[author's note: So this article took me longer to get written than I had planned because I had to write it three times. Thank you very much cloud storage (or lack thereof)... And then SRAM launched AXS 12 speed. The next article in this series should come more quickly.]
Introduction: 1X & 2X
While this article is about gearing for gravel, it's specifically about 1X gearing. 2X has several distinct advantages, but also a few notable disadvantages. On the bright side, it's much easier to get a broad range of gear ratios (though it's generally only about 14-15 distinct, usable ratios for a 2x11, so it's not actually twice as functional as 1X.) In particular, it's much easier to hit both the bottom end and top end of your desired ratios. On the downside, gravel is precisely the type of scenario where a dropped chain from a misfired front shift is more likely. And on several popular bikes, the added bulk of a front derailleur ends of being a limiter on tire width. I think most regular readers of this site know that I am a huge fan of 1X. I have literally not shifted a front derailleur in over four years. I don't even own a bike with a front derailleur anymore and haven't for some time. But I also recognize that as someone relatively light, relatively strong, and who generally likes a relatively low cadence, I'm also the perfect candidate for a 1X drivetrain; I'm incredibly biased and won't pretend to be otherwise. I love 1X drivetrains for their simplicity while riding.
However, most of what I've written here will also apply to a 2X drivetrain, at least as far as picking ratios. You'll just have fewer decisions to make when it comes to getting your gearing right; your compromises will come elsewhere (thought they may be totally worth it to you). You'll probably want a smaller ratio than what you'd run with a typical road compact setup. 48/34 rather than 50/36 or possibly even smaller for the big ring. Unfortunately, you'll run into the limits of a 110mm bolt circle when trying to get your small ring smaller, which is part of why 2X doesn't really crush 1X in terms of overall utility for gravel.
From a practical standpoint, most people at most gravel races seem to be happiest if they have at least a 1:1 ratio (meaning front chainring and rear cog are the same size) as their lightest gear. For 2X folks with a 36T or 34T that means an 11-36 cassette in the back. For 1X folks, you're almost certainly going to be running the standard "do it all" 10-42 (or 11-42 if you can't be bothered with the XD Driver body) with a 40T or 42T chainring. This is about as close as you'll find to a "default" setup for gravel. If you want a "set it and forget it," this will be it.
In Depth 1X: Rings & Cassettes
But, if you're a gearing nerd like me - something that has only gotten worse with my embrace of track cycling, this misses the idea of what's "optimal." And this is really where I fault the component manufacturers. SRAM only makes one "gravel" cassette with a 10T cog - the 10-42. But I'd love a 10-36 or 10-32 for races where I want tighter ratios between shifts; I really don't understand why the 10T hasn't trickled down to more road specific cassettes. I do, however, appear to be (somewhat) alone here - and I get that; people just want to set up their bike and be done. 3T's awesome 9-32 cassettes (yes, plural) the Bailout (9-10-11-12-13-15-17-19-22-26-32) and Overdrive (9-11-12-13-15-17-19-22-25-28-32) are no longer being sold. Gerard Vroomen is an innovator of the highest order, but he's also a pragmatist. He's got countless good ideas, but he only sticks with the ones that gain traction (see the introduction of the 2X Strada Due for further proof of how quickly he responds to market forces).
But 3T's approach here is worth exploring further. The Bailout was focused on the high ratios - 9-10-11-12-13 in order with the 32 as an aptly named "bailout" gear; the Overdrive was focused on the low end with more even spacing - 22-25-28-32 - in the lighter ratios. The key lesson here is that you can tune your gearing to suit both your riding style and the course... if you have the cassettes available.
