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Technical FAQ: AXS chain roller size explained, AXS/11-speed compatibility, wires in tires

Zinn dives into SRAM electronic groupset inter-compatibility and tries to get to the bottom of one of the most frustrating sources of flats.

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

This report filed August 30, 2022

Dear Lennard,

I’d like to follow-up on this discussion that you so graciously published on the website. So, my follow-up questions are:

  • Besides the larger rollers of the AXS chain, is the gear/chain pitch different?
  • Why are the rollers larger? Is it to get more “grip” on the 10-tooth cog?
  • Ultimately, what matters to me is this (related to question 1): Is the roll-out/gear inches of a 12-speed SRAM the same as a Shimano 12-speed in, say, a 50-11 gear combination?

Nigel

Dear Nigel,

No, the chain pitch is the same.

Here’s the skinny on the Flattop chain roller diameter from SRAM’s road PR technical communications specialist, Brook Fowler:

“Our road drivetrain squeezes 12 cogs in slightly less space than our MTB drivetrain; we found that space by thinning the plates on the Flattop chain slightly while almost maintaining inner width dimensions (which helps maintain cog thickness and contact stresses on the teeth).  The takeaway is the Flattop chain is narrower on the outer dimension and pretty much the same on the inner dimension — this is achieved with thinner plates that were made slightly taller to maintain cross-section area. The taller link dimension is what actually drives the larger roller diameter of the Flattop chain.”

Yes, rollout is the same.

― Lennard

Also read: Technical FAQ: Steel wires in tires, thru-axles, 10/11-speed upgrade, cleat fore-aft

Dear Lennard,

Long time reader and learner from Kansas but who is eager to tackle some of Colorado’s climbs. As such, I’m looking for every opportunity to lower my gearing. I currently have Sram Red eTap 11-speed with the short cage rear derailleur (28T max cassette).

In addition to downsizing my chainrings to a 50/34, is it possible to swap my rear derailleur for the newer Force or Rival AXS eTap with 33T max? Will my older 11-speed shifters communicate with the new stuff?

Joey

Dear Joey,

Again, SRAM’s road PR technical communications specialist, Brook Fowler has been kind enough to answer this question as well, saying: “The 11-speed eTap shifters will not work with the 12-speed AXS derailleurs. Additionally, the first generation of RED eTap front and rear derailleurs are mechanically designed for 11-speed gearing and are not compatible with 12-speed chains, cassettes, and cranksets. Mixing and matching 11-speed and 12-speed drivetrain components (derailleurs, cassettes, chains, chainrings) are not mechanically compatible.

However, some of the 12-speed AXS shifters are backwards compatible with 11-speed eTap derailleurs. SRAM RED and Force eTap AXS shift levers or the eTap AXS BlipBox with firmware version 2.29.4 or later can be paired with RED eTap 11-speed front and rear derailleurs using the eTap pairing process. Rival eTap AXS shift-brake levers are not compatible with 11-speed eTap derailleurs due to differences in their electronic hardware and do not support this firmware feature.”

If your foray westward into our Colorado mountains proves too much with a 34 X 28 low gear, then you might consider using a Road Link to get more gear capacity. You will need a longer chain, of course, along with the Road Link and the larger cassette.

― Lennard

Dear Lennard,

I have a follow up question on the letter about thin steel wires in tires. I’ve had many flats caused by these also. My question is how can these tiny thin wires be strong and stiff enough to penetrate a tire and a tube to cause a flat? If somehow they were sticking straight up in the road it seems they would just be crushed. It seems more likely they would be lying flat which would make it nearly impossible to then penetrate the tire. Any thoughts?

Mark

Dear Mark,

Well, that’s a good question. My guess is that you do regularly ride over thousands of these wires that are lying flat, and that it is a rare one that is somehow oriented in such a way that it pokes into one of your tires. It would be interesting to, with a magnet, go through the contents of what a street sweeper pulls up off of the road. I would be surprised if the magnet didn’t get immediately furry with countless little pieces of steel wire, meaning that there are so many that you constantly ride over that even a low-probability occurrence like one standing up can still occur frequently due to the sheer number of them.

If that were to turn out to be true, then the question is, what would the conditions have to be for the occasional one to give you a flat? Perhaps every now and then one drops into a space alongside one of the gravel chunks in the asphalt and ends up sticking straight up. Where the car tires are rolling, I imagine you are correct that vertical wires would be crushed (or picked up by car tires). On the shoulders, though, they would tend to sit there for awhile. Around here, when we get a torrential downpour that creates rivers of sediment depositing sand and gravel on the road, the sand and gravel ridges quickly disappear from the car-tire paths, but they stay on the shoulders perhaps indefinitely until a street sweeper remove them. So those wires could sit on road shoulders a long time, un-crushed, awaiting your bike tires.

When I used to build steel, fillet-brazed frames, I would rough out the shape of the brass fillets with a flame-shaped carbide burr bit. I also used to shape the ends of the chainstays and seatstays where I had brazed them to the dropouts using the same tools. This produced millions of tiny, sharp slivers of brass and steel on the floor that sweeping was not 100% effective at removing. Even though the slivers were lying flat, you could not walk barefoot in the shop without getting some sticking straight up into your feet. And the floor was very smooth concrete, as opposed to the rough surface of an asphalt or concrete road.

I suppose the movement of the surface of the skin as it pushed off from the floor to take the next step might have stood some of the splinters up to the point that they stick into the foot. If that’s the case, perhaps the flexing of the rubber tread as it rolls along could also pull the occasional wire into a vertical position, letting it stay standing up on the tire as that section of rubber leaves the road and goes around and back to contacting the road again, pushing that wire straight in each time it hits bottom. Interesting to think about; all just conjecture.

― Lennard


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also 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