Custom/modified Drivetrain 2x/3x Considerations and Planning

Question

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" 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" 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 :)

Answer 1

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.

Answer 2

Ok did some further research, final clue came from @MaplePanda .

The e*thirteen 11-speed TRS cassettes look promising. They are available in various sizes, one of them is 9-42. I will pin that for my final setup.

Will try to get a full Shimano XT M8000 2x11-speed (36/26t) setup to work with the e*thirteen TRS 9-42. According to several user-reviews, it seems to work quite well to use the TRS as drop in replacement, although Shimano 11-speed derailleurs were designed for 11t max-top-sprocket.
Other M8000 rear derailleur specs like max-capacity and max-low-sprocket look good.

Another plus is that I dont have to mess with front derailleur customization that way. Also the M8000 series shiftlevers have the optical gear display option, perfect!

That setup calculates to 18-116 gear inches (29x2.0, 170mm) and 646%. Fine by me :)

Edit: Noticed e*thirteen also offers 12-speed cassettes (9-46), so I could try with M8100 XT 12-speed too.
12-speed setup calculates to 16-116 gear inches (707%). Here the cassette exceedes all 3 specs of the corresponding derailleur, would be test to see if it works reliably. And sadly the M8100 XT does not offer optical gear display shifters anymore.

Thanks to everyone for their opinions!

Edit2: Finished the bike build and been riding for about 3-4 months now. Worked out really great. Went with the M8000 2x11 36/26t, SGS derailleur, e13 XCX 9-42 and CN-HG701 chain. E*thirteen also advised against using the XCX cassette in combination with Shimano 2x12 on their website. The only thing I want to mention is: I did not manage to get a perfect shift into the 9t sprocket. Sometimes the chain needs a few hops to settle down. I guess this may be due to the fact the derailleur is designed for 11 tooth. And chain movement feels little rougher in that 9t. But that is totally worth it IMO, use the 9t rarely after all - but so glad to have it. Great gear range in total, absolutely awesome. Btw: From adjusting the shifts and looking at the derailleur/chain positions, I also get the impression it may even work to put in the 24t front of the M8000 34/24t. Might try that at a later point.

Answer 3

I use a modified 26/36/48 crankset on my touring bike. I removed the 26t chainring, as I do not have a use for it. I would ideally pick a 11-32 8-speed cassette, but with the 47cm chainstays a 32-tooth rear and 48-tooth front setup would mean 116-link chain is not long enough. Chains only little longer than 116 links are not available from Shimano (you have to buy 138 link chain then at considerably greater expense than 116 link chain), so I have to accept a 11-30 8-speed cassette. Of course some other chain manufacturer could perhaps sell 118 link chains, but then I cannot use the superior reinforced connecting pin system of Shimano and have to use the horrible quick links that are impossible to disconnect without pliers if the chain is full of dirty sandy oil mixture. Furthermore, having to carry few quick links would mean more extra weight for my emergency tool kit when compared to carrying few reinforced connecting pins, and a portable mini chain tool is necessary anyway. The reinforced connecting pins are also much cheaper than the quick links.

The crankset is actually intended for 9 speeds (it's a Deore Hollowtech II crankset and those are hard to find for 8 speed setups -- I wanted the Hollowtech II feature because the bottom bracket has durable bearings and durable spindle at the same time, being of the outboard type). The rear derailleur is also intended for 9 speeds (it's some old Deore XT). Both of these work just fine in the 8-speed drivetrain. The shift levers are Ultegra 8-speed bar-end shifters. I don't remember what front derailleur I use other than it's intended for double chainring setups. Anyway, the front shifter is of the friction type so the shifter works with all cable pull ratios. Besides, a double front friction shifter is essentially equal to an indexed shifter because the limit stop screws constrain its movement, while at the same time allowing trimming which an indexed shifter system would not allow.

I find that the 36t/30t low gear gets me just fine up very steep hills if I ever encounter one -- most of the time I don't and thus don't need to use the loweest gear I have. At the same time, 48t/17t and 48t/15t allow leisurely flatlands cruising and 48t/13t allows high speeds when I need them. The 48t/11t gear is unused, but if I had the option to switch the 11-30 tooth cassette to 13-30 tooth I wouldn't as I prefer large jumps between gears so that a large change in gear ratio is felt when I operate the shift lever. The 11-30 tooth 8-speed cassette has 15.41% jumps between gears and a hypothetical 13-30 tooth 8-speed cassette would only have 12.69% jumps.

As an added feature, I installed the drivetrain with a full plastic chainguard allowing me to have non-oily trousers after riding the bike, without having to put the end of the right trouser leg inside my sock.

Recently, I have been using an e-road-bike more than the touring bike. The e-road-bike unfortunately has 10-speed drivetrain, but fortunately, the cassette is 11-34 tooth so it still has 13.36% jumps which I can tolerate although I would prefer larger jumps. The drivetrain setup in the e-road-bike is stock as I have not modified it. It doesn't seem to be feasible to install a full chainguard to the e-road-bike so I have to do without.

My preference for lower speeds is caused by lower expense of wearing parts and the observation that it's easier to have big jumps between gears by a lower speed count in the drivetrain. I am aware of a study that found that the higher speed chains are counterintuitively more durable than the lower speed chains due to improved materials despite their narrowness, but my understanding is that the increased durability of these new chains does not offset their higher cost.