531colin wrote: ↑26 Jul 2021, 7:51pm
45mm from the frame centre to the centre of the gap between the 2 chainrings is a "road" chainline.
Your GRX F. mech. apparently expects the chainwheels to be 2.5mm further away from the frame. ie. 47.5mm chainline.
2.5mm isn't very much....If it was me, I would be re-measuring!
This is exactly what my theory had suggested to me is the crux of the problem ie. the the gravel FD is 2.5mm further out from the frame than a road FD would be, BUT, the chainset is a road one and therefore, relative to the FD, 2.5mm closer to the frame than it should be.
531colin wrote: ↑26 Jul 2021, 7:51pm
When you say
the inside do you mean its rubbing on the inside of the F. mech. cage or on the outside?
(or on the inside of the outside, if you get my drift). In other words, is your mech. too far in towards the frame or too far out away from the frame?)
AND another question....if the mech. doesn't go far enough out away from the frame, will it go further if you push it with your finger?
I find FD nomenclature a little confusing when discussing with people. I don't know if there is an "approved" way of talking about it so I'll describe it quite literally.
Let's say we are looking at the FD from BEHIND. ie. it has a right hand side that lies to the right of the chain, and a left hand side that lies to the left of the chain.
The FD shifts from big to little and vice versa absolutely perfectly and immediately. The limit settings are perfect (0.5mm between inside of right side and outside/right of chain when in the big ring as per Shimano manual).
When in big ring but shifting to 4th smallest cog in cassette, the chain rubs against the inside ie. against the left side of the FD, or to put it your way, the outside of the inside. I then have to engage high trim at this stage to move the FD more to the left/inside so there's a gap between chain and the left side/outside of the inside, rather than the usual case where you have to engage trim only when in the bigger cogs eg. 9/10/11th.
When I then adjust to make sure that the FD moves more to the left (towards the frame) so that the chainrub doesn't occur until the bigger cogs in the back, then of course when in big-small combo there is lots of chain rub on the right of the FD/inside of the outside. Other adjustments lead to chain rub in the little ring. I've followed all instructions, have tried microadjustments in multiple combinations, and two bike mechanics have tried the same with no luck.
In Youtube reviews of this bike, the reviewer mentioned he had chain rub out of the box too, as have multiple commenters. Tried to message a few of them and from the very few that replied, they've never managed to get rid of it as well, which to me suggests it truly is a compatibility problem and the 2.5mm does make a difference. Only problem is you have to actually buy the stuff and replace it before you can truly find out if this is true.
I called the shop I bought this from today. They are in Inverness, I'm in Manchester. Explained the problem and he said he couldn't imagine Giant would make a mistake because they are Shimano's biggest client apparently. I explained that Shimano apparently themselves say that only the GRX chainset and FD should be used in combination because of the 2.5mm difference between gravel and road, and that this problem is unfixable. He's forwarded the message to Giant to see what they say, but even if they are in the wrong, which I suspect they are, they do this with literally thousands of bikes and so are not going to say yeah, we'll pay for you to replace your chainrings, it will likely be some fob off reply which doesn't address the problem.
