Gravel Bike Tech: Hacking SRAM Force/Red/Eagle AXS drivetrains

When SRAM launched their new electronic Force & Red road groups and Eagle mountain bike group, an additional moniker appeared: AXS. It’s pronounced “access” and it is SRAM’s way of indicating that these parts can all talk to each other.

There are also additional AXS shift buttons, an AXS dropper post, and who knows what else may follow (AXS lighting comes to mind).

Why would you care as a gravel rider? Well, if you’re like me, it probably went something like this:

“SRAM brings out new electronic road groupsets, with 1x and 2x options.” – “Yes!”
That’s great, because the majority of new high-end gravel bikes are spec’d 1x, but until now that meant mechanical shifting or a cobbled-together Di2 setup mixing road and MTB parts that Shimano doesn’t want you to mix. So every firmware update is a potential game-over for your shifting.

“These new groups are 12-speed.” “Yes!”
12-speed for a 2x gravel drivetrain is no big deal. 2×11 gives you more gears than you need, 2×12 gives you “more more” gears than you need. But for 1x, there are riding locations and riding styles where 11 speeds simply aren’t enough and that 12th cog makes all the difference.

“The biggest cassette for the road groups is a 10-33.” “Noooooooooo!”
That’s pretty much the exact sound we made when SRAM invited us for a preview 9 months before the AXS launch. We begged and pleaded for a bigger cassette, but that simply was not in the cards given the available timeframe.

That maximum 330% range is OK for a road bike, but the mixed surfaces and often tougher climbs you encounter on a gravel bike simply require more range.

Pure SRAM Force or Red 1×12 drivetrains don’t work for gravel. The gear range is useless.

If you already ride a 1×11 drivetrain you know this, since you probably use an 11-42 or 10-42 cassette. Going from that to a 10-33 is like throwing away the biggest two cogs! Nobody who rides gravel thinks “I don’t need my two biggest cogs”.

“But there are third-party cassettes like Rotor’s 11-39T”
Correct, and that’s a great cassette, but the Force AXS & Red AXS rear derailleurs can’t handle those cassettes. There is very little margin to go bigger than that 33T with these derailleurs.

“But we have AXS now.” “Yessssssssss!”
That’s where AXS comes to the rescue, if you do it right. Because of AXS, road shifters can communicate with mountain bike rear derailleurs. And mountainbike rear derailleurs can shift mountain bike cassettes, all the way up to 10-50.

So what do you need to make these mixed drivetrains work?
The AXS parts may be able to talk to each other, that doesn’t mean everything works together. You need to take care of a few things for a successful mixed AXS setup or as it is known internally at SRAM: the Mullet (business up front, party in the back).

You have to use an Eagle chain, a Flattop road chain will NOT work!

• Shifters: Force or Red AXS, whichever your budget allows for. They are both great, my preference is the Force shifter for superior value-for-money.
• Flatmount brakes: Obviously the world has gone flat mount (don’t get me started). Normally the brakes are packaged with the shifters, so unless you put in extra effort, you’ll get Red brakes with your Red shifters or Force brakes with your Force shifters.
• Postmount brakes: Strangely, you won’t find these on SRAM’s own website, but the Red AXS group also includes a brand-new postmount brake caliper. So instead of using brackets, choose these if you still ride a postmount frame. Just search for “Red AXS postmount caliper” and you’ll find stores offering them. See below flatmount (left) vs postmount (right).

  

• Rear derailleur: The Eagle AXS rear derailleur comes in two flavors: XO1 and XX1. The latter is the top of the range, with a carbon cage, while the XO1 sits just below it with an Aluminum cage. In the end, they are quite close in performance and weight. But they are also close in price, so you may want to opt for the XX1. I did.
• Chain: This is the most important part. The Flattop road chain does NOT work in combination with the Eagle AXS rear derailleur. You have to use an Eagle chain. The reason is that the Eagle derailleur has standard pulleys, but the Flattop can only wrap around a pulley with teeth in one direction (the flat size doesn’t have enough space for full teeth. SRAM road AXS rear derailleurs have a special upper pulley to deal with the Flattop chain, but Eagle rear derailleurs do not.

