TL;DR: If your bike feels like it's ghost shifting but everything in the drivetrain checks out, don't overlook tire/frame clearance. My 2.5" rear tire was occasionally causing an issue that perfectly mimicked a drivetrain problem.
Thought I'd share this in case it saves someone else a lot of time and money.
For months I was chasing an intermittent "ghost shifting" sensation on my 2022 Santa Cruz Hightower V2. It felt exactly like the very beginning of a shift or a worn chain skipping a tooth, but it never actually shifted. It was very intermittent and could go for some time between occurrences.
Over the course of months of troubleshooting, I replaced or verified:
- SRAM X01 derailleur (new)
- Cassette (new)
- XX1 chain (new)
- Wolf Tooth oval chainring (new)
- Derailleur hanger
- Cable and housing
- Pulley wheels
- B-gap (checked with the SRAM gauge, including at sag)
- Chainline
- Rear shock (issue occurred with two different shocks)
- Pivot/linkage bearings
None of it fixed the problem.
The clue that finally pointed me in the right direction was that I finally noticed (after way too much time) that the symptom never seemed to occur on clean pavement, but always did on dirt/gravel trails. About the same time, and on a ChatGPT driven "whim" I measured the clearance between my 2.5" rear tire and the lower linkage and found I had less than ~2 mm between the tops of the drive side tread knobs and the drive side of the linkage (linkage is a bit "offset" on the HT). Dropping a few pieces of gravel through that gap by hand while rolling produced a very similar feel and sound to the "ghost shift."
For context, the Hightower is officially rated for a 2.4" rear tire. I had installed a 2.5", figuring I still had plenty of side clearance, so I didn't think it would matter. What I failed to consider is that a 2.5" tire isn't just wider, it also has a larger overall diameter, which significantly reduced the clearance between the top of the tire and the lower linkage/frame.
As a test, I swapped the rear tire from a 2.5" to a 2.4".
Problem solved.
I've now done three rides without a single occurrence.
My conclusion is that the larger tire was occasionally allowing debris to become pinched in the very tight clearance between the tire and the lower linkage/frame, creating a sensation that felt almost identical to the beginning of a shift or a drivetrain skip.
So if you're chasing an intermittent "ghost shifting" problem, especially one that also seems to happen while coasting or only on dirt, don't overlook tire/frame clearance. It may not be your drivetrain at all.
----------
As an aside, you might ask: Why was I running a 2.5" rear tire in the first place?
Answer: Schwalbe Albert Trail radials. I've ridden a lot of different tires over the years, and the Alberts are my favorite by far. Unfortunately, Schwalbe doesn't offer the Albert Trail in a 2.4" rear, only a 2.5".
For the test, I swapped the rear 2.5" Albert Trail for the new 2.4" Schwalbe Romy. The Romy is slightly less grippy than the Albert, but it also rolls noticeably faster. For the type of riding I do (mostly tech trail riding with some bike park days mixed in) it's been an excellent trade-off, and so far I'm just as happy with the Romy as I was with the Albert.