As I’m waiting for my S-works diverge frame to finally arrive (so far 1 month delay) I’m trying to put together my build. I own a set of Roval C38 wheels with 11-24 cassette and want to build a 2X. I also prefer shorter cranks not sure if its as beneficial for gravel as it is for road, I do have a set of brand new Ultegra 50/34 165mm cranks.
So My questions: 1. will gearing work with Ultegra 50/34 165mm crank, Ultegra FD Di2 and GRX RD-RX815, ST-RX815 shifters ? 2. Does it make sense to get sense to get a FC-RX810-2 crank instead of the Ultegra 50/34 , is there pros and cons to GRX crank? 3. Is 165mm crank too short for gravel since GRX only goes as low as 170mm? 4. Is it better to use GRX crank or can Ultegra function just as good? 5. Will the GRX crack function well with an Ultgra Di2 FD?
I have no personal experience, but GRX is designed with a 5mm wider spindle/Q factor, so I would suspect an ultegra crank/grx front derailleur (or vice versa) would give you some problems with shifting/alignment.
xtmtb wrote:But an ultegra crank and FD should work fine with GRX RD?
That seems to be the guidance I've read. Ultegra will work just fine with GRX in 2x as long as the FD matches the crank. Q Factor is the main difference.
So My questions: 1. will gearing work with Ultegra 50/34 165mm crank, Ultegra FD Di2 and GRX RD-RX815, ST-RX815 shifters ? Ultegra crank and FD should be fine. 2. Does it make sense to get sense to get a FC-RX810-2 crank instead of the Ultegra 50/34 , is there pros and cons to GRX crank? The GRX crank gives you the option of using smaller chainrings. The Ultegra can only go down to a 34T. As mentioned, the GRX crank has a wider spindle/offset chainline(2.5mm each side) for clearance with frames with wide stays for wide tires. If the Ultegra fits your frame, it fits. 3. Is 165mm crank too short for gravel since GRX only goes as low as 170mm? Depends on your leg length and pedaling style. The Diverge has a large BB drop so it's a good match for shorter crankarms. I've got the same crank length on my road and gravel bikes. 4. Is it better to use GRX crank or can Ultegra function just as good? I like having gearing lower than 50/34. That is a personal choice based on riding style and conditions. 5. Will the GRX crack function well with an Ultgra Di2 FD? The GRX designed chainline is offset +2.5mm. I don't know if the Ultegra FD has enough adjustment range to make it work.
Angstrom wrote:So My questions: 1. will gearing work with Ultegra 50/34 165mm crank, Ultegra FD Di2 and GRX RD-RX815, ST-RX815 shifters ? Ultegra crank and FD should be fine. 2. Does it make sense to get sense to get a FC-RX810-2 crank instead of the Ultegra 50/34 , is there pros and cons to GRX crank? The GRX crank gives you the option of using smaller chainrings. The Ultegra can only go down to a 34T. As mentioned, the GRX crank has a wider spindle/offset chainline(2.5mm each side) for clearance with frames with wide stays for wide tires. If the Ultegra fits your frame, it fits. 3. Is 165mm crank too short for gravel since GRX only goes as low as 170mm? Depends on your leg length and pedaling style. The Diverge has a large BB drop so it's a good match for shorter crankarms. I've got the same crank length on my road and gravel bikes. 4. Is it better to use GRX crank or can Ultegra function just as good? I like having gearing lower than 50/34. That is a personal choice based on riding style and conditions. 5. Will the GRX crack function well with an Ultgra Di2 FD? The GRX designed chainline is offset +2.5mm. I don't know if the Ultegra FD has enough adjustment range to make it work.
Thank you so much for all the detailed answers, helped out a lot.
I just switched to Ultegra r8000 cranks on my bike. I was previously using 172.5mm FSA cyclocross cranks with 46/34 gearing. I had a set of 6800 Ultegra 170mm cranks on my road bike so I tested the fit on my gravel bike and really preferred the length. I went with the Ultegra 50/34 170mm for the gravel bike instead of the GRX because i think they look better, and I prefer a narrow q factor. I purchased a 46t big ring for it because I think it's more useful gearing for me, but the 50t is good for road rides. (I'm in a mostly flat area with nothing bigger than moraine hills in the Michiana area) I'm using a Campagnolo Chorus front derailleur with Ultegra 6800 shifters and it all works well together.
FYI, I find 175 works when I need a lot of torque, and 165 when I need a lot of high rpm horsepower. So, I have 175 on my tandem and mountain bike, 165 on my track bike (>150rpm).
