Cranksets, chain-lines and all that
#1
Occam's Rotor
Thread Starter
Join Date: Jul 2013
Posts: 7,248
Mentioned: 61 Post(s)
Tagged: 0 Thread(s)
Quoted: 2366 Post(s)
Liked 2,331 Times
in
1,164 Posts
Cranksets, chain-lines and all that
The new GRX crankset supposedly has a +2.5mm chain-line relative to Ultegra, with the implication being I need to buy a new front derailleur if I want a GRX crankset.
Is this accurate?
I replaced my Di2 Ultegra crankset awhile ago with a White Industries square taper 46/30T.
Here are the chain-lines for the three cranksets:
Chain line
Shimano Ultegra 43.5 mm
Shimano GRX 46.9 mm
White industries 47.5 mm
So, two things are evident:
(1) I introduced a bigger change in chain line with my switch to the WI crankset. The Di2 Ultegra derailleur copes with this just fine, so presumably a Shimano crank with a slightly smaller difference in chain line would be even less problematic.
(2) 46.9 - 43.5 mm = 3.4mm, not 2.5. WTF?
Is this accurate?
I replaced my Di2 Ultegra crankset awhile ago with a White Industries square taper 46/30T.
Here are the chain-lines for the three cranksets:
Chain line
Shimano Ultegra 43.5 mm
Shimano GRX 46.9 mm
White industries 47.5 mm
So, two things are evident:
(1) I introduced a bigger change in chain line with my switch to the WI crankset. The Di2 Ultegra derailleur copes with this just fine, so presumably a Shimano crank with a slightly smaller difference in chain line would be even less problematic.
(2) 46.9 - 43.5 mm = 3.4mm, not 2.5. WTF?
#2
working on my sandal tan
Join Date: Aug 2011
Location: CID
Posts: 22,625
Bikes: 1991 Bianchi Eros, 1964 Armstrong, 1988 Diamondback Ascent, 1988 Bianchi Premio, 1987 Bianchi Sport SX, 1980s Raleigh mixte (hers), All-City Space Horse (hers)
Mentioned: 98 Post(s)
Tagged: 0 Thread(s)
Quoted: 3868 Post(s)
Liked 2,560 Times
in
1,574 Posts
2.5mm (or even 3.4mm) isn't all that much. Seems like you'd just need to adjust the FD for a little more throw and make sure it isn't too high.
#3
Senior Member
Join Date: Sep 2011
Posts: 4,764
Mentioned: 28 Post(s)
Tagged: 0 Thread(s)
Quoted: 1975 Post(s)
Liked 232 Times
in
173 Posts
The main difference is in the tire clearance. If it clears your tire right now it should be ok. It's especially an issue with the 4700/5800/6800/9000 long arm fds
#5
Senior Member
Join Date: Sep 2017
Posts: 7,768
Mentioned: 37 Post(s)
Tagged: 0 Thread(s)
Quoted: 6882 Post(s)
Liked 10,874 Times
in
4,638 Posts
Likes For Koyote:
#6
Occam's Rotor
Thread Starter
Join Date: Jul 2013
Posts: 7,248
Mentioned: 61 Post(s)
Tagged: 0 Thread(s)
Quoted: 2366 Post(s)
Liked 2,331 Times
in
1,164 Posts
As @Koyote said, I am just curious.
The Shimano GRX descriptions all make a big deal of the chain line difference, and how this necessitates getting a different front derailleur. When I made my switch, the chain line difference was significantly bigger, and there was no issue. So for others who might want to make the switch, it is worth keeping in mind that it might not be any more complicated than changing the crank.
#7
Senior Member
Join Date: Sep 2017
Posts: 7,768
Mentioned: 37 Post(s)
Tagged: 0 Thread(s)
Quoted: 6882 Post(s)
Liked 10,874 Times
in
4,638 Posts
I appreciate that info, @wgscott . I just took delivery of a new bike with Ultegra Di2, and we decided (at the last minute) to put on a placeholder crankset from the parts bin which will be replaced with the GRX 48/31 when it is available. My shop ordered the new GRX fd as well, just in case...But it seems (from your experience) that my road Di2 fd may work just fine.
I’m also expecting the RD RX805 rd to work, too.
I’m also expecting the RD RX805 rd to work, too.
#8
Senior Member
Join Date: Feb 2014
Location: Pasadena, CA
Posts: 4,847
Bikes: Schwinn Varsity
Mentioned: 22 Post(s)
Tagged: 0 Thread(s)
Quoted: 1931 Post(s)
Liked 741 Times
in
421 Posts
Could this + chainline be better for 142mm rear axels. I don't know what most gravel bikes are now. Some are 142mm.
#9
Very Senior Member
Join Date: Mar 2018
Location: Always on the Run
Posts: 1,211
Bikes: Giant Quasar & Fuji Roubaix
Mentioned: 4 Post(s)
Tagged: 0 Thread(s)
Quoted: 413 Post(s)
Liked 343 Times
in
244 Posts
Rather than change out your front DR, which might entail different cable run and set up, why not change out the BB for the proper length to make the existing front DR correctly align with the front sprockets? Or will that throw off the chain alignment to the rear cassette? Or am I way off base here? At my age I'm easily confused.
#10
Occam's Rotor
Thread Starter
Join Date: Jul 2013
Posts: 7,248
Mentioned: 61 Post(s)
Tagged: 0 Thread(s)
Quoted: 2366 Post(s)
Liked 2,331 Times
in
1,164 Posts
I appreciate that info, @wgscott . I just took delivery of a new bike with Ultegra Di2, and we decided (at the last minute) to put on a placeholder crankset from the parts bin which will be replaced with the GRX 48/31 when it is available. My shop ordered the new GRX fd as well, just in case...But it seems (from your experience) that my road Di2 fd may work just fine.
I’m also expecting the RD RX805 rd to work, too.
I’m also expecting the RD RX805 rd to work, too.
#11
Member
My understanding is that the GRX line is compatible with most Ultegra and 105 components. That was what I heard during a podcast with some Shimano reps. The goal was to make it easier to upgrade to different parts of the group set without having to do it all at once. If you want to switch to a GRX crankset, I don't see why you would need the front derailleur. As long as the derailleur you have works with the largest size chainring then you shouldn't have to switch out the derailleur. What one person mentioned on here is true: the new GRX derailleur is really only necessary if you need the extra tire clearance.
#12
Senior Member
Join Date: Sep 2017
Posts: 7,768
Mentioned: 37 Post(s)
Tagged: 0 Thread(s)
Quoted: 6882 Post(s)
Liked 10,874 Times
in
4,638 Posts
#13
Member
Same here. Not sure what the actual difference is between an Ultegra and a GRX front derailleur. Could be just a matter how the derailleur is positioned off the bracket, basically making sure they keep the components tight and away from the back wheel...especially when mounted off a seat post.