GRX to Ultegra crank - chainline problem?

Equipment
1

I have a 2021 Trek Checkpoint ALR 5 with GRX 600 2x11 crankset (46/30) with 47mm chain line and the GRX front derailleur that accommodates the wider chainline. I want to put on an Ultegra 8000 50x34 crank (Shimano says 43.5mm chain line) and front derailleur from my road bike. I plan to leave the GRX 800 rear derailleur in place.

I assume the Checkpoint is designed for a 47mm chainline. Assuming my tires clear (700x35 slicks for winter road - eyeballing it I should be okay on frame and tire clearance) the narrower chain line with the Ultegra crank/front derailleur, will this work well? Will shifting performance/drivetrain efficiency be compromised? Trek used to ship the ALR 5 with Shimano 105 kit pre-GRX (like 2019 model or so), so I’m thinking this may work.

I have the crank/front derailleur on my road bike, and it is my understanding the BB is compatible, so I could just try it, but want to avoid spending the time if it ultimately won’t work, or won’t work well.

Goal is to get bigger chainrings for winter road use. I’d rather have the 50/34 on my road bike or even 52/36.

2

I researched this when I was building my Colnago gravel bike. Asked about compatibility of an Ultegra crankset (I had a spare w/ Stages PM) with GRX for the rest of the group and was told they would not work together.

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3

Why not just buy a bigger ring for the GRX?

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4

I already own an extra Ultegra 50/34 crank and FD and I know that gearing works well for winter road use.

I can probably get by with the 46/30 with a smaller cassette in back. Don’t need the 34 in back for road and would prefer smaller jumps in the midrange so might just try an 11-28 or 11-30.

5

I reckon the shifting will be fine. Maaaybe you’ll get some rub on the front mech if you try to cross chain a bunch. The worry will be clearance as you say. Since you’ve already got the cranks just slot them in without messing with the front mech yet to make sure they clear everything. That’s a 5 minute job.

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6

The bcd should be the same. You can just pull the big ring off the ultegra and put on grx

7

Check what the maximum chainring size is for the GRX mech. The two chainsets both have a 16T range so that won’t be a problem.

Are there any BB spacers in place at the moment? If there are and they are on the NDS then you could arrange them so they are on the drive side and that will push the chainline out by Xmm. The other thing to watch out for would be clearance of the rings at the chainstay.