Shimano GRX- How intercomaptible with Ultegra/105

I have been thinking, which is not my strong point, that my gravel bike is missing a couple of gears at the bottom. It currently has 11-34 cassette and 50/34 chainrings.

I am more than happy with the shifters and brakes but I was wondering if the pull ratios are different on GRX preventing buying a crankset and fitting. Also, would I need to change front mech too to accommodate the 46/30 rings?

Being doing a bit of reading and I suspect I will need to buy the GRX front mech due to the 2.5 MM chain line difference.

I brought a GRX 46/30 front rankest and fitted it no problem using my tiagra front mech. it had plenty of adjustment and accommodated the 2.5mm difference without issue.

You could opt for a bigger cassette instead. My Ultegra mech is currently working fine with a 11-40. It’s outside Shimanos published recommendation but lots of folk are running it no problem.

I couldn’t get my 105 mech to work with GRX chainset, I guess it depends on frame and BB tolerances.

My experience is the opposite of Scotroute’s, personally I would consider a bigger cassette as a last resort, I’m forever having to adjust the shifting on my GRX 11-40 rear setup, it just seems so much more sensitive to small things and the bigger/lower hanging rear mech is a mud and undergrowth magnet! Will go back to 11-34 1x for winter CX duties then put 2x and front mech on for summer gravel duties 😎

I’d just go down the super compact crankset and if necessary the front mech route, all the new shimano 11 speed stuff has the same pull ratio AFAIK

30/34 is incredibly low gearing for a non-mtb, not saying its a bad thing mind, I’ve got it on mine 😁

There are a number of super compact chain sets that aren’t GRX, but are normal ‘shimano’ compatible. My topstone has a an FSA 46/30 with a 105 FD.

I’ve also got an 11-40t cassette on with the clutched ultegra rear derailleur with no issue.

#clydesdale with one partial knee replacement :/

105 Front Mech and GRX 46/30 chainset dint work for me either. I bought the GRX 600 front mech tho not fitted either yet as still running that bike in road mode (11-28 + 50/34). When in gravel config, I’m running 11-40 + 50/34 105 front and rear dérailleurs with a copy road link. All gear combos available, no issues.

My Diverge was pre-GRX so has a 105 groupset with a 48/32 Praxis chainset to get smaller chain rings. Works really well, so well I’m not really sure what the point of GRX is.

It is a difficult one. I imagine the easiest thing to do is to go for the cranks and is the FD is not capable of being adjusted to get the GRX mech. I am not sure I want to go down the 40 tooth cassette route, too may uncertainties and I think I may miss the closer ratios 11-34 gives.

30/34 is incredibly low gearing for a non-mtb,

That rather depends on what you’re using your non-mtb for. Many of us are doing what we’d have done with an mtb anyway.

My experience is the opposite of Scotroute’s,

Sure. But see also: disc brakes and tubeless tyres.😅

Aaah yeah, I’d forgotten…

I’ve one of those Praxis 48/32 chainsets on another bike with an 11-40 cassette. Ultegra FD/105 RD. Works fine, all combos no bother.

My wife spins out on compact chainsets as she has a low cadence. So I’ve just removed the GRX chainset from her new Diverge & fitted a 105 50/34 from her winter road bike. After a bit of fiddling the GRX front mech only rubs the inner cage in the 34 cassette & 30 chainring but it wont adjust any further in.
Is there any issues to watch out for going to the 105 chainset with the narrower chain line? There is plenty of clearance between the frame & chainrings.

I have an Ultegra 36/46 chainset, gives pretty decent winching gear with an 11-34 on the rear. As Scotroutes said, Shimano’s claimed ranges are pretty conservative.

my commuter bike (a cross bike that was really designed as a gravel bike 10 years + before they ‘invented’ gravel bikes – but now mostly dorbed in the rear rack and guards) I’ll be jumping on in a minute has:-

GRX 800 chainset (46/30) and front mech
105 11spd rear mech
105 10spd TRIPLE shifters
Miche 10spd cassette (12-28)

The front mech and shifter are set up using the middle and top ring positions on the STI and works fine. The miche cassette has a couple of snickerty sprockets that are annoying the hell out of me and might get sacked off but I think that might be a poorly timed chain change that is too new for the life left in the cassette.

It works fine with a 11-32 cassette too but I don’t need the range on the road. I think a 11-36 could probably be got away with too but tbh 10spd options are a bit more limited and I don’t plan to upgrade the STIs until they die.

Another +1 for bigger cassette.

Commuter is running 11-42 sunrace, ultegra mech and grx 42t chainring. Sweet as with minimal to 0 adjustment over 3k miles

Bike 2 is 48T wolftooth with 11-40 cassette & ultegra rear mech. Shifting again sweet as and well outside tolerance. Over 5k miles with that and all i had to do was fit a chain guide to stop the occasional chain drop. Again minimal adjusting once set up.

Both bikes get me up 25% although the 48t definitely borders on unpleasant at that grade!