• Cassette for gravel use: The whole point of the mixed road/MTB drivetrain is to get a bigger cassette than 10-33T. We have tested every combination in the lab and on the road, below are what we recommend:
  • Eagle 10-50T cassette. The advantages are that it is light and offers a huge range, the disadvantage is that the range is really too big for most gravel applications. It’s great for MTB, but in gravel you could use a bit less (which is why the 10-42T is such a great cassette for 11-speed mechanical groupsets, that 420% range is magic).
  • Eagle 11-50T cassette. In my opinion this is a better cassette for most people. The smaller range means some of the steps are a bit smaller. On the downside, this cassette is not available in SRAM’s highest level, so the lightest 11-50T cassette is 250g heavier than the lightest 10-50T cassette.
  • Rotor’s 12-speed 11-46T cassette. This is a great hack of the system with a 418% range, so basically the same as that magical 420%. And with an extra cog so the steps are smaller. It shifts flawlessly with a SRAM Eagle AXS rear derailleur and chain.
  • Rotor’s 12-speed 11-39T cassette. Yes, this one also works with the Eagle AXS rear derailleur and Eagle chain! The range is small, but bigger than the 10-33T (355% vs 330%). It’s great for riders (I happens to be one) who don’t care about their top gear because they are a relaxed descender. You can use this cassette together with a small chainring (maybe 36T, depending on your strength and terrain) and enjoy very nice, small steps between the gears.
  • In the end, which cassette works best for you on gravel depends on where and how you ride. The 11-50 is a good middle ground, so we spec it on our Force-Eagle complete bikes. Be sure to always set the limit screws and B-tension correctly for the chosen cassette.
• Cassette for road use: If you also want to use your gravel bike for serious road riding/racing, Rotor’s 12-speed 11-39T is great, and again the small steps will be appreciated. Just combine it with a bigger ring than you would for gravel, so maybe something in the  40T to 46T range
• Cranks: There are a few options here:
  • The new SRAM road cranks come with DUB axles, with a 28.99mm instead of 30mm diameter. Most frames fit either axle size so who cares, but it means yet another standard. Other than that, the cranks are fine, except the chainline is pretty low at 45.0mm, so it favors the small gears over the big ones. I prefer a chainline around 47mm but this is not a crucial difference. No major difference between Red and Force. Both models come with chainrings optimized for Flattop chains, but they work perfectly well with the Eagle chain as well.
  • You can also opt for the power meter version. Luckily for 1x customers, the meter is separate from the rings (except for the 48T and 50T rings, but those are too big for gravel anyway). So that means you can swap out your ring without having to buy a new powermeter (It’s incredible to me that for 2x cranks, the power meter is permanently attached to the chainrings. So if you need new rings, you also need a new power meter).
  • Last but not least, you can also hack the groupset towards an even lighter and faster setup by using 3T’s very own Torno crank. It is designed specifically for 1x and tested to withstand the rigors of gravel riding. Because the mixed setup uses the Eagle chainring, all five chainring options offered with the Torno (36, 38, 40, 42, 44T) will work flawlessly. You can see it installed below (and in the action photo near the top of this article).

• Chainrings: Which size do you need depends on which cassette you use, where you ride and how strong you are. The most popular chainring with the 11-speed 10-42 cassette is the 38T. Second most popular is the 40T and third is the 42T. Keeping that in mind, I would recommend the following for the three main cassette options for mixed AXS drivetrains:
  • With 10-50, I would recommend to use the same 38, 40 or 42T rings as before. So 38T for the toughest terrain and the least powerful riders, 42T for the strongest riders. Nobody needs more than 42×10 for gravel, taking into account where you ride and the fact that a gravel tire has a slightly bigger rollout than a road tire. That then leaves 42×50 as your smallest gear, small enough to ride up the side of a building.
  • With the 11-50, you can scale up your chainring a bit. To keep the same top gear, you would need a 42, 44 or 46T chainring respectively, but some might decide to “split the difference” and get some extra bottom gear, so that would mean going with a 40, 42 or 44T chainring.
  • With Rotor’s 11-46, you would want to get the same gears as on a 10-42, which means you need to scale up your rings 10%. So that gets you to a 42T ring for the least powerful riders and a 46T ring for the strongest riders.

Some final thoughts
Yes, I know some manufacturers spec gravel bikes with that 10-33 cassette despite what I write above. That doesn’t change the fact it doesn’t work.

Of course everybody is entitled to their opinion, but the real problem is that once you’ve gone down that road, fixing it gets REALLY expensive for the customer. As you’ve seen above, it requires not just a new cassette but also a new rear derailleur and chain, so roughly 800 USD/Euro.

If you have any comments or questions, or would like to suggest topics for future episodes of Gravel Tech, let me know below. If you’re interested in a gravel bike with Force/Eagle eTap AXS drivetrain, check out the Exploro Force/Eagle and the Exploro Force/Eagle/Torno.

More a Shimano fan? you can read my thoughts about Shimano’s GRX gravel drivetrain here.