ok, The TR mafia has convinced me I can build My Gravel bike from a frameset (likely Open UP/UPPER). My LBS will help me and due to supply issues has recommended I stick with SRAM for the drivetrain.
The obvious choice is XPLR, but I do spend some time on pavement in a paceline AND I have to do a lot of steep climbing with a relatively low FTP. So I want a 2x with a very easy lowest gear. Because I want a 2x I think i need to go with Force eTap and i’m wondering if there is any difference in terms of chain security and ability to handle really rough trails vs XPLR…
any thoughts on how to build this machine??
Nah, the force rear derailleur has a “orbit management” system (which is a fancy clutch) so you shouldn’t get too much chain slap.
Orbit™ chain management technology keeps the drivetrain quiet and secure
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If you want 2x with the widest gearing, then the obvious choice is SRAM’s 43/30 crank with a 10-36 cassette. The gear range is an insane 516 %. I have included the 10-44 cassette coupled to a 38-tooth chainring for reference. As you can see, you get about an gear and a fourth at the top with the 2x setup and a virtually identical lowest gear.
I have a 10-36 cassette on my road bike (set up as 1x), and the gear steps feel identical to an 11-speed 10-32 cassette. So if you prefer the feeling of a road cassette, the 10-36 delivers. However, let me say that if you have knobby tires on your gravel bike, I think the top gears of the 43/30 + 10-36 combo are unrealistically hard, you won’t use them. At 90 rpm in your 43/13 with 40 mm wide 700c tires gives you a speed of about 40 km/h. On road tires not a problem, but I think with knobby tires, you’ll be putting our a lot of power to stay that fast.
Unless you plan on having two wheelsets, one with road tires, I don’t think you need that tall a gear.
PS All SRAM eTap AXS rear derailleurs have a damper, which works very well and does reduce chain slap.
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Super helpful
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Glad I could be of help.
Just three more things: if you go 2x, you want the Wide rear derailleur, which officially accepts anything from the 10-28 to the 10-36 cassette. If you (against what you stated in your post) want to go 1x, you should take the XPLR rear derailleur, which fits the aforementioned 10-36 and the 10-44 cassettes. AFAIK the XPLR rear derailleur is incompatible with a 2x crank.
Also, the 10-33 cassette will have even tighter gearing than most road cassettes, although here you obviously max out at 0.91 as your lowest gear. If you happen to end up with two wheelsets, perhaps you want a smaller cassette on the road wheelset (e. g. 10-33 or 10-30 if you go for 2x).
Lastly, if you want to spend your money in a smart way, opt for a Rival drive train and Red or Force cranks. The shifting mechanism is to my knowledge identical between Rival, Force and Red, so shifting is going to be as good as with the more expensive drivetrains. The clunker of Rival is the crank, it is quite heavy and you can easily save quite a bit of weight for relatively small money.
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One more question
Is there any way to get the 44 cog (or anything larger the 36) with a 2x drivetrain?
Not officially supported but people managed. Won’t be great for a road focused machine
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Nope, at least not officially.
If you want even lower gears, you either need to use a third-party crank or go for a 1x mullet setup.
What kind of terrain do you have in mind? Usually, when you need mountain bike climbing gears, you don’t need very tall gears and a 1x setup is almost always better than a 2x setup when the terrain gets super rough and/or super steep. The latter can be dealt with by route choice.
Many people find 2x better when the riding is closer to road riding, i. e. you want wider tires with a little bit of tread on them and want to be able to ride on all surfaces.
In my mind the question you should be asking yourself: do you want a jack-of-all-trades or a rather specialized weapon. A 43/30 with a 10-36 cassette should allow you to get pretty much anywhere and with the right tires you will be able to hang with your road bike buddies. What kind of routes do you plan on riding? Do you plan on having any other bikes in addition to your Open Up/Upper? Or do you plan on having two wheelsets instead? Do you enjoy having the wrong tool for the job, i. e. being underbiked in certain circumstances? Or you do love having a scalpel that slices through thin, soft things like warm butter? The latter requires a more focussed setup. Either way, you will have to make some trade-offs.
