DOS

Got a flat today on my relatively new disc wheel road bike. I am still getting used to operating the thru axle so it took me a bit of fiddling to get the axle reinstalled. Well, once I got rolling, I discovered my shifter (Shimano Ultegra r8000) which had been working fine, was jammed.The cable head had slipped out of the recessed cavity inside the shifter, and I want to make sure I know what might have caused that so I don’t do it again. I can’t figure out how I could have affected cable tension enough to dislodge the cable end while I was changing the flat..Two best guesses:

Right after I flatted, I was focused on operating the thru axle and failed to shift to the smallest cog before removing the wheel. The wheel came off easily enough but I shifted down to the small cog with the wheel off to make reinstalling easier.Could shifting without the wheel caused the cable to slip?

While I was struggling to get the thru axle to seat correctly, I pulled the wheel in and out of alignment with the drop outs a few times. In manipulating the position of the wheel, maybe I pushed the derailleur (also R8000) in some way to cause the cable upstream to slacken and cause the head to pop out of alignment? The cable routes internally so the only visible section is the small length that passes through the derailleur pinch bolt.

Editing to add new theory: The internal cable routing emerges from the frsme right at the junction of the chainstay and dropout, with housing passing rigt over the thru axle cap. In struggling with the thru axle, I am guessing I dislodged or otherwise interfered with the housing and cable, causing enough slack to make cable end in shifter pop free.

Alternative theories welcome.

andrewclaus

If you shifted up while the RD was in a lower gear with the chain not moving, the cable end could have popped out and jammed. I do this all time to inspect the cable ends in my shifters.

Retro Grouch 

Try to get into the habit of shifting into the smallest rear cog before you remove your wheel. That way your shifter, shift cable, derailleur and chain will all be in alignment when you reinstall your wheel.

jadocs

Doesn't sound like that's what happened. The OP said he removed the wheel without shifting and it came out easy. Prior to reinstalling, he shifted to the small cog to make assembly easier.

Andrew R Stewart 

I think the second scenario is the more likely. When screwing around with the wheel reinstall and through axle install the der likely got pushed on. Which will want to push the cable back up the casing and possibly all the way to the lever. When the pushed out of it's seat cable head gets trapped between the cable spool/pulley and the lever body insides.

If this is right and if the lever wasn't then down shifted with force the repositioning of the free cable head is not too hard. If the lever was jam shifted and the cable head is wedged up tight, good luck. Andy

DOS

Weeeellllll, I can’t swear that @andreclaus scenario didn’t happen. There were three of us trying rater pathetically to figure out how the thru axle worked and it was apparent after reinstalling the wheel that first time that we hadn’t gotten it right. Its possible, after reinstalling the wheel the first time and running through some shifts to check to see if all was well, that shifting without turning cranks occurred as we went to remove wheel to start the reinstall process over. .

DOS

Once the cable was disconnected from the derailleur, it was easy to get the head to pop back out and reinstall. All seems well now. I just need to make sure I don't repeat the error. I think between learning to manipulate the thru axle correctly and @andrewclaus advice, I should be good.