Greetings-
A friend and I are upgrading his commuter Trek FX Hybrid nine speed triple from Shimano lower level chainrings and bb to XT rings and bb with external bearings. The original setup specs a 47.5mm chainline but the XT specs 50mm. Using the provided spacers we can obtain either chainline. Which would be the correct setup?

Either or in between are OK. Textbook "perfect" chainline would have the center of the chainrings lined up with the center of the cassette. From there, you might tweak it toward on end of the range or the other for a variety of reasons. You move out a bit to clear a wide chainstay, or you might move in a bit to reduce Q-factor.

My bikes are set toward the outside of the range since I use the outer end of the cassette the most and this yields the best chainline in the combinations I ride the most.

My '88 and '91 Bianchis (assuming they're stock) are like this -- presumably so the chainline is best when in the upper gears.

Thanks-
we suspected 47.5 was likely best bc he uses the middle ring the most on the streets of phila. We needed to see what the group thought- lots of knowledge/ideas here. Complicating factor is chain rub. When on the big ring-small cog, the chain rubs the front der outer plate. Can adjust this out with cable tension, but then the chain rubs the inner der plate when in the middle ring-big cog position. Can't have it both ways. Installed a tiny washer between the cage plates at the rear of the front der for extra clearance-this helped a little but rub remains. Hence, the chainline question. Can this be solved, or is it too much to ask for on a nine speed triple? There is no trim function.

Yea, textbook: chainline is middle chainring lines up with middle cog
on rear-gear-cluster
double crank the space between the chainrings is the center point.

3 ka-thunk STI shifting lacks the post shift trimming of the FD cage position
of Friction shift-levers.
It's a trade off ..
the rubbing begins at the margins of cross chain combinations.

middle to middle cogs , low to bigger, high to smaller, cogs
was engineered to have clearance.. in STI FD cage shaping

If you're game, you could try tweaking the derailleur cage with a smallish shifting spanner, with a screwdriver through the hole in the handle.

You have to be pretty damn careful to actually make an improvement, though. It's not hard to break that brittle steel, either.

But I've had great success doing this a couple of times.

I'd imagine a very slight bend high in the inner plate would make the most of the washer you've added.