Internal Cabling Problem? I've been having some bike issues with a 2016 Trek Allant 7.4 . I'm looking for any documentation about this problem, or any known solutions that don't include externally routing the cable.

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Long story short is that the gears aren't shifting properly. I've seen it up on the mechanic's stand and within a few minutes he was able to downshift on the shifter and the derailleur didn't budge at all.

When I ride you have to downshift one past the gear you want and then upshift again to reliably get it into gear. 

If you downshift a few gears at once, like you might when going up a hill, downshifting one more than you need and then up again doesn't always seem to get you into the gear that's selected on the shifter.

What happens then is that the derailleur changes the gears at some unexpected time to the gear that is actually selected on the shifter. There's a crunch that I'm sure isn't great for the cassette or the chain, and your weight gets thrown off balance.

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The mechanic I spoke to said that this was a problem with the design of the Allant 7.4. The shifter cable comes out straight at the same angle from the chain stay. It gets bent upward and sits in contact with the quick release nut. The mechanic told me that the problem was due to the sharpness of the angle that the cable was routed after it exited the chain stay.

The same mechanic told me that this problem was addressed in later models. I've attached a photo of the Allant 7.4 and FX-4 routing and you can see that the FX-4 angles the cable upward at the exit from the chain stay. It looks like the FX-4 routing is along a much more gentle curve as well.

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The shop I bought the bike from has replaced every single component involved in the rear gears (possibly not the external sheath at the back? The manager told me this couldn't be the cause of the problem). The cassette, derailleur, chain, shifters, shifter cable core (now using a thinner core) have all been replaced.

The bike has been looked at by 5 mechanics: 3 mechanics from the store that retailed the bike and stock only Trek bicycles, and 2 mechanics who are Trek employees. 

This experience has really put me off internally routed cabling and Trek bikes in general to be completely honest.

Does anyone know of any documentation around this issue? Are there any known fixes?


https://archive.trekbikes.com/au/en/2016/Trek/allant_7_4#/au/en/2016/Trek/allant_7_4/details


https://www.trekbikes.com/au/en_AU/bikes/hybrid-bikes/fitness-bikes/fx/fx-sport-4/p/21559/?colorCode=grey


The cable doesn't flare out to the right that much, I think my phone's camera is giving a bit of a fisheye effect up this close.
I agree with Nathan Knutson's comment that this looks like cable friction, but I suspect confirmation bias has everyone looking at the wrong portion of the cable routing.


  The mechanic I spoke to said that this was a problem with the design of the Allant 7.4. The shifter cable comes out straight at the same angle from the chain stay. It gets bent upward and sits in contact with the quick release nut. The mechanic told me that the problem was due to the sharpness of the angle that the cable was routed after it exited the chain stay.


The rear derailleur cable routing isn't perfect, but it does not look that bad either. There is nothing inherently wrong with the cable housing touching something.  The problem can arise if the housing is severely kinked, which doesn't appear to be the case, I beleive the manager is correct in that this is likely not the culprit.

You also said:


  The shop I bought the bike from has replaced every single component involved in the rear gears (possibly not the external sheath at the back? The manager told me this couldn't be the cause of the problem). The cassette, derailleur, chain, shifters, shifter cable core (now using a thinner core) have all been replaced.


This still leaves a couple possibilities:


Cable housing from the shifter to the frame has a kink.  You made no mention of whether or not the cable housing (cable outer) was changed.  If the housing is a bit short and/or the handle bars get over rotated this can damage the housing.  This however should have been obvious from inspection, and I would hope a reputable shop would have looked here first, long before cassettes, derailleurs and shifters!
Inside the frame, you are missing a sheath or the cables inadvertently became entangled during assembly. If the cables are tangled, then this would cause extra friction. You might be able to see this by activating one shifter and looking for movement on the other derailleur. Friction could also be increased if an internal sheath is missing. Most internally routed systems will run the cable inside a sheath to reduce friction. If is missing, then the friction is higher. 
The shop changed the inner cable, but depending on how this was done, they might have missed these type of problem (e.g., you pull the new inner cable through by using cable that is currently in place).   


Typically, internal cable routing results in straighter cable runs and should provide better shifting for longer than external routing.