Lastly, this doesn’t just apply to gearing, but also tire choice. The type of (gravel) riding you can do on e. g. 38 mm wide Panaracers or Schwalbe G-One Allrounds is different from situations where you have put on fast mountain bike tires (think Schwalbe ThunderBurts). The latter just won’t ride very nicely on road. It’ll work, but you can tell that this isn’t what the tires were made for.
To help you, I can recommend the Youtube channel Rides of Japan. He owned an Open Up for a few years and switched to another gravel bike. In addition, he got a proper road bike, too. His Open Up was set up 1x, but currently he runs 2x on his road bike. Most of the time he used his gravel bike as an all-surface bike. In Japan we have lots of smaller mountain roads that have become impassable due to things like mud slides. With a road bike, you might end up in double track terrain. With a gravel bike, you can keep going.
Here is my advice: I’d start with figuring out what gears you actually need. Prioritize climbing gears over gears at the top end. To give you an example, I’m a reasonably fit rider, and I can make 1x12 work in hilly terrain with pretty much no compromises. Top end gears are not a problem, especially if you are not super fit (as you wrote about yourself in your original post). To give you an idea, my top gear is 42:10, and I essentially only use it on downhill segments as an overdrive. 42:11 is rarely used. When I am cruising at speed (think 37–50 km/h) on (false) flats, in a peloton or with favorable wind conditions, I am typically in 42:14–42:12.
If I were to spec a gravel bike, I probably wouldn’t both with any gear over 38:10. I’d spin out at about 60 km/h (at 120 rpm) as opposed to 66 km/h. Even if I were a gear grinder, I’d still do 45 km/h at 80 rpm. In fact, I wish SRAM offered smaller chain rings than 38 teeth. Just be honest with yourself for a moment: how often are you faster than 50 km/h and wished you could put power to the pedals?
Figuring out climbing gears is way tricker. You’d have to tell us how steep and long the climbs are, and what your W/kg is. In my experience, in case we are talking about proper mountains you can never go low enough. Oh, and make sure to have gears when you are putting out Z2 to low Z3 power and not just FTP.
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Can’t really agree here.
Yes, the Rival crank is heavy.
But: to my knowledge there is no Red level wide crank, and the wide cranks are needed for the 43/30 chainring.
Additionally, the rival has a cheap and easy power meter option, which SRAM Force Wide doesn’t have.
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Correct, which is why I wrote “Red or Force” — and my recommendation was Red cranks for a 1x setup, Force if @Jkauffman goes 2x (43/30). I should have been more specific, though.
That’s correct, that’s the one downside of the Force 2x 43/30 crank. You could get pedal-based power meters, though. On 1x, you can get a Quarq power meter, though. If you are cool with a one-sided power meter (I used one for several years), then a Rival crank will be a great option.
My point was that with a Rival + higher-level crank setup, you can get the same shifting with much of the weight savings of a higher-level groupset at a reasonable price.
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Did you consider a Mullet Drivetrain? Like the S-Works Diverge has? It has Force Axs Shifters and a 1x Drive train with a MTB GX oder xx1 Derailleur in the back. For me this is the best gearing for a Gravel bike especially since i have a Roadbike aswell.
Right now i have a 42 Frontring and a 50t in the Back 
but i may change the 42 and make it smaller
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The problem with pedal based Power meters for MTB/ Gravel is, they are usually pretty heavy, expensive and have high stack.
I run a SRM X-Power that works hit and miss, and weighs around 170g more than the Eggbeaters I usually run.
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No, that’s true, and if you want a crank-based power meter, then the Rival chainset is really an interesting option. The Rival power meter is also surprisingly cheap, AFIAK cheaper than 4iiii or Stages one-sided power meters.
I personally also opted for a crank-based power meter. Since I am on 1x, the Quarq DZero was a really easy choice. One of the best and most boring purchases I have made. Apart from having had to change the battery once, it has been in continuous operation for over a year with zero failures.
PS Thank you for making me clarify what I wanted to say. 
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Pedal based obviously boasting the big advantage of being a multi-bike system.
If you don’t care for that. Quarq all the way. I even run Quarq on my Shimano bikes.
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