I rode Dirty Kanza on 48x11/42. I chose this over 44x10/42 (the more "typical" Kanza gearing) because 48x11 has theoretically less friction that 44x10, and in a long race, friction theoretically adds up. Did it make a difference in the end? Probably not. Could it have? Theoretically... From a practical standpoint, there wasn't really a meaningful difference, but as gravel becomes more competitive, I think that may change. With the Cannondale-EF having announced that they will take on DK200 this year as part of their formal racing schedule, the wattage savings from better drivetrain setups may indeed be a meaningful difference as the pointy end of these races becomes even pointier. (In the case of Kanza, I'd now say that lubrication is really the key - 206 miles in those conditions just trash drivetrain lubrication and the logic that may prevail on the road. I'd run a much heavier, much thicker, much more durable lubricant even at the expense of possible penalties for friction. Kanza just punishes your chain.)
When it comes to 1X, I have more options than I need, but that's also because I care about sussing all this out for the rest of you! For the front, I have 38-40-42-44-46-48-50-52-54T and even a prototype 56T XSYNC front rings. And I've ridden - and raced - on almost all of them (the lone exceptions being the 38 & 40T rings, though there are plenty of nearby trails where I'd make use of both; they are just new ones in my collectiom). I like having choice. Including triathlon, road, and gravel, I've paired various front rings with 11-25, 11-26, 11-28, 11-30, 11-32, 11-36, 11-42, and 10-42 cassettes. Now, of course, it's easy for me to enjoy having all these gearing options because I was privileged to be a sponsored athlete. And I certainly don't think everyone needs to go own all these. The two "essentials" are the 46T (for road-heavy courses; I used this at Belgian Waffle) and the 42T or 40T (for more technical, more MTB-heavy courses). Plus the 10-42 (or 11-42). You can ride almost anything with these ratios. I do think you absolutely need two front rings - one for "road" and one for "trail." But which two is likely a matter of personal preference and power output.
A Note On Chain Length
One might think that running a big front ring and a super wide cassette would be the best of all worlds. But you do (potentially) run into issues with chain length. Standard SRAM chains have 114 links. KMC chains are 117 links. SRAM Eagle chains have 126 links, showing the importance of link count for super-wide drivetrains. You can run (roughly) a max of 88 total teeth (chainring + largest cog) with SRAM's 114-link chain with a standard-ish chainline. Maybe you can get away with 90, but it's not ideal. I had 90 on my Dirty Kanza bike - 48/42 - and it shifted ok, but Kanza is flat enough that I didn't expect to use my 48-42 pairing much, which is when the chainline really got out of whack; Ben Collins ran 90 on his Diamondback Andean for tri since he liked a fast cadence (54-11/36) and also said it was fine, but again I doubt he expected to be in 54-36 much. So 90 is doable, but I think you are pushing the envelope. In my experience, I think 86 is really the sweetspot. And 88 is probably as much as you really want to do. 90 works, but you definitely don't want to be in that largest cog if you can avoid it. KMC chains, however, come with 117 links which gives you quite a bit more usability. But, as you'd expect, all OEMs recommend the use of their own chains and not aftermarket ones.
(This topic came up in discussions between Dan and I as he said there is something called a "mullet" configuration, which involves a road crankset and an Eagle backend, but I couldn't find an mentions or examples of this setup on a gravel bike (with drop bars). If you run this setup, let me know in the comments.)
SRAM addressed a lot of the above limitations with their cassette offerings with the introduction of their 12-speed AXS groupset. The 10-26, 10-28, and 10-33 cassettes all fill the exact gap that I referenced above. In part, this is because Force1 has proven to be a much more adept groupset for both road and tri than I think SRAM expected. It was, originally, meant as a CX-specific groupset. It came out before the real boom in gravel and "mixed-road" events. And certainly it wasn't designed with tri/TT in mind. But with only the addition of a few more rings and short and medium cage derailleurs, it proved very capable. And I think that's typical of SRAM's development process, speaking both from being a part of some of it from an early stage and also from being on the outside looking in.
(The only part of the AXS project I did any testing for was the new brake rotor.)
SRAM somewhat cautiously tests the waters, and then jumps all in. I think AXS is really SRAM's second groupset. The first was their very first 10-speed group. I never felt like any of the 11-speed groupsets was really a final product. There were incremental improvements, but it wasn't the same sort of wholesale jump that we see here. 10 and 11 speed groups were fairly similar. AXS almost nothing like either of them.
