Custom/modified Drivetrain 2x/3x Considerations and Planning Planning a drivetrain setup for a new bike:
Currently running old Shimano 3x9 44/32/22t on 11-34t, which gives me 19-115 gear-inches (about 620%).
I used 29x2.0, 170mm crank to calculate my current setup at https://sheldonbrown.com/gear-calc.html.
The old 3x9 provides me with nice little jumps between the gears, which is something I definitely do not want to miss.
Turned out to be not too easy to find something nearly close the gear setup of my 12 year old bike...
Requirements:
I definitely want the option to mount a set of 29x3.0&quot; tires, which may already cancel out 3x cranksets (even though full boost is used?). And I want to stay in the sub-20 and 110+ gear-inch range, preferably even going a bit lower than 18 gear-inches.
Also I do not want to use drop bars/road-shift-levers. The ideal rear-levers would have gear-indicators, but only found some Shimano/Sunrace-SRAM variants for 11speed.
Added current frame considerations:

Surly ECR 29+ (Large)
Jones Steel Plus LWB Diamond (Large)
I know that at least the ECR is good with 2x and 29x3.0&quot; tires. Both support 148mm boost.

Seems like 2x still allows for comparable small steps, while having the extended 600%+ range.
SRAM/Shimano 2x11 of either brands uses 11-*t cassettes, which ends up in a significantly lower max-speed with the corresponding dual chainring cranksets.
The only off-the-shelf option getting close, seems to be the Shimano M9100 XTR 2x12 (10-45t,38/28t). And to some extend, the 2x12 XT/SLX (10-45t,36/26t) drivetrains. But those further decrease the higher gear-inch range (of course, providing better climbing gears instead)
I was thinking of going with XTR M9100 and maybe try to fit a 26t granny, when derailleurs are working fine with it.
But the full M9100 XTR setup is pretty expensive, also in replacement parts...
What are your experiences with custom or modified dual chainring cranksets, maybe even as far as 15-20t difference (e.g. 40/20t)? It should allow for using the cheaper 11-34 or 11-36 cassettes. I guess, I wouldn't even have to go 12-speed that way.
Which combination of manufacturers/models/series of cranksets, cassettes, front/rear derailleurs and matching shift levers did you came up with to make it work?
Hoping for an interesting exchange of known-good custom drivetrain setups. Maybe someone has additional ideas, which I failed to consider yet?
Best Regards,
Wiener :)
In my reading of your question, the big limiting factor is the 29x3. Your question isn't just about what FD/chainring jump hacks function and how well, but also of those which will also work with your clearance needs. The answer to that question is generally not very many, but it's going to depend on the chainstay length of the frame in question, whether the cranks are boost, etc.
Contemporary double MTB FDs of the dainty, short variety that are made to give as much clearance as possible do not play nice with hacking the jump that much. I don't have a ton of experience doing it, which admittedly is what your question is driving at, but if you look at one you'll see that their total top-to-bottom cage space is limited and the contact points for the chain are not large. Maybe you can get it to function okay with 2t extra jump, maybe not.
What always works is just going with a larger cassette range than the total capacity of the RD can handle and accept having a slack chain in some small-small gears. This has been the standby approach to cheating the range on derailleur systems forever and it applies here just the same. Then you're not forever playing games trying to make a front derailleur make a shift it wasn't designed to.
Don't discount just going Rohloff. At 526% the range isn't quite what you're looking for, but obliterating all clearance problems forever is huge.