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I just received a bike from a bike shop. It has a GRX800 front derailleur (FD) and rear derailleur (RD). When taking a look at the derailleur setup to check if it is alright, I found something very odd.

When I am on the outermost chainring and release tension from the RD (shifting up, to the smaller sprockets), the FD moves significantly inwards (as if I'm shifting down). In fact it moves so much, that I have to choose between the highest gear working, or the T-trim position working.

  • Before going back to the dealer I'd like to know if this might be a feature I'm not aware of?
  • If not, what could cause this?
    • Maybe "entangled" shifting cables within the frame?
  • If yes, why would this be? I would rather expect lower RD tension to increase tension on the FD follow the chain which now moves outwards.

I'm happy to hear your thoughts on this!

EDIT: Here is a video of it.

Moving derailleurs

EDIT#2: I returned the bike to the bike shop, but was able to take a photo of the internal routing through a little hole in the frame which I didn't see before. Here it is, for you to look at :)

twisted pair shifter cables

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  • Only way I can think happens is if the chain is holding the FD back. If that case, the FD is badly out of adjustment. Does the chain run on the FD cage?
    – mattnz
    Sep 3, 2023 at 1:39
  • No, this happens when the chain is not in contact with the FD cage and the RD is shifted to the smallest sprocket.
    – Daniel
    Sep 3, 2023 at 1:47
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    That's weird - if it were the other way around (changing chainring affects rear derailleur angle) then that would make sense, but not like this. Does your bike have internal cable routing? cos this kinda smells like bare wires crossing somewhere and friction at work.
    – Criggie
    Sep 3, 2023 at 2:50
  • Yes, the bike has internal cabel routing. So I guess my first guess was correct. Thanks, I'll let the shop take a look!
    – Daniel
    Sep 3, 2023 at 9:56

1 Answer 1

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The cables are touching or entangled in the frame. This happens sometimes when re-routing a bare internal cable from scratch - it can be hard to avoid when shooting blind. The real error was letting it out the door that way. Have the shop correct it.

The physical explanation in your example is that the RD cable is wrapped around the FD cable inside the frame, such that the more tension is on the RD cable (the further up the cassette you are), the more tension gets added to the FD cable (the further it's pulled away from being in a straight line). Thus releasing tension from the RD cable also makes the FD cable closer to a straight line, which is the same as reducing the tension on it, hence moving the FD in.

It's moderately likely that both cables have acquired kinks from this. Minor kinks on bare sections of shift cable aren't the end of the world and usually inconsequential, but aren't exactly what you want on a new bike, and you'd have to get a visual on them to know they were minor anyway. Ideally you'd get two fresh ones.

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