Tech FAQ: 12 Speed Component Compatibility
This article originally appeared on Velo News
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Shimano recently updated their compatibility chart. As for the latest generation of calipers that came with 12-speed groupsets (i.e. BR-R9270, BR-R8170, BR-R7170), the previous generation Ultegra, DA, 105, GRX STI is compatible with new calipers (but new STI are not compatible with old calipers). My question: Although the new calipers are compatible with the previous generation STIs, will we actually get the improved pad clearance functionality with this combination? This would seem to indicate that the improved pad clearance had nothing to do with the action of the STI, but then why wouldn’t they be compatible the other way round (i.e. old calipers, new ITS)?
As we all deal with the continuous availability of components, understanding cross compatibility is essential!
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Here is Shimano’s official response:
“The increased pad clearance of the new braking system is a product of the caliper design where the pads are actually spaced 10% wider than the previous version. The latest system’s quick pad engagement 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 mixes up the new brake lever that has SERVOWAVE technology with the previous generation caliper that actually has a larger piston size, the system would generate an undesirable power curve and likely cause issues. control for the cyclist. 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 actually works because the pistons of the new caliper have a smaller diameter. Using this combination will create more free travel in the lever action and ultimately won’t produce as much peak braking force, but braking performance will still be smooth and controlled.
Regardless of compatibility, Shimano recommends using the same generation brake lever and caliper for the best braking performance available.
Now that you’ve confirmed that the gear/chain pitch is the same, what would be the actual consequences of using a 12-speed SRAM cassette – with a 10-tooth cog – on an otherwise 100 setup? %Shimano? Reasons could include, for example, the rider thinking that Shimano brakes are better, or the same for Shimano hoods, power meter, ecosystem, etc.
To benefit from a large Shimano 52 ring with a 10-tooth SRAM cog – which is a larger gear than a 100% Shimano 54/11 but with less cadence jump between gears, the best of both worlds – would be it is better to run the SRAM chain, or the Shimano chain?
Would the shifting be completely off or close enough to be manageable? Would the chain slip at full sprint power? How would the durability of the transmission be affected?
Finally, have you or someone you know actually tried it? I think this would be the perfect setup.
I haven’t tried that. I know a Flattop chain will jump on any cassette other than AXS, because the big rollers won’t drop in the valleys (I’ve experienced that). Whether a non-AXS 12-speed chain will work on an AXS cassette is another question. Although it may seem that since the teeth are always the same spacing, the chain’s standard diameter rollers will ride further forward in the larger AXS tooth valleys while perhaps still holding firmly onto the teeth, the String width could be an issue.
From the SRAM response last week, it appears that the cog spacing in an AXS cassette is less than other 12-speed cassettes; this was the reason the chain plates were so thin, thus imposing higher plates and rollers. Again, I haven’t tried it, but based on this answer, it would seem that, first of all, a non-AXS would drag on the larger adjacent cog. Second, if SRAM installs the 12 cogs of AXS (and XPLR) road drivetrains in a tighter space than other 12-speed cassettes (as this answer indicates), the Shimano 12-speed derailleur may not align with each sprocket, because each lateral pitch from sprocket to sprocket would be smaller in the AXS cassette than the Shimano derailleur pitches.
I just measured the width of a SRAM Flattop chain at the chain pins with a digital caliper, and it’s 5.0mm. A SRAM Eagle mountain bike chain has a nominal width of 5.25mm.
That would, I think, answer your question about which chain to use – the Flattop chain would be needed not only for optimal tooth engagement in the AXS cassette, but also so that it doesn’t drag on adjacent sprockets.
This could indicate that each sprocket in an AXS cassette is 0.25mm closer to the next sprocket than in a Shimano or SRAM Eagle 12 speed cassette (due to diameter differences it is not easy to measure spacing between the sprockets of a cassette, just as it is difficult to measure the spacing between the front chainrings). If so, multiplying that 0.25mm difference by the 11 cog spaces would make the entire AXS cassette 2.75mm narrower than other 12-speed cassettes. If true, that’s enough of a difference to ensure that the Shimano 12-speed derailleur won’t line up with every cog on the AXS cassette.
Finally, the Flattop chain will not go all the way down into the tooth valleys of Shimano front chainrings. While there may be enough tooth engagement that chain skipping isn’t a risk, it does at least mean that the chain follows a larger diameter circle, which means the effective size of your gear has increased.
Your response to Joey from Kansas regarding gearing his Red 22 with lower gears for riding in the Colorado mountains was correct (of course) regarding the direct answer to his question – no he can’t replace his rear derailleur by a new 12-speed one and expect everything to go well together. However, SRAM made an eTap WiFli 11-speed rear derailleur that would definitely handle a 32-tooth rear cog and maybe even a 34. There are still a few new ones at retailers and one could probably find one good used on ebay. The Red eTap version isn’t cheap but can be an adequate solution, and it wouldn’t have to make the jump to the 12-speed AXS.
This is a very good solution; Thanks for that.
Lennard Zinn, our longtime tech writer, joined VeloNews in 1987. He is also a custom frame builder and supplier of huge non-custom bikes , a former U.S. National Team rider, co-author of “The broken heart” and author of numerous books on cycling, including “Zinn and the art of road bike maintenance,” “DVDas good as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Introduction to Cycling: Maintenance Tips and Skill Building for Cyclists.”
He holds a bachelor’s degree in physics from Colorado College.
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