Shimano 9/10 spd compitability question

[Feanor]

Hi all.

Opportunity arises to upgrade road-bike drivetrain from existing tiagra 9x3 to 10x3.
The Tiagra stuff is needed on another bike.

Now, when Shimano went from 9 to 10 spd, I know about what happens at the back end.
We also get a slimmer 10-spd chain.

Q: Did anything change at the front to take account of the narrower chain?   Like did the chainrings get slimmer, or closer together?   Did the front derailleur cage get narrower?   I think the answer is 'no', but I'm not sure.

The reason I ask is that if I do the change-over bit-at-a-time, will the new chainset which came from a 10-spd groupset be happy with the wider 9-spd chain for a few weeks?

[Little Jim]

I can't answer alll your questions, but I am happily running a 10 speed chainset with 9 speed chain and front derailleur, and all 9 speed on the rear.

[Polar Bear]

All that changed is that the outer dimensions of the chain got narrower as did the front cage inner dimensions.   You should have no significant issues running a ten speed chain over 9 speed chainrings and through the 9 speed cage.   

Oops  .  ... Or for that matter funning a 9 spped chain over ten speed chainrings.   Being slightly narrower, you may get some rub at the extremes from a nine speed chain through a ten speed cage.     

[tiermat]

Hang on, that is just what I have done for the last 1500 miles on the InBred!

9 speed rear block
8/9/10 speed rear mech (105 5600)
9 speed chain
10 speed chainset (RaceFace Evolve XC)
10 speed front mech (Ultegra 6600)
9 speed shifters (Tiagra)

No no issues whatsoever in what you want to do.

[frankly frankie]

I think there is a small difference in the spacing of the chainrings, and therefore in the indexing of the front lever.  But mismatches will work without too much trouble, as is obvious from other replies.

I put a Shimano 9sp chainset on an otherwise all-shimano-10sp bike and front changing was a bit problematic.  But only a bit.