Okay. Deep breath. Things are about to get technical.
After tires and wheel sizes, gravel bike drivetrain options are the biggest topic of our discussions with customers. And with good reason- there are a lot of technical aspects to consider. Add in personal preference, ergonomics, and past experiences and there is no one answer that will suit everyone. So at the risk of inducing analysis paralysis, let's dive in.
Let's start with a few terms (as they'll be used for this discussion) so that we're on the same page:
- Ratio: In short, it's the comparison between front chainring and a given rear cog. A 42t chainring and 21t cog will reduce to a 2:1 ratio. It's not used much because the numbers rarely work out that cleanly, but as starting point, we tend to target somewhere in the neighborhood of 1:1 for a low gear (42:42, 34:34, etc. etc). Much lower and traction and balance can become issues- as anyone who's struggled to stay upright at a walking pace will attest.
- Range: The difference between highest gear ratio and lowest gear ratio (high divided by low) expressed in percent. I know- range should be (high-low)/low. But the bike industry collectively decided to apply its own definition. For single-ring drivetrains it's straightforward: a 10-42 cassette gets you 42/10 or 4.2 or 420%. With a double, you'll need the highest and lowest ratios (as well as a pencil), but the idea holds: it's a number that can be used to communicate breadth of ratios available to a rider.
For reference, a typical road drivetrain (Shimano Ultegra 50/34 x 11-32) has a 428% range.
- Step: The difference, in percentage, between one cog and the next in a cassette. When shifting from cog A to cog B the step would be (B-A)/A. For example, a 15-to-17t shift would be a 13% step. Why not just count teeth? Well, an 11-to-13 2-tooth step (18%) will have a much bigger impact on cadence and feel much bigger than a 25-to-27 step (8%).
- Cadence: How fast the rider is spinning the pedals, in RPM. Generally, most riders are comfortable between 80 and 90RPM and can spin up towards 105 or more- but it gets awfully bouncy. 60-70RPM is a pretty slow cadence, seen standing on steeper grades.
How do these things come together? Let's look at a few options, one familiar (Shimano Ultegra R8020 2x11) and two newer gravel-oriented (SRAM Force 1 1x11 and Shimano GRX 810 2x11), all with 700x40 tires.
A Shimano R8000 Ultegra group is a fantastically refined set of components. Designed for road riding and racing it has most of the performance of the range-topping Dura Ace group at a fraction of the price. The widest-range crankset is a 50/34, and with a the widest-range 11-32t Ultegra cassette you can approach a 1:1 low gear. Swap in a clutched Ultegra RX rear derailleur and chain retention is excellent too.
Looking at the table above, you can see that our Ultegra double can comfortably cover speeds from 5 to over 40mph. Do you need to be able to spin happily at 37mph? If you do a lot of group rides with strong riders it could be nice- though 40mm tires probably aren't your friends at those speeds.
You'll see that a number of ratos and rows are greyed out (six, to be exact). Those are basically redundant gears- somewhere around 2:1 most riders swap from one chainring to the other- and unless they're faced with an unusually flat and long road don't shift between rings to find the perfect ratio. It's no bad thing (we'll get to steps in a bit), but there is a fair amount of redundancy in most double- and triple-chainring drivetrains. Overall, the drivetrain above has a 428% range.
Now let's shift to SRAM's Force 1 (and the similar Rival 1) group that we spec most often. It only has a single chainring and derailleur. While that doesn't necessarily save a whole lot of weight, single-ring drivetrains do away with redundancy and without a front derailleur to rattle against and more aggressive clutches are generally quieter and more secure on rough terrain.
SRAM's Double Tap shifters use a single paddle to handle shifting: a small tap will release a gear for a downshift while a longer push will upshift one, two, or three cogs at once. One suspects that absent Shimano shifter patents they might have chosen something a little more intuitive- but it is easy enough to adjust to. Feel-wise, SRAM sits somewhere between Shimano and Campagnolo on the smooth-to-agricultural (or Lexus-to-Ferrari?) scale. We spec 10-42t cassettes on our Force 1 and Rival 1 builds for the range they offer.
Because we're shooting for a 1:1 low gear, the low speeds are the same (60RPM netting a brisk walking pace) but sustainably topping out at around 36mph (about the same as a 50x12). The Force and Rival groups have 11 non-overlapping ratios, down about a third on the Ultegra's 16. Overall, the Force 1x group has a 420% range- 98% of the Ultegra group above.
