the sci guy 

It's a GRX derailleur. It'll shift down to the 2nd highest, but then even though the shift lever pushes over, nothing happens. It will go all the way up to the big ring, though. It also skips around a lot as you move up and down through the cassette - it won't stay on certain cogs or will want to bounce back and forth between them.
It all started because I tweaked the barrel adjuster a tad as it was initially skipping a tad on a few and sounded a little clicky like it wasn't just right. Then it snowballed from there and it's all gone to hell.

halp pls.

fietsbob

All the engineering done to speed shifting works against you,

if the derailleur hanger alignment and index shifting adjustment, is not spot on.

dsbrantjr

It sounds like you need to start at the beginning and use a systematic approach to the issue. Here is a good procedure: https://www.parktool.com/blog/repair...eur-adjustment But as fietsbob suggests a properly-aligned derailleur hanger is the basis for smooth shifting.

Erzulis Boat 

The classic "Won't go to the smallest cog"- Very common.

Remove the chain, and start from scratch.

You need to get both the hanger interface and both limit screws (high/low) in order first.

There is a trick to the limit screws adjustment. On the smallest cog, you need to actually have the top derailleur pulley go "past" (as in farther outboard) than directly below (in line) the cog. Typically, it is about a cog width of outboard offset that does the trick.

The reason that you cannot get into the smallest cog is that due to deflection the chain is fighting itself basically, and even though technically it should work, it simply doesn't!

Align the upper pulley of the derailleur directly in line with the biggest cog however, as this issue only manifests itself with the smallest cog. Once you have both stops adjusted, go through the gears (chain is still not installed) about 10 times to settle everything in, and to triple check.

Miele Man

Check your cable housing to see if the strands are poking through the ferrule or if a ferrule is missing.

Cheers

BoraxKid

After you verify that your derailleur hanger is straight, I would undo the shifter cable at the pinch point, reset your barrel adjuster, and follow the Park Tool video instructions for setting up a rear derailleur. This should be the fastest path to restoring your bike to its pre-barrel adjuster change state.

It sounds like you turned the barrel adjuster the wrong way to start with, and now you have lost your reference point for correcting it. The issue of not reaching the smallest cog could be caused by too much cable tension (from the barrel adjuster changes you made), an improperly set limit screw (which you did not mention changing, so less likely), a bent hanger (could have caused the original problem), or some other reasons (cassette precession, wheel not properly seated, play in the hub bearings, etc.). Start with the derailleur setup video and work from there. Good luck.

the sci guy 

I didn’t want to go all-in and take the chain off and start from scratch since this is a new out of the box bike and I was hoping it would pretty close to all set to ride.

I futzed with it a little more tonight and I got it to shift smoothly up and down the cassette but it still won’t go to the last cog. The shifter paddle pushes all the way but nothing happens. The chain appears to run smoothly through every other gear. The hanger is not bent as far as I can tell.

I’ll check again tomorrow but if I twist the barrel so it skips to the last cog, then adjust it each time moving back up the cassette, will that help?

trailangel

Align the DR hanger before proceeding. You are wasting your time.
New out of the box is not straight.

tFUnK

Check the b limit screw also. Even if it's new, it sounds like either something is wrong (eg, hanger) it it was never adjusted properly to begin with.

dsaul

It sounds like you turned the barrel adjuster far enough out to skip one whole cog, so you can shift all the way up until it hits the limit but you run out of clicks going down the cassette. Shift down until the shifter doesn't click any more and then turn the barrel adjuster in(clockwise) until the chain shifts to the smallest cog and start from there.

In my experience, the derailleur hanger has to be bent pretty badly to prevent you from being able to get it to shift correctly. As long as it is not noticeably bent and it hasn't taken a recent hit from a fall, its probably not the problem

trailangel

In my experience, when the derailleur hanger is bent very slighty, it will prevent you from being able to get it to shift correctly.

Erzulis Boat 

Ok. You do not want to remove the chain. Fair enough. If the limit adjuster isn't set properly, NO amount of playing with the barrel adjuster will fix your issue. And, if it's a barrel issue, this will get you back to ground zero. Win/win!

1- Run the shifter all the way down until it won't "click" any more.

2- Loosen the barrel adjuster so that the cable is slack between where the cable housing is anchored to the derailleur body and the clamp for the cable.

3- Go back to the lever, and try to click it down some more. IMPORTANT.

4- Start rotating the cranks, and see if it still doesn't want to engage the smallest cog, we are almost done!. At this point, it will either not drop down at all, or start going "Gung...Gung...Gung" (in a metallic fashion ) Like it wants to drop down, but it just won't.

5- Find the limit screw on the derailleur that allows the body to go farther outboard (away from the bike). Start by rotating it about 1/2 turn, and rotate the cranks again. If it is still not there (as in going into the smallest cog), allow more movement outboard until it hits the stop. KEEP THE CABLE SLACK!

