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 gearing 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 for a gravel bike (42:42, 34:34, etc.). If you go much lower, 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 single 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, a SRAM Eagle mountain bike cassette 500%.
- 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 two-tooth step (18%) will have a much bigger impact on cadence and feel much bigger than a 25-to-27 two-tooth 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 Gravel Bike Gearing Affects Speed
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.
Shimano Ultegra R8000: Speed by Gearing Ratio and Pedal Cadence
Looking at the table above, you can see that our Ultegra double can comfortably cover speeds from 5mph 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 if we're honest with ourselves 40mm knobby tires probably aren't helping you reach or maintain 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 identically-geared Rival 1) group. It only has a single chainring and derailleur. While removing the extra hardware doesn't necessarily save as much weight as you might think, 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 than road-oriented models.
SRAM Force 1: Speed by Gearing Ratio and Pedal Cadence
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 usually aiming for a 1:1 low gear on our gravel bikes, the low speeds are the same (60RPM netting a brisk walking pace) but Force 1 realistically tops out at around 36mph (roughly the same as a 50x12). The Force and Rival groups have eleven non-overlapping ratios, down about a third on the Ultegra's sixteen. Overall, the Force 1 group has a 420% range- 98% of that offered by 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 minor efficiency losses in a 10t cog when compared to an 11t, 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 recently-released GRX group. As an 8xx-series group it is roughly on par with Ultegra, with more gravel-oriented gearing and borrowing a bit from the same-level XT mountain group.
Shimano GRX 810: Speed By Gearing Ratio and Pedal Cadence
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 as a rule these differences are probably more noticeable from an office chair than from the saddle. The high gears are lower with the stock chainrings than Force 1 but so too are the low gears, which should make the 474% overall range (with the 46/30t crankset we spec) welcome on steep climbs and for loaded touring.
Steps and Progression Are Equally As Important When Considering Gravel Bike Gearing
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 cadence (somewhere around 80RPM), recent studies suggest that most of us are pretty far off the lower cadences thought to be most efficient, so conventional wisdom may be due an update. Even so, muscle memory is a powerful thing.
So where do our gravel drivetrain options land? Let's have a look:
Looking at our gravel drivetrain options, the single-ring GRX 10-34t and AXS 10-33t cassettes do well in terms of step size, but many riders will find them lacking at one or both ends of the speed spectrum.
It's with the Force/Rival 42x10-42 drivetrain that range becomes manageable for most riders- the steps are on average a little bigger than road groups' (around 13%), but most of us can adjust our cadence by roughly 11rpm when shifting from one cog to the next.
GRX 11-speed and SRAM AXS 12-speed doubles both have significant range, with GRX biased towards lower speeds (steeper climbs and heavier loads) with the road-oriented Force and RED AXS groups unsurprisingly favoring higher speeds.
The recent introduction of SRAM AXS "Mullet" drivetrains, which combine road wireless shifters with mountain 12-speed cassettes and derailleurs provide a massive range with single-ring simplicity. The 12-speed 10-50t cassettes actually share the first eleven ratios with Force/Rival 10-42t cassettes, so steps are identical until the lowest gear. Paired with a 44t chainring, the Mullet groups have a road-friendly top end while matching GRX in the wall climbing department.
So How Should We Build Your Gravel Bike?
Let's start by saying 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 generally prefers simplicity and range at the expense of step size.
Who Might Like a 1x11 or 1x12 Gravel Drivetrain?
Gravel riders with a background in mountain biking, where 1x drivetrains have firmly taken hold, will probably find a single chainring setup more natural. Additionally, people relatively new to cycling tend to find the simplicity of the 1x setup appealing. And it's hard to argue against the aesthetics of a single 'ring- bikes just look cleaner.
Riders from areas with more sustained climbing and longer, steeper descents are less likely to notice the larger steps of a 1x system. This terrain often requires more time in a few low gears on the climb, and more coasting than spinning on descents. Plus, the chain retention of a well-designed single-ring drivetrain is unmatched on steep, bumpy descents of mountainous areas.
Interested in a 1x setup? We currently spec SRAM Force 1 and Force AXS "Mullet" single-ring groupsets on our J.Guillem Atalaya Gravel Bikes. The mechanical Force 1 groups are built with 42t chainrings, electronic Mullet groups take advantage of wider-range Eagle cassettes to add in a bit of top end by way of a 44t 'ring.
Who Might Like a 2x Gravel Drivetrain?
If you're coming to gravel riding from a background road cycling, then you'll probably appreciate the smaller steps of narrower-range cassettes paired with multiple chainrings. You might also like the ability to find the "perfect" cadence when riding in a pack or on long, flat smooth or pavement stretches.
Flatter or more gently rolling terrain also lends itself to 2x gravel drivetrains. The smaller steps will allow you to maintain your preferred cadence through more subtle changes in the terrain. With less runoff scouring the road, surfaces tend to be smoother and you'll be able to keep up with the pack when the hammer drops. And as noted above, a 2x system -and Shimano GRX in particular- is a great choice if your ambitions include loaded touring.
More road-biased riders will also love SRAM's AXS 2x12 groups. They shift beautifully and have plenty of low end for all but the steepest dirt climbs. With twelve closely-spaced cogs to choose from, the perfect gear is always within reach.
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. Feel free to contact us to start the process.
Still having trouble pulling the trigger on a drivetrain? Know that no matter the manufacturer or model, there will probably come a time when you can't find the perfect gear—but we're fortunate to live in a time where the options from most major players work very, very well.
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.
Speeds are based on our most common configuration: 700C wheels with 40mm (as measured) tires.
Last update: December 12, 2019
Want to go even deeper into technical gearing specs? We did even more math. Read our article, There's No Free Lunch in Gearing.