If I don't need to spend large amounts of time over 35mph, then it's a no-brainer- right? Of course, nothing comes for free and there are other factors to be aware of. Yes, ergonomics and aesthetics play a role and there are efficiency losses in that 10t cog, but the biggest complaint with single-ring drivetrains tends to be the steps between gears. More on that in a moment.
Finally, we have Shimano's just-released (and not-yet-readily-available) GRX group. As an 8xx-series group it should be on par with Ultegra, with more gravel-oriented gearing and an XT-level clutched derailleur. So its pedigree is excellent.
While its use of existing Shimano cassettes is admirable, it does drive some compromises. The 11-34t cassette is the widest approved with a double chainring and it has wider steps than most road cassettes at the high end of the range (smaller cogs) and smaller steps at the low end. Its progression stands out on paper (see visualization below) but without some miles it's hard to say if the difference will actually be noticeable on the road. The high gears are lower with the stock chainrings than Force 1 but so too are the low gears, which should make the 479% overall range (112% of Ultegra) welcome on steep climbs and for loaded touring.
Steps & Progression
Have you ever been in a car with a poorly-calibrated automatic transmission and overworked engine? One that hunts between gears, revving frantically in the lower and gutlessly lugging in the higher? That's an extreme example of what we're talking about here. Now, just like some motors are perfectly happy working across a broad range of speeds, some riders can easily adjust their cadence or speed by 5-10%. Others find larger steps jarring and prefer to keep their pedals turning within a narrower band. While most of us have a fairly narrow preferred range (somewhere around 80RPM), recent studies suggest that most of us are pretty far off the lower cadences believed to be most efficient, so conventional cadence wisdom may be due a revision. Still, muscle memory is a powerful thing.
So where do our gravel options land? Let's have a look:
Sticking with our examples from above, the road-focused Ultegra has the smallest range and average step at 291% and 11% (80 to 88 RPM, on average when downshifting), respectively. the Shimano 11-34T GRX 810 cassette has a 309% range and a 12% average step (80 to 90 RPM). The SRAM Force 1 10-42 cassette has an average step of 15% (80 to 92 RPM).
The two SRAM AXS groups above manage to keep their average steps in check by... adding a cog. As nice as they are, twelve-speed gravel groups currently live at the high end of the market, complete with wireless electronic shifters. Their performance is excellent, but the cost and complexity of life at the bleeding edge won't appeal to everyone.
Let's just say that no one company or drivetrain has found a magic bullet- just as no one is missing anything obvious. Drivetrains reflect a combination of company philosophies, engineering strengths, and market demands. Shimano favors smaller steps and more gradual progression at the expense of range (single chainring) or redundancy & complexity (double chainring) while SRAM prefers simplicity and range at the expense of step size.
Those familiar with single-ring drivetrains from mountain biking and those new to the sport generally appreciate the simplicity and security of singles and tend to be willing to accept larger steps between ratios. Those with a longer road background tend to appreciate the small steps of narrower-range cassettes and tend to take the weight and complexity of doubles as a given. No matter where you fall, we're happy to work with you to create a bike tailored to your terrain, riding goals, and personal preferences.
And no matter the manufacturer or model, there will probably come a time when you can't find the perfect gear- but we're at a point where anything on the market will work very, very well.
Below are the cassettes that we can spec on our Atalaya gravel bikes. In a perfect world, each model would follow a smooth exponential curve (not a straight line), representing a consistent step between each cog and the next. The realities of whole-number tooth counts and concessions to things like shift ramps make reality a bit less pretty, but it's clear what everyone is going for here and how their philosophies are applied. The cassettes with 275-350% range are typically paired with two chainrings, those from 375% and up with a single. And if that isn't enough, we went ahead and did the math on over 30 different drive trains on the market.
To the extent that I was able to keep this simple, it was necessary to limit the discussion to mainstream options and manufacturers' recommendations. Yes, it is possible to exceed most companies' recommendations, but there are always caveats (and some even require the sort of modifications or adaptations that we have designed for others). So as fun as they can be we'll save the frankenbuilds for the time and desire to tinker.
The Shimano's GRX 1x11 "812" groups use 11-speed mountain cassettes and are not unlike Force 1 groups with 9-13% less range and slightly tighter cog spacing.
Want to go even deeper into technical gearing specs? We did even more math. Read our article, There's No Free Lunch in Gearing.