Technical FAQ: Compatibility of 12-speed components

Published September 6, 2022 01:22PM

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Shimano recently updated its compatibility chart . In regards to the latest generation of calipers shipping with 12-speed groupsets (i.e. BR-R9270, BR-R8170, BR-R7170), the previous generation Ultegra, DA, 105, GRX STI’s are compatible with new calipers (but new STI’s aren’t compatible with the old calipers). My question: although the new calipers are compatible with the previous generation STI’s, will one actually get the enhanced pad clearance functionality with this combination? This would seem to indicate that the enhanced pad clearance had nothing to do with the action of the STI, but then why wouldn’t they be compatible in the other direction (i.e. old Calipers, new STI’s)?

As we all deal with continuing availability of components, understanding cross compatibility is critical!

“Increased pad clearance of the new braking system is a product of the caliper design where the pads are actually set 10% wider apart than the previous version. The quick pad engagement of the latest system is a product of the brake system as a whole, so both the new caliper design as well as the new brake lever.

In the scenario where someone would mix the new brake lever that features SERVOWAVE technology with the previous generation caliper that actually has a larger piston size, the system would generate an undesirable power curve and would likely cause control problems for the rider. Therefore, they are incompatible.

On the other hand, using the previous generation brake lever without SERVOWAVE with the new caliper design with 10% more pad clearance does in fact work because the pistons in the new caliper are a smaller diameter. Using this combination will create more free stroke in the lever action and ultimately does not produce quite as much maximum braking force, but the braking performance will still be smooth and controlled.

Regardless of compatibility, Shimano recommends using the same generation brake lever and caliper for the best available braking performance.”

Now that you have confirmed that the gear/chain pitch is the same, what would be the real-world consequences of running a 12-speed SRAM cassette — with a 10-tooth cog — on an otherwise 100% Shimano setup? Reasons could include, for example, the rider thinking the Shimano brakes are better, or the same for the Shimano hoods, power meter, eco-system, etc.

To get the benefit of a 52 Shimano large ring with a SRAM 10-tooth cog — which is a bigger gear than a 100% Shimano 54/11 but with less cadence jump between gears, the best of both worlds — would it be better to run the SRAM chain, or the Shimano chain?

Would shifting be completely off, or close enough to be manageable? Would the chain slip under full sprint power? How much would drivetrain durability be impacted?

Lastly, have you or anyone you are aware of actually tried it? I think it would be the perfect setup.

I have not tried this. I know a Flattop chain will skip on any cassette other than AXS, because the big rollers won’t drop into the valleys (this I have experienced). Whether a non-AXS 12-speed chain will run on an AXS cassette is another question. While it might seem that, since the teeth are still the same spacing apart, the standard-diameter rollers of the chain will roll further forward in the larger AXS tooth valleys yet perhaps still hold firmly on the teeth, the chain width could be an issue.

From the SRAM response last week, it is apparent that the spacing between cogs in an AXS cassette is less than on other 12-speed cassettes; that was the reason for making the chain plates so thin, thus mandating the taller plates and rollers. Again, I have not tried it, but based on that response, it would seem that, first of all, a non-AXS would drag on the adjacent larger cog. Secondly, if SRAM is fitting the 12 cogs of AXS road (and XPLR) drivetrains into a narrower space than other 12-speed cassettes (as that answer indicates), then the Shimano 12-speed derailleur might not line up with each cog, because each lateral step from cog to cog would be smaller in the AXS cassette than the steps the Shimano derailleur is taking.

I just measured the width of a SRAM Flattop chain at the chain pins with a digital caliper, and it is 5.0mm. A SRAM Eagle MTB chain is nominally 5.25mm wide.

This would, I think, answer your question of which chain to use — the Flattop chain would be necessary for not only optimal tooth engagement in the AXS cassette but also for not dragging on the adjacent cogs.

This might indicate that each cog in an AXS cassette is 0.25mm closer to the next cog than in a Shimano or SRAM Eagle 12-speed cassette (due to the differences in diameter, it is not easy to measure the spacing between cogs in a cassette, just as it is a challenge to measure the spacing between front chainrings). If so, then multiplying that 0.25mm difference by the 11 spaces between cogs would make the entire AXS cassette 2.75mm narrower than other 12-speed cassettes. If that’s true, that’s enough difference to ensure that the Shimano 12-speed derailleur won’t line up over every cog across the AXS cassette.

Finally, the Flattop chain will not drop down fully into the tooth valleys in Shimano front chainrings. While perhaps there is sufficient tooth engagement that chain skipping is not a risk, it does at least mean that the chain is following a larger-diameter circle, meaning that your effective gear size has increased.

Your reply to Joey from Kansas concerning gearing his Red 22 with lower gears for riding the Colorado mountains was right (of course) as far as directly answering his question — no, he can’t replace his rear derailleur with a new 12-speed one and expect everything to play well together. However, SRAM did make an 11-speed eTap WiFli rear derailleur that would definitely handle a 32-tooth rear cog and maybe even a 34. There are still a few of these out there new from retailers and one could probably find a good used one on Ebay. The Red eTap version is not cheap but may be an adequate solution, and he wouldn’t have to make the leap to 12-speed AXS.