Originally Posted by
SoSmellyAir
The 17C to 19C shift misses about once every four or five times and leaves the chain rattling on the 17C cog. Another push on the STI lever moves the chain onto the 21C cog, skipping the 19C cog altogether. No issues with the 19C to 17C shift. Using the 17B cog (from the 11-28 cassette) instead of the 17C cog (from the 12-25 cassette) moves the problem onto the 16A to 17B shift, where the missed shift occurs slightly less often than the missed 17C to 19C shift. Again, no issues with the 17B to 16A shift. So it seems slightly better to have only a single tooth difference at the downshift from one donor cassette to the other.
I soon got tired of the missed downshift (between the donor cassettes) and the compensating effort to feather the shifter just so to pull off that downshift. The whole point of a custom cassette was to reduce disruption to my cadence. So now I use only the 12-25 cassette, leaving me with an extra, slightly used 11-28 Ultegra cassette. Yes, that means I usually have to shift onto the small chain ring before I come to a stop.
Yeah, I'm way late to this party. Still, the topic is of interest, and your experience is unusual, so I wondered:
What you describe sounds to me exactly what happens when you replace only the cassette in a long-established drivetrain, and the rest of the parts get offended and sulky and won't play with the new kid. But I've also heard of full overhauls failing in exactly this way.
This is why all that one can say about mixing cassettes is, "It almost always works." The "almost" is doing all the work there. Yours didn't. It's an exception. Exceptions are interesting. I'd be curious to know more.
--Shannon
PS: I'm a confirmed necromancer. Mostly because of my deep and abiding loathing of VBulletin's search engine. (Actual quote from the developers: "Search sucks. Deal with it." That was more than 15 years ago.) Reviving dead threads that have relevant information in them is almost always preferable to starting a new one, because the more threads there are to search through, the worse search sucks.