I have a bike that was originally a Tiagra triple group. Last owner changed out the front for a Ultegra double. Seems okay but a little chain rub on crossover gears. Anything I can do? Any advantage to changing out the front derail?P1010003..jpg

check the chain line, it might be off. try changing the BB axle to a shorter one. ( one for a double ) .

the inner side of the shifting cage is deeper on the triple.
maybe the compact double type, will work better they too lower
the pushing side of the cage, as the chainring sixes are different by radius.

If the chain rubs on the side of the cage its up to you to re center it so it doesn't,
not the mech to do it automatically ..

and yes chainline for a double you have to fit a shorter BB axle,
where the middle ring was,
is to be the center of the space between the 2 chainrings.

I think the BB was swapped also ... I'll have to check, thanks for help.

Going by the pic, that's a 2-pc Hollowtech crank. The BB cups are the same for double and triple.

Hmmm .. does this photo lend clue?

The chainline for a double should be 43.5 mm, from the center line of the down tube to a point mid way between the two chainrings.

Your first pic looks like the der cage is a little high. I set mine 1-2mm above the big ring teeth, and have no rub issues. Of course, proper adjustment of stops, cable tension etc. is assumed.