ScottChapman

So, I am running a SRAM Force-22 compact (was 50/34) on my gravel bike.

I opted to swap the big chainring for a 46 to get better gearing so it is currently a 46/34.

However, the chainring is really only compatible with a 36 inner (46/36 not 46/34).

I figured this couldn't be THAT much of a difference and am riding it anyway.

What I have discovered happens, much to my surprise, is that sometime when I shift to the BIG chainring the chain drops on the INSIDE of the chainrings.

Is that at all what you would expect??

Thanks in advance!

SSRI

did you re-position the FD when you changed to the smaller 46T ring?

fietsbob

Add a N gear jump stop , or K edge chain catcher..

HTupolev

It's not "expected" per se, but also not surprising. Pins and ramps work great in their optimized use case, but they can create weird behaviors elsewhere. The fact that it's "only" a two-tooth difference isn't necessarily helpful, since that creates far more than enough misalignment to destroy the effect of the pins&ramps.

Pins&ramps actually involve pretty precise design... they're specifically optimized with the pedaling motion in mind. Sometimes modern cranksets experience weird shift failures (such as throwing the chain off the inside) if someone attempts to do something weird like upshift while slow-pedaling in the small ring and a big cog.

As SSRI mentions, make sure your overall front derailleur adjustment is good.
And, as fietsbob says, a chain catcher might also solve the problem.

Psimet2001 

As mentioned - the rings are designed with ramps and pins to work together with the ring sizes stated.

The best you can hope for is proper FD setup. Let me be clear on this - that doesn't necessarily mean "textbook" FD setup.

I see a ton of FD issues that are brought to me out at races or in the shop. A of of times after a factory race mech has worked on them. I am laughingly referred to as the "Shift Whisperer" by those that work around me or with me.

Almost all FD's are set too high. Slam that FD. It's in the right place when it almost touches the large ring while shifting. If it actually hits the large ring then it's a tiny bit too low. Obviously the other part is the "twist" on the cage alignment to the rings. If you are having problems with the chain shifting to the big ring then toe the front inside edge of the cage out slightly towards the big ring. It should be enough to help the shift but not enough to allow rubbing once the shift has been completed.

Third part is always tension. Don't attach the cable to the anchor until you know for a fact that your low limit is set to within a partial mm of rubbing the chain when you are in the largest cog in the rear. Then attach your cable but make sure you have all of the slack out of the cable. I often use a cable puller to get the tension (commonly referred to as a 4th or 5th hand tool or some such nonsense) fully without slack.

If you have done all of that and it's still having a hard time shifting then check to make sure your chain isn't too worn out (too much wear can lead to a lot of lateral flexing when shifting. causing it to hesitate or even drop when shifting the front.). Based on your initial comment that it was originally built as a 50/34 frame I am guessing the braze on for the FD just doesn't go low enough to allow the FD to get low enough and into the right position. If that is the case then pull the FD from the mount and check to see if there is a second hole in the FD that allows it to be mounted lower in the braze on slot.

macca33

It'll come down to set-up of the FD, I'd reckon. I agree the pins and ramps may not be 'perfect' with that chainring combo, but it'll be the FD allowing the chain to drop - either positioned too high, or there is too much play in the limit screw adjustment.

cheers