For the modification of the Fnhon folding bike from 11 speed to 12 speed, I will be using the SRAM AXS drivetrain. One quirk of that drivetrain is the unique AXS Flattop 12 speed chain, which is not compatible to any other drivetrain.
Although SRAM does have its own 12 speed MTB drivetrain, it uses a different chain from the road/gravel 12 speed drivetrain. From what I understand, the Flattop chain has larger chain rollers, which means that it will not fit onto any of the existing chain rings or cassettes, which are matched to smaller rollers. In fact, roller size have not changed for a long time, even though the chains are getting narrower as we go to more speeds. For chains with 11 speed and below, they are mostly cross-compatible across manufacturers, even though performance may differ slightly.
Therefore, I had to get a new chain ring for this 12 speed conversion, even though I already have a few narrow wide chain rings available. I first used a Wolf Tooth narrow wide chain ring 5 years ago, and my most recent one was the cheaper Stone narrow wide chain ring.
I find both brands make good narrow wide chain rings, and both of them do make new narrow wide chain rings that are specifically stated to be compatible to the SRAM Flattop 12 speed chain. However, there was not a lot of good information on the Stone 12 speed chain ring, which was why I decided to get the Wolf Tooth 12 speed chain ring instead. Also, the Wolf Tooth version is apparently backward compatible to older chains as well.
In this post, I will do a comparison of the chain rings and chains, between 11 speed and 12 speed versions. Note that measurements will be off by a little bit, as the proper measurement tool to use would be a micrometer instead of a vernier caliper that I have.
Old 48T Drop Stop chain ring, vs the new 44T Drop Stop B chain ring.
Even with a side by side comparison, I couldn't really tell a difference in the teeth profile just by looking at it.
The new profile (in front) apparently has a deeper valley and a sharper tooth to match the 12 speed Flattop chain, which has larger rollers and a narrower inner width.
The narrow teeth on the new chain ring (right) has a sharper point than the older version.
Old tooth profile is around 4.64 mm from front to back.
New tooth profile is about 0.1 mm smaller. However, this measurement may not be accurate as there is no clear measurement datum on the tooth profile.
Thickness of older narrow tooth profile, 1.95 mm.
Thickness of new narrow tooth profile is noticeably thinner, at 1.7 mm. This is to match the new 12 speed chain with a narrower inner width.
Thickness of old profile wide tooth is 3.64 mm.
Thickness of new profile wide tooth is 3.34 mm. This also means lower tooth durability.
For the chain rings, all I can compare are the thicknesses of the narrow and wide tooth profiles. Comparing the old to new profile, the new profile has a 0.25 mm thinner narrow tooth profile, and a 0.3 mm thinner wide tooth profile. As it is thinner, it is compatible to older chains that are wider. However, it also means less chain stability as the chain has more space to move laterally on the chain ring teeth.
Next, let's compare the chains. The difference in chain dimensions is why a new chain ring was required.
SRAM Force AXS Flattop 12 speed chain, vs the Shimano Dura-Ace 11 speed chain. Comparison will be based on dimensions only, not other factors.
Putting the 11 speed chain on the old Wolf Tooth chain ring gives just a little bit of free play. Good for chain retention.
Putting the 11 speed chain on the new chain ring, with narrower teeth means there will be more free play. It is compatible, but chain retention ability might drop a bit.
In this case, the new chain ring is compatible to both older 9/10/11 speed and also the new 12 speed Flattop chain, as claimed by Wolf Tooth. It is NOT compatible to Shimano 12 speed chain.
However, the reverse is not true, as you can see below. The older chain ring is NOT compatible to the new 12 speed Flattop chain.
New 12 speed Flattop chain on the old chain ring. As the chain rollers are larger, they are not able to rest properly on the chain ring teeth. Completely incompatible.
12 speed Flattop chain on the new chain ring, fits perfectly as it should. This is why a new chain ring is needed for the Flattop chain.
Flattop chain rests nicely on the new chain ring. The chain link plates look really thin here!
Diameter of Shimano 11 speed chain roller is 7.65 mm, which is the same as almost every other chain.
Diameter of SRAM 12 speed Flattop chain roller is larger at 7.9 mm. This is why it does not fit properly onto existing chain rings or cassettes.
External width of Shimano 11 speed chain is 5.62 mm. This is the width that gets narrower with more speeds.
External width of SRAM 12 speed Flattop chain is even narrower, at just 5.05 mm!
Internal width of Shimano 11 speed chain, between outer plates is 4.06 mm.
Internal width of SRAM 12 speed Flattop chain, between outer plates is 3.68 mm, almost 0.4 mm narrower. This does not really fit onto the old profile wide tooth which is 3.64 mm as shown earlier.
Internal width of Shimano 11 speed chain, between inner plates is 2.32 mm. This is the same across most multi-speed chains, from 9 to 11 speed.
Internal width of SRAM 12 speed Flattop chain is slightly narrower between inner plates, at 2.21 mm.
Thickness of Shimano 11 speed chain outer plate is 0.78 mm. The plate thickness decreases as the chain goes to more speeds.
Thickness of SRAM 12 speed Flattop chain outer plate is even thinner at 0.65 mm.
Thickness of Shimano 11 speed chain inner plate is 0.73 mm.
Thickness of SRAM 12 speed Flattop chain inner plate is really thin, at just 0.58 mm!
With a comparison table, all the chain dimensions can be seen clearly. Once again, note that these dimensions are based on my own measurements, in which there will be a small error. Also, there are manufacturing tolerances, so this is by no means a standard dimension.
1) Just a small 3.3% larger chain roller makes it completely incompatible to older chain rings, as the difference accumulates quickly across the chain links.
2) 12 speed Flattop inner chain width is reduced slightly, which necessitates narrower chain ring teeth as well.
3) Chain link plate thickness of the 12 speed Flattop chain is really thin, which is why they needed more material on the plates for strength, giving it the distinctive Flattop shape.
Side Note: The Campagnolo Ekar 13 speed chain is even narrower at 4.9 mm!
I think this is a pretty good comparison showing how different this new AXS 12 speed Flattop chain is, compared to other chains. There will be serious compatibility issues if you try to use this new chain on older chain rings or cassettes.
Now that the Shimano 12 speed road groupsets are available, there is an opportunity to mix and match components. However, the Shimano 12 speed chain (common use for road and MTB) is also unique, as it has longer inner plates with a large chamfer, which introduces other compatibility issues. I might have a chance to do some 12 speed cross-compatibility checking in the future, when I get my hands on some new 12 speed components.