But that's also the problem with talking about AXS as a "solution" here. AXS offers virtually no backwards compatibility. It's also very expensive. For many triathletes, their gravel bike is a training bike only. If they are going to spring for an AXS groupset, it will probably be for their tri bike. Likewise, overhauling your gravel bike because that's where your focus is may be a totally viable option if you are happy with whatever's on your tri bike. There's relatively little crossover in terms of parts - wheels being the big one - between a tri bike and a gravel bike. So maybe AXS ought to go on the training bike. The tri bike is a pretty specific tool - I never found myself wanting with 2x10 or 1x11 - meant for relatively specific courses. So I think it does fine with pretty simple parts. Gravel/mixed-road tends to be much, much more diverse. And, of course, if I was writing this for a gravel (or even bike) website instead of a triathlon site, I think AXS would represent a more ideal solution than it does for an audience where this is almost certainly a second bike.
In any case, AXS represents an interesting talking point, in that many - if not all - of the limitations of gearing are addressed both with the introduction of another cog and the introduction of three compelling new cassettes. AXS is a group set fully designed with 1X and also both mixed-road and TT/tri in mind. And the completeness in this regard is evident.
The final place that AXS really opens things up - and you can see this in Dan's discussion with Ron Ritzler from SRAM - is the convergence of drivetrains. The "mullet" configuration - Eagle in the back; Red 1X in the front - absolutely is doable with AXS. This is where compatibility of components becomes important. Using a Red 12spd "brifter" (brake+shifter) with a 12spd Eagle derailleur becomes an option. Again, though, that's a pretty expensive option (currently) for a "frankenbike" setup, but these things do trickle down. There's no reason to think that the 10-50 cassette won't become a viable option for gravel in the future.
Wheels & Tires
Putting the number of cogs and cassettes aside, where gravel starts to get really interesting is that gearing is not just about the gears. It's also about tires. The effective gearing difference for the same setup on 35mm vs 45mm tires is significant. Shift to 650B, and you (typically) will give yourself even more low end. In the case of gravel, ratios are really inadequate as a means of expression. What makes the most sense is either gear-inches or gain-ratio (an idea developed by the late, great Sheldon Brown and which helps explain some of the phenomenon - I think - behind shorter cranks). In either case, you need to account for wheel size. A 40mm 700C tire has a circumference of about 2200mm. A 50mm 650B tire has a circumference of about 2100mm. That's a big difference. The actual circumference will of course depend on the rim, pressure, and particulars of tire manufacturing. But both 700C 40mm and 650B 50mm are "big" tires. And yet there is still about a 5% difference in size between them. That means that you're 44T chainring on your 700C setup becomes a 42T on your 650B. Of course, if cassettes and chainrings are expensive, that's nothing compared to needing an extra sets of wheels and tires, so again, I think if you want a default here, it's probably going to come down to riding style and preferred course. 650B is probably a better choice for folks who are more focused on offroad and who want to lighten their gearing up. Plus, 700C wheels are easier to find in used/beater configurations if you want something as a backup.
The 650B versus 700C topic is something I'll cover in the future, and it is separate from any discussion of gearing. But it's also impossible to talk about gearing without talking about the fact that both 700C and 650B are now ubiquitous in the world of "gravel." Will you be happy with 650B wheels for something like Belgian Waffle? Well, with faster rolling 650B tires coming out from manufacturers like Compass and some others, you might be happier than you'd think. I think often when people describe 700C as being "faster" than 650B it's because they don't properly account for the gearing differences between the two.
I can offer no great conclusions in terms of gearing, because it is very individual. But I think that's one of the best things about gravel. As with geometry and tire design, gravel seems to have spurred both a wave of innovation and also a wave of nostalgia for important concepts - like steering geometry and gear ratios - that seemed to have been a bit forgotten as the sport of cycling expanded. Certainly not everyone needs an in-depth understanding of these things, but I do think that a basic grasp of the concepts involved can help with making better decisions about what you do - and what you don't - "need" (or want).