Help me build a 2x11 crankset with a 40t/42t big ring and bashguard

Question

I'm in the process of sourcing parts for a new MTB bike. The frame supports e-type top swing front derailleurs, and 92mm pressfit BBs.

And I want to have a 2x11 crankset with a 40t/42t big ring and 28t small ring, and a bashguard, because the resulting gear ratios fit my riding needs perfectly (I have an old bike with a 42x28 crankset and a 12-36 casette). Also, I want to avoid grease marks on my pants and legs, hence the bashguard.

I thought this would be easy, however it seems lately the industry has shifted to adopt either 1x setups or 2x with 24-34 to 28-38 cranksets that don't support bashguards. That doesn't really suit me, not only because of the missing bashguard support, but because the highest gear, 38x11, is just a tad smaller than my current 42x12, and I still have room for improvement.

What I'm considering ATM is

1. Build my own crankset sourced from parts on Aliexpress, because, although I find some chainrings on other online stores, I can't find a crank that supports bashguards. And Aliexpress is loaded with offerings, but I don't quite understand the specs, and I don't know how to check whether the specs of chainrings and cranks are going to be compatible both between one another, and the BB. So how do I build a compatible crankset?

2. Taking a cheap Alivio or lower-end crankset with a bashguard and remove the smallest chainring. I might end up with a 40t/30t crankset in that case, but that's acceptable. Are there trade-offs here, and if so, what are they?

I'm overwhelmingly inclined to go with 1., but still considering 2., and alternative solutions are welcome; I just want a Shimano-compatible 2x setup with a top gear greater than 42/12 and a bashguard.

BTW, I'm aware the front and rear derailleurs have capacities when it comes to min/max sizes of front chainrings and the difference between the smallest and biggest one; I've found compatible derailleurs.

Answer 1

Regarding removing a ring from an lower end triple crank. That won't work: the two rings you are left with will be biased toward the outside of the bike; the spacing of the rings on a 8 or 9 speed crank will be too wide to work well with an 11 speed chain.

You should check that you frame has chainstay clearance for larger rings. This can be a problem on newer bikes that are designed for small chainrings.

To select a crank that is compatible with your frame you need to consider two things, the bottom bracket standard and the chainline.

You say you have a frame that takes a 92mm pressfit BB, that's either a BB92 compatible with Shimano 24mm axles or a 392EVO compatible with 30mm axles. See here for a summary of BB standards. If you have a 392EVO and you want a Shimano crank there are adapters to step down to 24mm axles, or manufacturers like Wheels Manufacturing sell BBs that adapt from a frame BB geometry to a crank BB geometry.

Chainline is connected to the rear hub spacing. Wider hubs require wider chainline. Manufacturers specs will tell you what chainline a crank provides and rear spacing (sometimes called OLD for over-locknut-distance) compatibility. As an example Shimano provides extensive and detailed specs. Here is the MTB cranks spec page.

Answer 2

Poking around the BBG bash guard site, I realized that there is already a solution a lot cheaper and simpler. Buy a 104 BCD double crankset that works for your bike and bottom bracket. Then buy a spacer ring, longer chainring bolts and a 104 BCD bash guard. The spacer ring and longer chainring bolts will allow you to mount the bash guard in between the outer chainring and the crank arm. This will likely only work on MTB cranks since they tend to flare out to provide the extra chain stay clearance needed.

These pics should help.

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I thought about this even more and realized there is more elegant solution than either my original ones below. A 10 speed Shimano 105 Road triple crankset. It comes with a 30t inner, but the inner is 74mm BCB and you can go as low as 26t. The outers are 130mm 5 hole standard, finding a 40t for that will be very simple. You can either grind off the teeth of the 50t it comes with for a bash guard, or find an old cyclocross one. You will likely need some kind of chain catcher for the drop from 40 to 28t. The one problem with this solution is that it is a road crankset and may not fit in a slightly wider MTB standard BB. Clearance for the crank arms at the chain stays may also be an issue as well. MTB cranks have a wider Q-factor for just this reason.

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Buy an older 10spd Deore Triple MTB crankset with a 104 BCD. You will need to buy the chainrings and since a 42 is an outer chainring, you'll have to grind it a bit to use it as an inner chainring. The BCD is the same but the "step" where the chainring rests is slightly higher. Finding a bashguard for a 42 will be a problem. Generally 36t is the largest for 104 BCD. You can buy a larger chainring and grind the teeth off if you can't find one. Although I did find a 40t bash guard from RaceFace. The other alternative is to buy an older road 110 BCD triple. Now that I think about it this is probably cheaper and easier with one caveat. 28t/42t chainrings will be relatively easy to find and you can likely make a bash guard from the outer chainring that comes with the crankset. The problem with this solution is finding one with a 24mm axle that will work with press fit bottom brackets.

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Answer 3

If you're willing to shell out some more cash, modern cassettes with 10 or even 9 tooth cogs are available. 38/10 or 38/9 is significantly steeper gearing than 42/12. This will require a special hub (either XD for SRAM or MicroSpline for Shimano) and you will need to move to 12 speed.

Your idea of taking a 3x crank and removing the biggest ring will not work well; the chainline will be terrible and you'll struggle to shift to the smallest cogs well.