How to achieve 24t-38t front chainrings

Question

After staring at many a graph and crunching far too many numbers, I've concluded that a 2x, 24-38t chainring setup would provide the best gear ratios for me. I bike near the edge of a valley with extended, steep climbs and long flat regions, so I need both small gear jumps and low climbing gears. I'm hoping to use a 11-34t, 11 speed cassette.

Unfortunately, Shimano's FD-M80__ series of 2x front derailleurs allow for a maximum front chainring differential of 10 teeth, significantly less than the required 14 teeth differential. Can I overcome this issue by using a 3x front derailleur with a 2x system? Can I exceed the 10 tooth capacity with minimal consequences? Should I just go with a 3x system? I'd prefer to stick with a 11-34 cassette to help narrow in a good cadence on multi-hour climbs. Wanted to add that I'm riding on a Specialized Sirrus 4, which does not have boost spacing.

Here are some links to relevant parts:

Planned crankset: https://bike.shimano.com/en-US/product/component/deorext-m8000/FC-M8000-2.html

Planned large chainring: https://bike.shimano.com/en-US/product/service-upgradeparts/deorext-m8000/WP-Y1RL98090.html

Planned cassette: https://bike.shimano.com/en-US/product/component/grx-11-speed/CS-HG700-11.html

Candidate Shimano FD: https://bike.shimano.com/en-US/product/component/deorext-m8000/FD-M8020-H.html

Update 9/21/24

Thank you for all your input! For users coming across this thread, I think the following combination of parts would work best:

• A SRAM 10 speed, 38-24t crankset. Specifically the S-1000 2x10 crankset (model id FC-S1000-A1) or the GX-1000 2x10 X-GLIDE crankset (model id FC-GX-1002-A1)
• A SRAM GX 10-speed front derailleur (model id FD-GX-HD-A1)
• A Shimano 105, 11 speed, 10-34t, 11 speed cassette (model id CS-HG700-11)
• A KMC 11 speed chain, because KMC chains are supposed to be compatible with both SRAM and Shimano systems.

Answer 1

This setup with its 14t jump has most often been done with 104/64 triples, mountain triple FDs, and a bashring or guard in the outer position.

Cheating the capacity number on mountain double FDs mostly doesn't work at all. When the FD is at the lowest height it can be to clear the big ring, it will be simply too high for the small ring position. The chain contact points will be all wrong and it will drag on the cage in a bunch of gear combinations.

There are some mountain double FDs that do have the 14t capacity you need, like the nominally 10-speed Microshift FD-M612. Cheating 10 to 11 speed on the chain doesn't really matter because they're close in width. For what you're describing I would probably start by using that FD or another with similar numbers.

If you do what you propose with the Shimano crank, the chain pickup areas on the 38 will not be located as intended. Since there are no barriers to just getting the matched 38/24 104/64 sets from Wickwerks, Origin8, and others, there's a strong argument for doing so and keeping it simple. There's also something to be said for being in the 104/64 ecosystem if you're going to be running a made-up drivetrain.

Edit: One detail to make sure you're getting right when buying parts for these setups is which side of the 38t ring has the counterbore for the chainring nut/bolt, and whether it will agree with the crank spider you're using.

Answer 2

The working principle of front derailleurs is that the cage and its ramps should align with the position of chainrings - and the parallelogram as well. They can be designed to take some variation into account, but there are limits. A triple chainring front derailleur is unlikely work, because the extreme positions are further appart than what you would have with a double chainring.

There's a 40-26 crankset in CUES U4000 range and a 36-22 crankset in the U6000 CUES and SRAM ranges (and matching derailleurs - that may work with your customized crankset), but if you want XT, that's a few ranges below. CUES uses standard 11-speed chains, so I wouldn't have any issue to use a CUES crankset within a non-CUES 11-speed system.

If you want to narrow steps also when climbing and remain with Shimano, going with a stock M8000 3x11 seems the best option, but I would stick with the recommended 11/40 cassette: having a 22 ring would allow you to stay more in the middle of the cassette, where steps between gears are typically smaller. SRAM 2x11 seems a good option as well (thanks Warren Burton — in the comments).

But just a note on the necessity of being in the "right gear" when climbing: there's a trend now in gravel/road, to have small steps on the fastest gears, and bigger steps on the larger gears, and it's somehow more "natural", if you want a more consistent variation between gears: if you keep a 2T increment between gears, you'll have a 20% variation from 13T to 11T, but only a 8% between 27T and 25T. 11/42T cassettes are somehow more homogeneous from that point of view (I recently made the change from 11/34 to 11/42 and do not regret it).

Answer 3

I've attached the Shimano Line Up Charts to answer the 3x & 2x M785 chainring combinations question.

I've recently finished a 3x8 Alivio to 2x10 Shimano XT M78x series upgrade with a 38-26 AK big ring paired with a 24 AM small on M785 cranks, with a top pull FD-M786 derailleur. The chainring shifting is superb. In fact, the front & rear shifting is flawless as well. I have raced 3x9 XTR & sram 2x10 back in the day when this was the newest technology & allowing for memory fade, I don't recall these earlier setups functioning any better than my upgrade. Couldn't be happier to get rid of the 8sp 17-20T jump right in the middle of my level road cadence sweet spot.

Since @Renaud described how the 2x chain ramps are designed particular to the chainring pair, I was curious how my "almost" compatible setup would perform, but if there is a difference between the 38-26T & 38-24T design, I'm not suffering from it.

Also, since I'm a high cadence climber frequently involved in 1+hr climbs, I have to buck the trend of the big gaps for climbing ratios. I would presume this might be common for us high cadence types which is why I'm still shying away from 1x tech. (Although Sram has just released a 13sp cassette & derailleur!)

Answer 4

There's a 104 bcd double (2x) crankset that has the option of a 38-24 chainring set up. The Shimano FC-M785, has an option that is 38-24 (also 38-26). This is a ten speed, XT crankset, Hollowtech2 style, which is the modern design of a 2 piece crankset. Most commonly found online as a used item, very usable a majority of them. There's probably some of this model available as N.O.S. As this was Deore XT upper level model, there may be lower tier models like Deore or SLX that have 38-24 2x cranksets. I can't really confirm that there are more than just FC-M785 2x cranksets offering 38-24 chainrings. The front derailleur, 2x days of mountain biking were short lived--or skipped over, for many--in lieu of 1x and wide range cassettes. The lower tooth count, 2x mountain cranksets were in their last year's of service to much of mountain biking except maybe XC (at the time, now also predominately 1x). They went to ten tooth count difference with large rings 34-40 even numbered counts. Then 1x pushed that toward gravel This crankset can be used within 9, 10 or 11 speed rear drivetrains without compatibility problems. The front derailleur should be a 9, 10 or 11 speed Shimano 2x mountain derailleur that is spec'd for a 38 tooth large ring (and mounts appropriately to the bike, which in most ways is dependent on frame requirements which speak to the required diameter of the derailleur's band clamp or the specific type of direct mount system the derailleur is outfitted with, paired with a bike designed for direct mount front derailleurs). I'm sure there has to be other 38-24 cranksets but cannot say for certain.