Red AXS dérailleur jumping/hunting problem

[tootall]

I have a Red AXS rear derailleur on my road bike giving me some issues. I’ve had the bike since Jan 1 and put 2,000 miles on it. The problem has just recently started. The issue is every once in a while, on an inboard (down) shift, the derailleur will do like a double take/fake shift. It shifts, comes back, and then completes the shift. This causes the chain to move to the target cog and then immediately jump back to the previous cog, before finally moving on to the target cog for good. I’m confident it’s not an alignment, limit screw, trim, or b-gap issue, because I’ve triple checked all those things and it shifts perfectly fine otherwise. I’ve tried freshly charged batteries and multiple batteries with the same results. I just updated firmware and same results. If you watch the video you can see the derailleur move back and forth on a single shift attempt the first and last shifts, even when I’m not pedaling. Anybody seen this before and have any ideas? I’ve done some searching online and found a similar problem reported on the WeightWeenie’s forum but no good solutions.


https://youtube.com/shorts/MuAGob8Fy...tZQHan_xgxHMXV

[bikerboy337]

My understanding is that when not pedaling it will do that to avoid motor burnout. When you shift and don’t pedal, it’s stressing the rear derailleur as it’s an electronic moto…. then moves to the previous gear to avoid damaging the motor….does it do that when you’re pedaling

My initial guessing bad shifting if all else is set properly is worn chain or cassette? But your video is what’s supposed to happen to keep the derailleur safe…

[tootall]

Originally Posted by bikerboy337

….does it do that when you’re pedaling

It does happen while pedaling, which is when I notice the chain jumping back and forth between cogs. I couldn't figure out what was happening while riding, I kept thinking it was a trim issue, so I was working on trim in the stand (pedaling the bike) when I was finally able to see the derailleur jumping back and forth. I couldn't pedal AND shift AND take the video at the same time but the jumping back and forth behavior I saw pedaling in the stand is the same as that in the video.

The chain and cassette were both brand new on 4/29 and have 260 miles on them so they can't be worn. I did start noticing the problem only after replacing them so I at first assumed it was an alignment, limit screw, trim, or b-gap issue, but as I said I've checked those to death and they're all good. I keep coming back to the new chain/cassette as the only change before the problem started though. The B-gap is set per Sram's recommendations (12mm gap with a 36t max Red derailleur and a 10-28 cassette), but maybe it's too tight for the motor so it's binding and protecting itself? I'll check that again again and maybe open it up some to see if the problem stops.

[ChainNoise]

Stupid question but KISS...Is the cassette lock ring tightened?

[tootall]

Originally Posted by ChainNoise

Stupid question but KISS...Is the cassette lock ring tightened?

I agree with the KISS thoughts, it's an XDR cassette so there's no lockring per se, but I'll double check that the cassette is tight.

On the other hand, I don't know why a loose lockring or thru-axle (which I know is tight) would make the derailleur jump back and forth unless it was contributing to the derailleur feeling like it was binding as bikerboy noted.

[eddief]

I know you mentioned trim but not micro adjust. Maybe the same thing here. But on my 1x AXS if it is acting up on the cassette, while riding I can micro adjust to fix fiddly things as you mention. Have you tried that?

Next thing would be chain wear. Worn chain often equals shifty shifting.

[tootall]

Originally Posted by eddief

I know you mentioned trim but not micro adjust. Maybe the same thing here. But on my 1x AXS if it is acting up on the cassette, while riding I can micro adjust to fix fiddly things as you mention. Have you tried that?

Next thing would be chain wear. Worn chain often equals shifty shifting.

Yeah, I was referring to micro adjust when I was saying trim. I would think if the micro adjust was off though it would shift poorly across the cassette, rather than just do the pumpfake thing occasionally. But either way I’ve adjusted it thoroughly and found the definite sweet spot at 18.

Chain and cassette were new together 260 miles ago so can’t be chain wear.

[eddief]

my last micro adjust issue was just on one cog.

[Dave]

There is no cassette lock ring with AXS. Assuming that a RD hanger alignment tool was used, that really only leaves micro adjusting. The adjustment affects all sprockets, not just one. Videos of the RD movement without turning the crank are really stupid. No RD will shift a stationary chain. If a shift pushes the chain to a larger than desired sprocket, thenback to the intended smaller sprocket, you need a micro adjust on the right lever. Try two clicks to see if the problem changes to the opposite direction.

[tootall]

Originally Posted by Dave

Videos of the RD movement without turning the crank are really stupid. No RD will shift a stationary chain.

Since like most people I only have 2 hands, I couldn't turn the pedals AND activate the shifter AND hold the phone to video all at the same time. When the problem presents itself while pedaling, the derailleur behavior is EXACTLY the same as shown in the video. The derailleur starts inward, bumps back out, then completes the move inboard.

If a shift pushes the chain to a larger than desired sprocket, thenback to the intended smaller sprocket, you need a micro adjust on the right lever. Try two clicks to see if the problem changes to the opposite direction.

The shift never pushes the chain to a larger than desired sprocket. It moves from the current sprocket onto the target sprocket (1 larger), then when the derailleur bumps back out it moves back to the original sprocket, then when the derailleur completes the shift the chain follows and lands on the target sprocket. This is when pedaling/riding.

Bottom line is if the derailleur doesn't do the double pump move, the bike shifts perfectly, up and down, across the cassette. It runs quiet. So the question is what's causing the double pump move? I maintain that I don't think it's micro adjust, it's set good; if I move it either way the chain starts getting noisy, the shift quality decreases and the double pump doesn't stop.

My best guess after reading bikerboy's comment (which I didn't know about before) is that I may actually have the b-gap too tight so it's binding the derailleur a bit too much. I measured the gap with a steel rule and set it per Sram's recommendations but maybe my measurement wasn't accurate enough. I've got a b-gap measuring tool on the way and I'm going to open the gap a bit before tonight's ride to see if the double pump stops.

[Dave]

The b gap isn't that critical, IMO. 12mm is a lot of gap, but it should only affect sprockets at the two extremes. I've run 10-36 cassettes with 33T capacity RDs and only 3mm of gap. Too much gap can cause the shifts to hesitate in the 10-11-12 range. It shouldn't affect the others a bit.

I always recommend trying a different RD. If the problem goes away, you may have a defective RD.

[eddief]

put it on your work bench and watched how is shifted off the bike? I wonder if you could learn anything from doing that?

[tootall]

Originally Posted by eddief

put it on your work bench and watched how is shifted off the bike? I wonder if you could learn anything from doing that?

This is a good idea.

[eddief]

is to make sure that the the stop on the derailleur is situated on the correct ledge of the derailleur hanger. I have managed to mount the derailleur but have that little thingy not put in the the right place.

[tootall]

After the suggestion to test it on the bench, I took the chain off for a cleaning and tried the derailleur. It hunts worse! So either my derailleur is messed up, or that’s natural behavior when there’s no chain as the derailleur freaks out with no tension on it. See below. Pumps 7/8 times on one of the shifts. Would like to know if this is normal, but I don’t have any other AXS derailleurs laying around and no shops within an hour of here.

https://youtube.com/shorts/5mTr_S-lJ...TbQPlLig2oPzDR