Noonievut

I want to understand this better, as I have a specific issue I'm trying to resolve.

First for my understanding
- turning the barrel adjuster (at the RD) CCW moves the chain inwards towards the frame, is that loosening cable tension?
- I don't understand how cable tension impacts shifting (I just know it does, but I don't understand why/how). I believe turning the barrel adjuster CCW moves the chain inwards, and CW moves it outwards, and depending on in which direction there is poor shifting or too much noise you turn it one way or the other (I know which way to turn it), but that impacts cable tension...so back to above on my lack of understanding around what's happening with tension (tighter = towards the frame would be my guess, as the cable is shorter)
- I understand better the two limit screws and the B screw

If when indexing gears to resolve some poor shifting, or the chain rubbing on a sprocket in the cassette, I am turning the barrel adjuster (at the RD) CCW, and it un-threads, do I need to shift into a particular gear before I thread the barrel adjuster all the way in and then loosen the pinch bolt to pull out cable slack (and then back the barrel adjuster CCW out a turn or so)? I ask because sometimes I have the indexing almost perfect (smooth shifting, no noise)...and that last turn CCW un-threads the barrel adjuster and I feel like I'm going back to square on in indexing...so back to my understanding of cable tension, I'm confused as to what's happening with the cable.

curbtender

Clockwise threads the adjuster in causing less tension. How about you say towards the wheel? Drop your derailleur down to the smallest cog and take slack out of the cable at pinch bolt. Then pop the gear up once and adjust cable to catch the next cog trying to center it in the middle between 1 and 3 cog.

cxwrench

You have it backwards. And you have a basic misunderstanding that most people have. The cable always remains the same length, the housing effectively becomes shorter or longer depending on which direction you turn a barrel adjuster. The cable doesn't actually become tighter or looser but that's how many people describe what they're doing. If you turn the barrel adjuster on the rear derailleur CW the housing gets shorter and the rear derailleur pulleys move towards the frame or 'high' (harder) gears. If you turn the barrel adjuster CCW the opposite happens. The housing gets longer and the derailleur pulleys move towards the lower (larger or easier) gears.

You are correct about unscrewing the barrel adjuster. If it comes out you should shift to the highest gear (smallest cog), undo the cable, screw the adjuster back in all the way, then back it out about a full turn. Then verify the shifter is in the high gear position attach the cable. Actuate the shifter once and observe what happens. If the chain doesn't make the shift you obviously need to turn the adjuster CCW. If it does shift and you have some noise I will gently push the derailleur with my thumb...if the noise stops I need to turn the adjuster CCW. If the chain immediately goes to the next larger cog I need to turn the adjust CW to center it over the second cog.

In layman's terms: If you make the cable 'tighter' the derailleur will move to the larger cogs, if you make it 'looser' it will move to the smaller cogs. You can watch what happens when you turn the barrel adjuster, it's very easy to understand.

Noonievut

Thanks cxwrench

I'm getting it...still needs to sync in though. Your layman's terms make 100% sense (I see it moving, now I know tighter = CW = derailleur will move to the larger cogs)

Two follow-up questions (bike isn't in front of me so I'm not actually trying this as of yet):

"shift to the highest gear (smallest cog), undo the cable, screw the adjuster back in all the way, then back it out about a full turn"
- when you say undo the cable, I assume you mean at the pinch bolt? Or do you just mean the barrel adjuster (it's already 'out', so nothing to undo)

"Then verify the shifter is in the high gear position attach the cable"
- I would already be in the small sprocket (highest gear) as I would have moved it there when I shifted to the highest gear...are you saying that when I do the previous steps it might shift on its own to a lower gear?

Andrew R Stewart 

cxwrench's reply has it pretty much spot on. (With the exception of the Rapid Rise/"Demise" Shimano ders of a few+ years ago. These are reverse acting designs and thankfully now off the market).

As to which cog does Shimano suggest doing this "index adjustment of cable casing length"- They say the second smallest cog. I find I generally also check and fine tune on the second largest cog. Now Campy use to say, for their early Ergo designs, to adjust cable/indexing on a central cog. This to distribute the tolerances evenly across the cog set. I don't remember if SunTour ever gave instructions about this and I also don't remember if SRAM does.

Do know this all assumes that those other aspects are within spec first. Like B screw and guide pulley/cog gap. Like hanger (or really cage) alignment. Like cable anchor done right. Like cable friction issues (and this is the biggie as it changes over time and exposure). Andy

cxwrench

No...you're still backwards. CW 'loosens' the cable moving the derailleur to move to the smaller cogs. CCX 'tightens' the cable, moving the derailleur to the larger cogs. You need to put your eyeballs on this until you understand what's going on. The derailleur will move in the direction you're turning the barrel adjuster.

Undo the cable at the pinch bolt, yes. Turn the barrel adjuster all the way in, then back it out a turn. No, the shifter won't move on it's own but you always want to verify it's in high gear position, just a good habit to get into.

70sSanO

While the cable remains the same length, the cable housing essentially gets longer and shorter using the barrel adjuster which moves the rear derailleur. A longer housing kind of shortens the cable (not really but an illustration) but makes it tighter and a shorter housing lengths the cable and makes it looser.

The only thing I will add is the rear derailleur return spring. Maybe think of it less than CW and CCW, think of the rear derailleur spring wanting to move the derailleur toward the smallest cog (standard top normal RD’s).

So when you turn it CW the spring is pulling the derailleur toward the small cog, and CCW is forcing the cable against the spring tension to move it away from the small cog.

One thing that might help, once you have it set up pretty close is to set it in a middle cassette cog and turn the barrel adjuster CCW and watch it try to move to a lower cog. Then CW and it will try to move to a higher cog.

John

Noonievut

Thanks everyone!

This is a Shimano GRX RD (with a clutch), should the clutch be on or off when going through these steps...or does it matter?

dsbrantjr

Thank you, folks referring to "adjusting tension" instead of indexing drives me nuts.

mtbikerjohn 

I believe Shimano calls for having the clutch off when setting your adjustments.

Chuckles1

Getting the cable clamped at just the right point makes the rest easier. I shift to smallest cog with barrel adjusters close to all the way in (clockwise), and manually push the derailleur a bit inboard on RD while pulling cable taut and tightening clamp. I think there's a tool I lack, third hand tool or some such, that makes that maneuver easier. It sometimes takes a couple tries, but cable tension is critical to good performance.

cxwrench

I can't remember the last time I used a third hand tool for this. You can do the same thing by 'cheating' the high limit screw like you would on a front derailleur. Turn it in a full turn or turn and a half, attach the cable, then back it out. Usually getting the cable attached correctly to a rear derailleur is easy if it's limit screw is adjusted properly.

Noonievut

OP here. I went for a long ride this morning and everything held up well after following the tips shared here - thanks!