7-8 shifting mystery solved! (Shimano XT 12-spd)

[Jeffsy29]

As some of you read, I had wrecked my shoulder a few weeks ago and off the bike since. During that ride however, even before the wreck, I experienced issues up-shifting (harder) from 7th to 8th gear, or downshifting (easier) from 8th to 7th. Nothing would happen either direction just a dead-zone from 7-8 or 8-7. I'd workaround by shifting 2x to 9th and back up 1 to 8th. Unfortunately, 7th and 8th are in a sweet-spot for a lot of flowy trail work so I was in that range all the time...irritating! Strangely, it shifted FINE everywhere else, even under load (as designed by Shimano for e-bike).

I read that the new microspline 12-spd is subject to shifting issues with even minor deviations from alignment spec. due to the narrow tolerance. From hub, to cassette, to spacers (cassette install), derailleur hanger, RDR adjustments, cable tension....it all needs to be pretty perfect to work flawlessly (like we expect!).

I deliberately started with one suspect at a time and remounted the wheel to test:

1. Hub - This looked fine. Surprisingly, it just "pops" off! The spline mechanism and springs all come bouncing out if you're not careful. This happened quite easily while the cassette was mounted. I greased up everything and reassembled and went on to the cassette.
2. Cassette - Removed and confirmed the spacer sizing (thin vs. thick) and proper position. I wouldn't expect DT Swiss (or YT) to get this wrong since generally new you just slide it off the tube direct from Shimano onto the hub. They were correct size/position. Reassembled, retorqued and remounted the wheel. Fiddled with the barrel adjuster and no-dice...still had an issue, but this time seemingly between 8-9.
3. Derailleur hanger - Thankfully I threw a spare in my cart when buying the Decoy. Since I *did* crash I replaced it, though there was no sign of scratches on the derailleur. That didn't work either. In fact, there was some chain resistance with each ramp-up to the next larger cog (downshift) almost like someone gently squeezing the brakes. Maybe something goofy with the chainline during ramping? Dunno....
4. RDR adjustments - I did Hi/Lo/B limiters the first time during assembly. I thought I had them nailed since my first ride shifted flawlessly. Had to pull back the Hi limiter a little bit for safety. Lo was fine. And the B limiter "looked fine". I almost didn't even bother as this is the least-consequential RDR adjustment there is - so they say....
5. B limit screw - For the hell of it, I loosened this quite a bit allowing the pulley to get much closer to the 1st gear (big cog). 7-8 shift was starting to improve! If a little is good, then a whole lot must be better right? It was, I got at least another 3-4 mm closer to the big cog but far enough that there's no possible interference w/ the pulley. The screw is nearly fully backed out. But it's amazing how much greater chain wrap length you get by just a few mm adjustment. That extra wrap produces some damn-crisp shifting response.

Who knew? I've never cared much about the B-limit screw EVER. If it wasn't crunching into the cog, it was 'enough' space. And I've never paid attention to the shift-quality difference by purposely fiddling with the range. It's not subtle at all when it's too-tight vs. loose enough. Too-loose and you'll end up rubbing your pully or getting your RDR cage in your spokes on pre-boost wheels.

If anyone has odd (sloppy) shifting behavior that doesn't resolve with tension or indexing adjustment - check your B-limit screw!

 

[seamarsh]

As some of you read, I had wrecked my shoulder a few weeks ago and off the bike since. During that ride however, even before the wreck, I experienced issues up-shifting (harder) from 7th to 8th gear, or downshifting (easier) from 8th to 7th. Nothing would happen either direction just a dead-zone from 7-8 or 8-7. I'd workaround by shifting 2x to 9th and back up 1 to 8th. Unfortunately, 7th and 8th are in a sweet-spot for a lot of flowy trail work so I was in that range all the time...irritating! Strangely, it shifted FINE everywhere else, even under load (as designed by Shimano for e-bike).

I read that the new microspline 12-spd is subject to shifting issues with even minor deviations from alignment spec. due to the narrow tolerance. From hub, to cassette, to spacers (cassette install), derailleur hanger, RDR adjustments, cable tension....it all needs to be pretty perfect to work flawlessly (like we expect!).

I deliberately started with one suspect at a time and remounted the wheel to test:

1. Hub - This looked fine. Surprisingly, it just "pops" off! The spline mechanism and springs all come bouncing out if you're not careful. This happened quite easily while the cassette was mounted. I greased up everything and reassembled and went on to the cassette.
2. Cassette - Removed and confirmed the spacer sizing (thin vs. thick) and proper position. I wouldn't expect DT Swiss (or YT) to get this wrong since generally new you just slide it off the tube direct from Shimano onto the hub. They were correct size/position. Reassembled, retorqued and remounted the wheel. Fiddled with the barrel adjuster and no-dice...still had an issue, but this time seemingly between 8-9.
3. Derailleur hanger - Thankfully I threw a spare in my cart when buying the Decoy. Since I *did* crash I replaced it, though there was no sign of scratches on the derailleur. That didn't work either. In fact, there was some chain resistance with each ramp-up to the next larger cog (downshift) almost like someone gently squeezing the brakes. Maybe something goofy with the chainline during ramping? Dunno....
4. RDR adjustments - I did Hi/Lo/B limiters the first time during assembly. I thought I had them nailed since my first ride shifted flawlessly. Had to pull back the Hi limiter a little bit for safety. Lo was fine. And the B limiter "looked fine". I almost didn't even bother as this is the least-consequential RDR adjustment there is - so they say....
5. B limit screw - For the hell of it, I loosened this quite a bit allowing the pulley to get much closer to the 1st gear (big cog). 7-8 shift was starting to improve! If a little is good, then a whole lot must be better right? It was, I got at least another 3-4 mm closer to the big cog but far enough that there's no possible interference w/ the pulley. The screw is nearly fully backed out. But it's amazing how much greater chain wrap length you get by just a few mm adjustment. That extra wrap produces some damn-crisp shifting response.

Who knew? I've never cared much about the B-limit screw EVER. If it wasn't crunching into the cog, it was 'enough' space. And I've never paid attention to the shift-quality difference by purposely fiddling with the range. It's not subtle at all when it's too-tight vs. loose enough. Too-loose and you'll end up rubbing your pully or getting your RDR cage in your spokes on pre-boost wheels.

If anyone has odd (sloppy) shifting behavior that doesn't resolve with tension or indexing adjustment - check your B-limit screw!

Theres actually an indicator line on the back of your derailleur pully arm that shows you where to set the b tension screw... 11speed and 12 speeds are all sensitive to b tension being set correctly.

 

[Jeffsy29]

@seamarsh - Thanks for that tip! Now I need to go run downstairs and see if my guesswork is anywhere close to Shimano's specs (shifting sure works fine now though).

I had NO idea there was such a thing. I didn't read Shimano's manual because YT didn't put a big "RTFM" sign on that one . I've always just used ParkTool's RDR adjustment videos in a pinch.

Sure enough....there it is in plain English.


Good description of the same here:

 

[steve_sordy]

Sram provide a snappy looking set up tool for their Eagle 12-speed. It sets the gap at a mammoth 15mm (or is it 12mm? it's a while since I sold the bike with 12-speed).

 

[Gary]

The SRAM b tension guide tool doesn't take into account, hanger placement or chain length. Nevermind sag point OR chain growth effect on alonger travel FS bike.
I NEVER use it as the mech can be set up far more precisely by setting up b-tension manually, testing it and optimising for best shifting when sagged. (where most gear changes actually happen)