Originally Posted by
Chris_W
The ring guard should be preventing this from happening. If you've changed to a different size chainring, then maybe the ring guard needs to be changed to match, or it may be possible to modify the current ring guard in some way so that it does it's job better (after checking the FD adjustment, as mentioned above).
Read.
He said slips off TO the chainguard, not slips off it. This is a front derailleur high limit screw adjustment problem.