I have 12spd 105 di2, and despite following all of the online guides and significant trouble shooting i cannot seem to diagnose and solve an issue.
What is occurring: anytime i try and sprint (high load) in the bottom 2-3 gears of the rear cassette I end up with the chain jumping off the big ring at the front. This happens regardless of whether I am shifting or not.
Below I have attached a couple of chainline photos.
I have triple checked the hanger is straight
I have trimmed the FD inboard as much as possible so that in the smallest ring at the back it is rubbing at the front to try and avoid any ability for it to jump outboard.
I have triple checked the height of the FD to ensure its sitting in the right spot and not high enough to allow chain to sneak through
All components are new, chain is new.
I’m sort of at a loss of what to even check next, and this isn’t something i can test for in the stand. It will shift fine and no issues when i’ve got it in the stand, only happens when i try and sprint.
The only thing that I can think of may be the fact that everything is new except the cassette and cranks? A lot of the time a brand new chain on worn teeth can cause issues of this sort.
The other thing I was thinking could be chain length, but I don’t think that’s the cause.
Are those 12-speed chainrings? How much wear do they have? I’ve seen a lot of strange issues that resulted from throwing non-Shimano parts (chainrings, cassettes, chains, etc.) into a Shimano group. They’re designed to work as a system, and although you can get other parts to work, it’s usually a sub-par experience, even if only slightly so.
Also, I’d maybe take the cassette off and double-check that it’s installed properly. The 12-speed cassettes come with three or four spacers that are different thicknesses. If the spacing between some of the cogs is off, it won’t work properly.
Bought new front chainrings as I thought that was the issue, these are 12spd specific ones. So zero wear on them as well.
Have taken cassette off and checked it wasn’t that, its even now on a new rear wheel since I first installed the groupset.
When you say the spacers are different - are you referencing the spacers that sit between the smallest cogs on the cassette? I can pull them again but i never noticed a difference in their thickness that was noticeable and to be honest I basically just slid the cassette straight onto the free hub using the plastic guide.
Those are the Praxis Buzz chainrings and are compatible with 10/11/12 speed chains.
I run the same set with Ultegra Di2 12sp. They are good rings in general. Particularly for older cranksets with 130 BCD, these rings are one of few options for people who want to run 12s.
Based on my experience, these rings and 12s Di2 are more sensitive to chainline. I suspect the issue the OP is having will be chainline related. Probably some BB or crank flex under high load like an all out sprint of jump. In the right circumstance that could lead to a chain drop issue. Once the other alignment and cassette spacer stuff is checked out as OK, that would be my guess.
If possible, perhaps install a Shimano crankset with Shimano rings and give it a test.
Once the front derailleur makes the shift it should be out of the equation and not touching the chain. e.g once the chain is on the big ring the front derailleur shouldn’t be an issue. Unless it rubs.
the 12 Speed shimano chainline is 44.5mm, 11 speed is 43.5mm, which means you will need to move the crank outboard 1mm to meet the spec for shimano, I wouldn’t have thought it would cause these issues though.
Hi, pretty sure you have it the correct way but double check the logo’s on the chain are outboard (if a shimano chain) ?
I have had a similar issue with the chain jumping on the 11 at the rear due to a build up of wax not allowing the chain to properly sit in the teeth. So perhaps degrease and clean then traditional lube if using wax.
Which side does the chain drop? onto the small ring or to the pedal side? If it’s the former this should help rule out the chain line being the cause.
Good luck, main thing is to be patient and try one thing at a time, you will find a solution.
Just so I understand you here, are you saying i should trim the FD outboard so that there is zero contact with the chain when in the little ring?
I had trimmed it inboard slightly to make it so the chain couldn’t drop to the outside (take away the opportunity). I’ll retrim inbound slightly and see if it makes a difference again.
I’ve had issues with Praxis 12 speed chainrings dropping chains. Not worn (brand new chain, cassette, and rings), properly aligned. Changed nothing else except for the chainrings and the problem went away 100%. No idea why.
I’d recommend setting up the trim based on the Shimano website or the E-Tube app. BetterShifting also does a great job working through this.
Another easy thing to check is that your high-limit screw isn’t too far in. This would likely only cause issues when in the 11t, but it’s worth looking at.
Hopefully it is such that you need to move the chainrings further out, and in that case you can just put spacers on the crank axle. Not sure what to do if the chainring is already too far out.
Agree with the Better Shifting website and the excellent information there. Definitely follow the Shimano or Better Shift directions for set-up.
All I meant was once the chain is shifted from small ring to large (or vice-versa) then the front derailleur shouldn’t be relied on to keep the chain in place. The front mech moves the chain back and forth, but the chain alignment and tooth profile (in case of 1X) keep the chain on the ring.
Sounds like you already checked the obvious stuff and have a good handle on it. You could try to ride seated and be progressively less smooth on the pedals and see if you can recreate a situation where the chain comes off. Or perhaps on a trainer standing / sprinting. Definitely not something to troubleshoot on roads where you could hurt yourself.
On Edit - I might have missed it above, but have you called or emailed Praxis? They are good folks in my experience and might have something helpful. Free to ask.