6- Once again, go back to the shifter, and click it all the way down. We are just making sure again.

7-IMPORTANT. Make sure that the derailleur cable is still slack during this entire process. If it get's tight while you are adjusting the limit screw, you are not at the mechanical limit (metal/metal).

8- Ok, it's now going all the way down, and it's running smooth! The cable is still slack, so that means that we have a true mechanical stop. Almost done!

9- Tighten barrel adjuster to the point where it just goes taught.

10- Rotate the cranks, and go through all the gears.

NOTE- During this process, you might have to loosen the cable clamp to allow slack, as the barrel adjuster might be "bottomed" out. If it is, DO NOT leave it in this position (bottomed out) before you re-tighten the cable clamp. Back it out around one full turn, as this gives you some breathing room for fine tweaks to the system.

billridesbikes

Look, if the shifting was mostly fine and you tweaked the cable tension and now it’s not fine, the problem is the cable tension. Use a work stand and loosen the cable tension (clockwise on the barrel adjuster) until it goes into the smallest cog and then indexing the shifter tighten the cable until it shifts from cog 1 to cog number 2, continue through all the cogs. Repeat from the largest cog to the smaller cog making counter-clockwise adjustments on the barrel shifter.

If the index shifting still won’t work right go through this check list:
1) The derailleur hanger is bent
2) The chain is too long/short
3) cables are worn/ or have a kink
4) The sprockets and/or chain are worn out
5) cable incorrectly clamped to rear derailleur

Finally, if none of that works buy the latest Dura-Ace Di2 group set and enjoy adjustment free shifting.

the sci guy 

actually i don't run out of clicks coming down, as i said above i can still move the shifter paddle all the way to the left, but nothing happens, so it just stays on the 2nd from last small cog.

Iride01 

Is everything GRX?

GRX is intended for bikes built to handle wider tires and the chainline is moved out further. So if you are adding GRX into something that wasn't before, there might be something off in that aspect. Though it would seem most would favor you getting into the small rear cogs.

But otherwise, any time I hear about not being able to get to the small cogs I suspect cable frayed in STI or elsewhere as that has happened to the bikes I work on 3 times in the last 3 or so years.

Erzulis Boat 

If everything in your system checks out, I posted the fix above.

the sci guy 

Yes everything is GRX. Like I said above it's brand new out of the box bike - like it literally showed up at my house in the box and I put it together.

I read through that but the issue is that i can't run it down until it doesn't click anymore - i wrote in my post above that the paddle still moves the full pull but nothing happens. like it knows it has 1 more shift, but can't get it to do it. so i can't get past your step 1.

Erzulis Boat 

Then go to step 2 and loosen barrel adjuster to full slack. If it doesn't go slack, then loosen the cable clamp fastener.

Erzulis Boat 

That is why I have step 3!

Iride01 

Is there a bare cable you can hold onto while shifting down? I'll hold the cable running along the downtube between my fingers while shifting down when trying to find such things. If it you get more slack when you get to the problem cogs, then you know there is something up on the DR side with it not being able to get there. Still maybe a housing kink or badly shaped housing at the cut on the housing between your fingers and DR.

If you don't get any slack, then maybe your cable put up too short in the holdfast.


I've been ignoring the chain skips and sounds clicky parts of you comments as that might just be the result of the need for the things mentioned above. But maybe that needs to be explored more.

cyccommute 

First, eyeball the rear hanger to see if it is straight. It probably is but check just in case.

At this point, DO NOT GRAB A SCREW DRIVER!!!! Plug up your ears, oh Ulysses, and ignore the Siren’s song. Death on the rocks only awaits you if you grab that screwdriver!

I doubt this is your problem, however. It’s probably cable tension because shifting problems are almost always cable problems. In your case, I suspect that your cable is too tight. Shift to the high gear (lowest cable tension). The cable should be taut but not tight. If you pluck it with your finger it should vibrate but it shouldn’t be like a guitar spring. If the cable is tight, back off tension some and watch what happens with the jockey wheel below the cassette.

It should move outboard (if the cable is too tight). If the cable goes slack (and if the alignment is right) and the jockey wheel doesn’t move outboard, now you can grab that screwdriver that I know you want to hold, caress, and use (What is it about screwdrivers and bicycles that make people what to use one for “fixing” a bike?) Turn the high limit screw enough to move the derailer so that the jockey wheels are aligned below the cog.

Once you got the limit screw adjusted, grind the damn thing off so that you aren’t tempted to grab that irresistible screwdriver again! (Kidding but only slightly)

dsaul

It's been a while since I had a Shimano shifter, but I think the shifter paddle will still move after it runs out of clicks. If it doesn't click, you are in the lowest gear position on the shifter.

Iride01 

Yes, you are correct about that. At least with what I've experienced.

Also a good reason to hold on to the bare cable as you click the paddles to know how the cable reacts to each click. Sometimes even after having reached it's limit it might feel like it's giving a click still.