… on front chain rings (road)?

No idea…. But I run 9 slx speed rings on my 11 speed 105 and they work perfectly

Width, I think. At a guess, you’ll get more chainsuck if you do the above.

Yeah I think the 9 speed cranks (double) were about 1 mm further apart. The 11 speed chain is slightly narrower. Apparently the shifting ramps on 11 speed cranks are optimised to improve shifting but as I said the 9 speed rings work and shift absolutely fine and no chain suck for me so far.

Differences:

9 to 10 is 1
10 to 11 is 1
9 to 11 is 2
11 to 10 is -1
10 to 9 is -1
11 to 9 is -2

Pretty sure thats all the differences and combinations…

9 to 10 is actually 0.999999 recurring, technically.

IIRC, nominally derailleur chains are the same internal width; the external width is shaved down to fit more sprockets in, which is why the 9s rings ^^ will work on 11s, but may be prone to chainsuck (the narrower chain could catch between the rings if they are further apart)

Ok, so I’m currently 9sp 50/34 on a 9sp 27/12 cassette.

I’ll be going 11sp 105 50/34 on a 9sp 25/12 cassette. Bugger all real world difference?

@kryton57 – thats what i use, no problems, although i think its slightly more rattly up front, could be the 9speed links on the 11 speed rings, or could be the 11speed front der. is a little narrower.
shifting is perfect up front and no issues on the rear either.

Reggiegasket 9 + 1 = 10.