darrellc

Hi all, looking for help… I have a 2021 Trek Checkpoint ALR 5 (the aluminum one), 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)s. I have an Ultegra 8000 front derailleur to match that I’ll pull over from my road bike. I plan to just leave on the GRX 800 derailleur in back.

I assume the Checkpoint is designed for a 47mm chainline. Assuming my tires clear (700x35 slicks for winter road) the narrower chain line with the Ultegra crank/front derailleur, will this work well? Will shifting performance 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 will work.

Should I address this a different way? My goal is to get bigger chainrings for road use. Even the 48/31 on the GRX 800 isn’t enough, I’d rather have at least 50/34 or 52/36.

shelbyfv

Since you have the bike in hand, should be possible to do the measurements. Especially check that the larger chainrings will clear the chainstay when positioned that much closer.

Badger6

Definitely this! I bought a Specialized diverge in the spring, that was 1x with a 40t chainring on a GRX crank. I swapped the crank to the Quarq (same as a SRAM Rival) and put a 46t chainring on, the teeth have 3-4mm clearance where they pass the chainstay due to the slightly more inboard chainline and larger chainring.

Chris_W

The other way around often doesn't work, an Ultegra FD with a GRX crank, because the FD can't swing out wide enough. Not sure if your combination, GRX FD with Ultegra crank will have an issue, but try it and change the FD if needed - GRX and Ultegra use the same cable pull so the lever won't care which is installed.
Or, you could learn to pedal with a higher cadence and stick with the GRX crank - 46x11 is a good enough top gear for me and I get in the top 10 of some downhill Strava segments - being aero' (e.g., tight-fitting jersey) and not braking much are more important that pedaling at a lower cadence on the descents.

DaveSSS 

I think that your proposed change will cost more than it's worth. If you can't maintain 100-110 rpm on the flats, then you need
more practice. A 48/11 only requires 8 more rpm than a 52/11, to reach the same speed.

I have both 48/31 and 46/30 grx cranks. The chain rings are interchangeable. I'm using the 48/31 right now, but I have a sram 12 speed drivetrain and 48/10 top gear that's the same as a 53/11. I can pedal it up to 45 mph, but only a few brief descending stretches allow that, on the way up to 50+ mph.

According to shimano the grx chainline is plus 2.5mm, which should be 46mm. I have my cranks on a road bike and used 2.5mm spacing washers to reduce the chainline.

darrellc

Thx for responses everyone! Since I already own all the components (have Ultegra 8000 crank/FD on my road bike) and looks like plenty of clearance for reduced chainline and increased chainring size, I’ll just try it and see.