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Cranksets, chain-lines and all that

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Cranksets, chain-lines and all that

Old 05-11-19, 03:04 PM
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Cyclist0108
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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?
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Old 05-13-19, 10:01 PM
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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.
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Old 05-13-19, 10:24 PM
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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
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Old 05-14-19, 10:06 AM
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Why are you thinking of switching from a WI crankset to the new Shimano one?
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Old 05-15-19, 06:17 AM
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Originally Posted by TenGrainBread
Why are you thinking of switching from a WI crankset to the new Shimano one?
I'm not sure that the OP ever said (wrote) that he was considering that switch. It might just be curiosity. And I'm curious, too.
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Old 05-22-19, 09:36 AM
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Originally Posted by TenGrainBread
Why are you thinking of switching from a WI crankset to the new Shimano one?
I'm not, but I might get one for my wife's bike.

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.
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Old 05-22-19, 09:48 AM
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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.
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Old 05-22-19, 10:02 AM
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Could this + chainline be better for 142mm rear axels. I don't know what most gravel bikes are now. Some are 142mm.
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Old 05-22-19, 10:06 AM
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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.
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Old 05-22-19, 10:12 AM
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Originally Posted by Koyote
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.
One thing I should mention is my Ultegra Di2 front derailleur is clamp-mounted on a constant-diameter (steel) seat tube, so it might have more adjustability. Also, I have a 135 rear axle, fwiw.
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Old 05-22-19, 10:22 AM
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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.
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Old 05-22-19, 12:43 PM
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Originally Posted by wgscott
One thing I should mention is my Ultegra Di2 front derailleur is clamp-mounted on a constant-diameter (steel) seat tube, so it might have more adjustability. Also, I have a 135 rear axle, fwiw.
Ditto x 2!
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Old 05-22-19, 10:06 PM
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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.
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