Shimano Ultgera 8000 groupset always needing to be indexed

The R8000 front mech can be a bit fiddly to get the right base tension set. My LBS mechanic explained the best way to approach it was to over-tension slightly to get it set and then remove tension with the on-mech adjuster.

Would this have a bearing on the performance of the front mech as well?

Would this have a bearing on the performance of the front mech as well?

Would this have a bearing on the performance of the front mech as well?

Are you confident in the cable routing job done? 

Are all the outers cut flush and fitted correctly in the plugs etc? 

Otherwise, get the hanger checked. Mine was bent from new, which made the gears tricky to get right... 

Does sound like the hanger is bent. I bent mine with an inoccuous knock on a door frame as I carried it outside. Was very gentle, and thought nothing of it at the time, but it was the only thing that happened to it that could explain the bend

Definitely recommend a rear mech alignment tool, it sounds just like mine when the hanger is out. I wouldn’t even fiddle with indexing any more till I check alignment. 

Could be a twisted derailleur cage as well. Not just the hanger. 

Is the cassette on the freehub correctly ?. Any excessive play ?

How or where are you adding /reducing cable tension? If one of the adjusters (most likely at the rear mech) has been unscrewed past its thread, it can move about giving uneven adjustments when moving up and down the block. I'd definitely check the hanger alignment too.

So, is the problem going up the gears (so lets say to bigger sized cogs) or coming back down towards the smaller cogs? 

And if you correct the indexing, which way will the indexing go off?

Is it effectively losing cable tension, so you are always winding on tension, or is it more a case of you having to compromise between having enough tension in the cable to go up the gear, without limiting the run back down the gears? 

Generally speaking, gears that are dicks are either because your hanger is mal-aligned, or there is fundamentally too much resistance somewhere in the cable routing. For instance, you may have your inners crossed in the frame, or the run isn't smooth somewhere... maybe a snagging liner. 

Giant bikes are, in my experience, a little more senstive than others as the chainstays are typically short. This shortens the gap between the cassette and chainrings, which puts more pressure on chainlines and set up. 

Let us know the answer to the above and we can provide more pointers maybe? 

I know you've changed the rear derailleur cable but have you checked its smooth running through the cable guide under the BB?  

When my gears were all over the place recently it took me an hour to suss that my inner cable had jumped out of its channel.  

Best of luck.

Is your dropout properly aligned ? They're made of relatively soft metal, so dropping the bike bends the dropout rather than breaking the frame. You can't really tell by eye.

Our family has 9 bikes (don't ask ...) so I used the Park Tool aligner on them and 4 needed adjustment ...

I'd check the rear mech cable routing at the clamp bolt. It's quite hard to get wrong on R8000, but always a possibility. Also check that all cable ends/ferrules are properly seated against their stops.

Oh, just reread and cannot think why you need to be re-indexing as such.  That should be a once off for new equipment setup...and then a minor adjustment after a little while.  The rear index function doesn't really care about what happens at the front.

It could be that you are trying to get something working that shouldn't really work without some kind of rub.  Running big/big on most mechnical setups is going to cause an issue as the front derailieur will probably rub at some point (either small/big or big/big) depending on which of the 2 options you have done the setup for.  If you check the angle of the chain when running big/big you will see why the chain, jockey wheels and everything else is having a problem running it.

Why not have a look at the range you are running and see whether there is a better option, albeit at the loss of a close ratio at some point, e.g. if currently running 11-25 try an 11-27 or maybe a 12-27, etc.  

Just as an aside, is it worth checknig that you are using any trim options if you have them?  Most groupsets have a trim option for the FD, puzzling my Potenza has 3 trim options for the small ring but none for the big, whereas my Shimano setups (4700/5800) have trim for big and small rings.