Hey guys, first time posting, long time lurker...

I've come into an old trek 4100, frame is in mint condition, I've decided to upgrade everything else.
I know its not the best frame in the world, but I get a real kick out of the building it all myself part

Anywhooo...

I have to admit I've got a bit stuck, and wondered if anyone here could lend me a hand.
Everything has worked fine until I got to sticking the chain on and the rear derailer.
I've upgraded it from a 7 speed tripple, to a 9 speed tripple (with a new crankset and shifters).
When its on the smallest sproket and chainring the derailer is almost in its out of the box position (Will post some pictures when I get chance), so almost completely folded, and the chain rubs against itself.
Then in the higher gears it seems like the derailer is too streched out.
Any help would be massive appreciated!
And you get a medal for understanding what I've just written!

Cheers all

no medal as I , cannot guess.. you are weak on explanation.

take it to someone who can saee it.. AKA the bike coop or a shop.

you bought a rear wheel?

you buy a new chain. ?may be too short.

folded ?

a bit of tip$ to your mechanic is appreciated ..

I can picture exactly what your problem is; and it sounds like you still just have too much chain and will need to shorten it - - within reason of course, because (as you note) you don't want it too tight in a big ring/bigger cog situation. Are you sure you have a long-cage derailleur? One of our more knowledgeable experts may correct me but with normal mountain-triple gearing you should be able to cross-chain to your heart's content without exceeding a long-cage derailleur's capacity (edit: not that I'm recommending cross-chaining but it should be able to handle the ocassional 'forgot-what-chainring-I-was-in' moment).

I had a similar situation when I first set up my new bike last spring - - with the derailleur being completely slack in (2x) small/small - - which was further complicated by needing to accommodate some chain-growth from 6" of travel. In your case, just take a link out at a time until the small/small slack is gone all the while checking your big/big tightness.

https://www.youtube.com/watch?v=yDPc1WqT2PE