12spd XT rear mech and B tension adjustment

So I have fitted an all new 12spd drivetrain. I can’t get the mark on the rear of the mech to align with the top cog on the cassette. I wouldn’t normally concern myself with this but I’m struggling to get a clean shift in every gear. I have checked the alignment both top and bottom of the cassette. Checked the chain length and all seems as it should. Cassette is 10-51 with a SGS rear mech and a 34t up front. The frame is a transition spur if that helps.

I couldn’t get as far as the mark either on my SLX 12. Maybe 5mm or so short of it. But the shifting is good so are you sure the problem is not elsewhere and have you fine tuned the indexing?

I believe the only way to get nearer the mark is by changing chain length but I don’t know which way as I didn’t really need to, can’t get my head round it…

What did you check the chain length against? Shimano TechDoc?

DM-MARD001-05-ENG.pdf (shimano.com)

Also triple check you have the cable routed the correct way into the mech (ask me how I know to triple check it…🙄)

I had some customers bikes that had to be set and adjusted at sag to get 12s to work.

Chain length and sag both change the upper jockey wheel position.

@rootes
Regards chain length big cog plus chainring plus 6 links. 4 too short mech almost at full reach top of cassette , 8 links to long almost rubbing on itself on bottom of cassette.

I have tried fine indexing but its not helping. It seem to be the middle of the cassette takes effected. Yes all cassette spacers are present and correct.

Checked hanger alignment etc? also which level of shim 12ps you fitted? Yep and for full sus 5 or 6 links is correct and at max chainstay length, where that is will depend on the characteristics of the bikes suspension.

Complete XT drivetrain. I have the tool to check the hanger alignment so maybe that’s the next port of cool. The transition mech hangers do look quite robust.

So you need to take the shock off and cycle the suspension to find where the chain needs to be at its longest.

Mech hanger first job – if it’s like my Sentinel it’ll be a sram udh which is quite robust. Mine worked fine straight away – but maybe the Spur has more chain growth like some people have said – worth letting all the air out of the shock to see.

I would second checking you have the cable into the mech in the right way, in my haste I did mine without reading the instructions (I’ve worked on hundreds of bikes over the years) felt great in the stand, but not quite right riding. It was only when I went back to square one to try and sort that I realised. Symptoms were exactly as you described.

Is the spinny plastic bit on the right side of the B tension screw? Is your UDH in the correct position (As far anti clockwise when viewed from the outside as it will go)?

EDIT: Obvious check and I know you said it already but its definitely an SGS mech?

The spur frame has a standard rear hanger ie. Not UDH.
The cable is in the clamp as per shimano’s instructions.
I think as the cage is so far away from the cassette that’s what’s causing the issue. As I increase the B tension the mech moves even further away. Currently it is set with the screw wound right out. Increasing the chain length moves it further away to start with.
I will check the mech is SGS, that’s what I ordered.

So far away from the cassette? You have the opposite problem to what I would have expected.

Photos probably needed, two from the side on (both sides), one close up on the b screw from up and behind will do it.

Given the position of the cage pivot and the top jockey, a longer chain will bring the jockey closer to the sprocket. Def needs some pics!

also what cassette you using? 10-51 or 10-45?

I’ll sort some photos

Ignore the dirt I forgot to clean it yesterday. Last image show the line Shimano says to align the top cog with.

Got one from the rear and also of the b screw etc?

on first look I would have said the chain is a bit short looking

Are you sure it’s the m8100 not the m8120?

@oceanskipper hiya, that mech is def not an m8120

I’ll post a photo of a rear view tomorrow

Chain too long is my assessment. See my two for comparison. Both on the mark as per set up guide. 125mm and 140mm travel for what it’s worth. Looking at the angle of your mech at full stretch it has much more range for a shorter chain.

View post on imgur.com

Must be something up with the b adjust.

in your pics @davos the upper part of the mechs are more more rotated than the OPs. Ie the cable arm is more horizontal.

if the OP shortens his chain (which he says he has already tried) then it will move the line even further from the sprocket.

Have to remind myself what mine looks like.

an odd one for sure.

Worth a double check of this:

Has OP checked benpinnick’s suggestion:

Is the spinny plastic bit on the right side of the B tension screw?

Final thought following from rootes observation, could be that not enough cable outer pulled through the frame to allow the full range of movement so the mech can rotate further round.

I hope these help. I couldn’t get the perfect line of sight of the image with the mech at the bottom of the cassette.

Mech definitely looks to be rotated too far back.
Have you got a picture with the mech removed so we can see the angle the stop on the mech hanger sits at?

Once had a very similar problem and it was the geometry of the gear hanger that was causing it.
Had to file the Bbscrew stop on the mech hanger to allow the mech to rotate forwards.

just comparing your image with a random from the web, you mech does look quite a more more rotate around clockwise:

different Transition I know, but useful for angle.

Spur example: def more anticlock than on yours.

back off the b screw and loosen the main mech to hanger bolt and see you you can rotate the mech anticlock. see page 10 on here: DM-MARD001-05-ENG.pdf (shimano.com)

see on the transition examples above cable goes into the mech pretty much at or near vertical.

I’ll investigate further tonight and report back. I’ll check for all the afore mentioned points. Probably user error 😳

fingers crossed.

i think the stopper plate is missing – maybe still in the box of the mech?

If you zoom in on the photos you can see the stopper plate in place… I think!

i think the stopper plate is missing

If the stopper plate was missing the mech would be rotated too far forwards.
OP has the opposite problem.

I’d be making sure the stopper plate was sitting flat on the B screw flat of the mech hanger to start with.

So I have taken the wheel out and the stopper plate looks correct. Again ignore the dirt.

Stopper plate does look to be in the correct place.

If it were my bike I’d file either the stopper plate or the mech hanger down to allow it to rotate into the correct position.

check this out!

apparently new hanger to fix the issue…? see top post:

Transition Patrol, Scout, Sentinel, Spur & Suppressor, Giddy Up & SBG Post your bike and discuss here! – Page 709 – Pinkbike Forum

perhaps there is a revised shape hanger? What groupset did the bike come with?

If everything else checks out (cable, stopper, chain length etc), would make sense in your case that perhaps there has been a revision at some point to better suit shimano mechs?

Perhaps speak to your Transition dealer or Windwave to see?

or crack the tools at as Stu suggests!

I was going to suggest emailing transition to check the hanger is correct. At least it all makes sense now!