I have been running 11 speed Sram eTap with 53/39 chainrings and 11/25 cassette and 53 link chain length. I just built a new bike with Sram eTap utilizing 1X setup with 52 chainring and 11/28 cassette. I cut the chain for the 1X bike to 53 links, the same number of links on my 2X bike. On the 1X bike the derailleur will not shift to the 25 and 28 cogs with 53 link chain. It requires 55 link chain to shift upwards to the 25 and 28 cogs. . I am puzzled why my 1X bike requires 55 links when the gearing is similar to my 2X bike.
I assume the derailleur is adjusted properly because it shifts to the 25 and 28 cogs without the chain attached.

53t to 52t equals one Half a thread pitch shorter.
rear 3t increase equals 1.5 thread pitches longer.
basic bike-ology, 101
there are no half links (actually ONE full pitch length) for multi-speed bike chains, that I've seen.

and you count links wrong, or your bike is insanely tiny.

can i assume that you meant 106 links, and 110 links?... and you might want to go re-count now.
off bike, i count just the outer plates, then add one to account for the Quick link/Master link... then multiply by two.

Also check the chainstay length, the 1X bike might be a little longer.

Are they the exact same derailleur?

Generally, you don't go by the number of links because of the differences in chainstay length and rear derailleur design. You read the rear derailleur instructions.

You really need to go to Park Tool website and watch a good video on this.

The only time I've ever encountered someone who went by the number of links or measured length was an old-timer who used a 1" pitch Block chain.

You took away one tooth from the front and added 3 teeth in the rear and you're puzzled by why the chain needs to be longer? More teeth require more chain and chainstay length also affects the length of the chain. Never cut a chain until you have measured it on the bike.

The rear derailleur is the same. I just read the instructions from Sram for the derailleur and it says to add two extra links for a 1X setup. I will try that and see if it works.

Didn't you already say that it works with 55 (110) links? Or was that also based on the doc, not testing it?

Different frame, different chainstays - you can't expect the chain length to be the same simply because the gearing is the same. Just measure the chain against the bike you're fitting it to - big/big and add a few links, if you're not sure leave it a bit long and see how it works (easier to remove a couple of links than add them. BTW you're counting links wrong - an inner link and its neighbour outer link is two links, not one, new chains are typically 116 or 118 links.

Watch and follow instructions. Reliable source. Should clear things up for you.

That is odd that SRAM has 1x instructions for a 2x rear derailleur when they make 1x specific rear derailleurs.

Actually the instructions say for 2X systems add 1 outer and inner link and for 1X systems add 2 outer and inner links.

Yes it works with 110 links (55) but according to the Sram manual I should use 108 links. I now have done that and it works. I guess next time I should read the manual.

Yes. Chainstay lengths are the same.

Thinking about what it takes to change a chain length is easy. Look at the distances. As little as 3mm can force you to add or reduce a link. And one link usually means two, Ha. The chain length calculators just get you into the ball park. Just get it right and be satisfied, as I am sure you have done.

Of further note, all my bikes are Franken Bikes. I no longer get puzzled of the fact that even though they are set up similarly, chain length has been determined by trail and error...

I have no clue how many links are on any of my bikes because I don't need to.
I size the chain to fit the bike and toss the rest into the "extra pieces of chain that will probably not get used" pile.
Put it on the biggest ring & cog and make sure you have enough slack so the chain can ride over the teeth to the next ring/cog.



As far as chain stays the same length? That may be, but with some of the futuristic styling, where does the CS start?
Is the distance from the center of the rear axle to the center of the ring(s) the same?

Yes, "chainstay length" is generally defined by the center to center distance from the BB to the hub axle. On geometry charts, it appears that the horizontal dropouts are measured from the front location rather the back of the dropout, but that is just in the handful I've measured.

But does the OP know that?

No idea. But he does now.

You don't need to know how to measure chainstay length if you have the geo charts from the manufacturer. They all measure it the same way.

Dear OP
Ignore the usual blather of who GAF extraneous info, watch the video, put the chain on the bike, and cut the chain to fit the bike as it is currently configured. No measurements or calculations are necessary.

If you don't know what you're doing get some assistance from someone who actually does.

Have a nice day