Technical FAQ: SRAM 12-speed mullet drivetrains, unsticking a bottom bracket

How to mix and match SRAM chainrings, chains, shifters and cassettes, and also how to unfreeze a frozen bottom bracket.

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Have a question for Lennard? Please email us to be included in Technical FAQ.


Dear Lennard,
I had been reading with interest responses to questions about the SRAM AXS mullet build.

So, I noted that it says on SRAM site that you can use SRAM eTap AXS road levers and a Force eTap AXS chainset, shown with single-ring front, with an Eagle AXS rear mech and chain.

If the front ring on the road Force chainset can support the chain from the Eagle groups, how does that fit with the bigger roller size of the flat top road chain? Surely, if it were not compatible, it would not work with the road chainset?

The image here even shows the Force chainset with AXS Eagle rear mech and an Eagle chain.

Has anyone actually tested this? And then I thought, if it works with the front ring and the AXS Flattop chain, then why would it not work when you swap in the rear road cassette? Is it width or really roller?

So, how can the roller spacing be different on a road AXS chainset from the cassette? And if they are not, is it that the Eagle teeth on the AXS cassette are wider that the road cassette? What happens if you stack the Eagle and road AXS cassettes on the table next to each other?

And then I thought, has anyone tried pairing a front mech from AXS tap road, with an Eagle rear mech; does it shift?

Dear Chris,
Roller spacing is not different on any of those chains or cassettes; the parameter that is different is the increased diameter of the AXS road Flattop chain rollers. Since the AXS road chainrings are made for the bigger rollers, and since the tooth spacing is the same, the smaller rollers of the Eagle chain fit into the valleys just fine. I have tried it with a single chainring, and that definitely works.

I suspect that the issue on the rear is the number of teeth engaging the chain. I have tried the Eagle chain on the road AXS cassette, and it works in the stand and on short test rides. We built a custom bike for a guy who wanted two sets of wheels, one with road AXS 10-33 cassette, and one with an Eagle 10-50 cassette. I accidentally test rode it with the Eagle chain on with the road cassette, and it did work. I think it is not recommended due to so few teeth engaging the chain on the small cogs leading perhaps to skipping or high wear problems. This customer has to change the chain each time he changes wheels; he does not have to change the chainring.
― Lennard

Dear Lennard,
You had previously indicated that the new SRAM eTap AXS 12-speed Flattop chain is compatible with existing wide/narrow chainrings, like from Praxis Works, Wolf Tooth etc. I am currently using the Rotor QX1 oval narrow/wide 52T 110BCD chainring, and it fits really well with the SRAM Force AXS flat top chain and SRAM Force 10-33 12-speed cassette. I’ve been happily using it for the last 2 months now.

I have a question about compatibility as well: have you checked if the SRAM XO1 Eagle 12-speed chain is compatible with the SRAM Force 10-33 cassette?

Dear Arnold,
Regarding your first point, the Flattop 12-speed chain is definitely not compatible with existing road wide/narrow (X-Sync) chainrings other than, apparently, your Rotor ring. We put an 11-speed SRAM 48T, 110BCD X-Sync chainring on an eTap AXS-equipped bike we were building, and the chain immediately came off. Since it is 11-speed (SRAM does not make this size with this bolt circle diameter in 12-speed), the fat teeth are too fat to allow the chain to drop into the tooth valleys, so it derails immediately, even with a straight chainline to a gear in the middle of the cassette.

For this bike, we needed a 110mm BCD chainring for a road bike (48T) and couldn’t use SRAM 12-speed MTB X-Sync chainrings. We called the SRAM tech support line to find a solution, and SRAM has no chainring with these specs compatible with the Flattop 12-speed chain. I didn’t think about Rotor at the time, but fortunately, Wolf Tooth now offers a road 12-speed AXS-compatible wide/narrow ring—which we got—and it worked perfectly with the chain.

I have not tried the Flattop 12-speed chain on an Eagle 12-speed MTB X-Sync chainring (which aren’t available in road sizes, either on the tooth count or on the spider bolt circle pattern).

The Eagle 12-speed chain is not compatible with road SRAM AXS 12-speed cassettes, like the Force cassette you mention. That’s because the tooth valleys in AXS road cassettes are wider to fit the larger rollers of the road AXS chain. On the bike I mentioned above, the customer wanted two sets of wheels, one with a Force AXS 10-33T cassette, and the other with an Eagle X01 10-50T cassette. In order to make it work, every time the riders switches wheels, he’d also have to switch chains; he will have to readjust the b-screw on his eTap AXS electronic derailleur each time, too. He can use the same Wolf Tooth 48T chainring with both chains, employing the Eagle chain with the Eagle cassette, and the road AXS chain with the road AXS cassette.
― Lennard

Dear Readers,
For any of you who has had to deal with completely frozen-in bottom bracket cups, there is almost always a way without resorting to hacksaws or Dremel tools. Here is a story with a happy ending stitched together from a couple of back-and-forth emails with someone having such a problem:

Dear Lennard,
I had a Colnago CT1 that I recently sold. This frame has a titanium bottom bracket and the drive side cup has virtually welded itself to the bottom bracket. The mechanic has tried penetrating oil, a large pry bar, etc. to no avail. He doesn’t want to heat the frame up because the paint is nearly perfect. I thought of using a Dremel tool, carefully, but there’s a concern about the threads being damaged.

unfrozen bottom bracket
unfrozen bottom bracket
unfrozen bottom bracket

We got it out! We used the “bolt method.”

Sheldon Brown is one good resource for how to do this without frame damage, as is the video below.

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is a custom frame builder and purveyor of non-custom huge bikes, a former U.S. national team rider, co-author of The Haywire Heart, and author of many bicycle books including Zinn and the Art of Road Bike Maintenance (DVD), as well as Zinn and the Art of Triathlon Bikes and Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn

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