't.

If the OP has this one, then it doesn't.
https://bike.shimano.com/en-US/produ...-M9050-GS.html
Max. front difference 10T

Well in the time it took me to answer the OP came back with part of the info. However is this a rd-9070-SS or -gs?

Iride01 

I only found a RD-9070-SS.
Those are what you need to watch. You might exceed them, but if you are ordering a really expensive crank, then you might consider whether you wish to take the chance you do when you go outside of spec.

​​​​​​​https://bike.shimano.com/en-US/produ...D-9070-SS.html

JamieDE

Rear mech is RD9070

Iride01 

It's been a while since I've been into eTube also. I don't remember if it let you give custom sizes or not. I know it had several selections of Shimano's offerings.

Kontact 

If you have the short cage SS version of this derailleur, you've exceeded both the max capacity and max cog size by 2t. You should be using a 11-28.

You might be able to fudge this into working by using a too-long chain, but if you get it wrong you'll destroy the frame when you shift into 46 front 30 rear.

JamieDE

I’m currently running an 11-30 cassette and a Rotor chainset 53/38, so I should be ok with a 46/30 taking into consideration advice here. Thanks all 😊

Kontact 

Maybe. But you did add a tooth as 53/38 is a 15t crank.

Polaris OBark

It will work fine with a 16T difference, which is standard for Shimano road cranksets. I have 46/30T and a larger cassette on the back on two bikes with Di2, and both are flawless.

As for syncro-shift: If the gearing isn't explicitly in E-tube, pick whatever is closest and manually program where you want the shifts to occur, as noted in the link I posted in the initial response.

Polaris OBark

FEATURES

• Programmed automatic motion
• CFRP pulley plate

• Saver (fuse) function
• Inner structure is protected from strong impact
• Wide Link design to reduce shifting deflection
• Increases rigidity, precise shifting over an extended time period
• Max rear sprocket: 28T
• Max front difference: 16T
• Total capacity: 33T
• Weight: 217g

Iride01 

Maybe.

Kontact is correct that with a 11-30 cassette you'll be out of the max (total) capacity of 33 for your new crank. However Di2 won't shift to the last two small cogs on the rear when you are in the small front. So that'll probably be a 13 cog and that possibly makes you okay.

Not certain what Shimano says about figuring total capacity for Di2 and other setups where they said not to use those cogs when in the small.

Learn what the numbers mean and then you'll make better decisions about what to get.