Hi, I can't shift into the lowest gear and actually, the shifter won't even go far enough to click onto that setting. I already tried loosening the L limiter screw (completely out) and that didn't have any effect. I'm thinking maybe there's some gunk in the shifter that's preventing the trigger from going all the way or from clicking into place on the last setting. Any ideas? I'm hoping there's an easier solution that taking apart the shifter.

You may want to check your cable tension if the shifter wont move into the lowest position. It may be too tight.. Once that is resolved, double check the lower limit screw adjustment again. Also make sure that your chain has adequate length.

Steve

Could be you connected the cable to derailleur with the shifter set on the second rather than first setting. Disconnect cable and make sure shifter is shifted all the way to first setting then reconnect

What has been said before. Make sure 1st speed is set before setting up cable. Did you make sure the number of speeds matches with your cogs (I know it can seem obvious but still...).

Hi All, thanks for the suggestions. I checked them all and unfortunately none worked. I'm assuming it's either shifter gunk or some internal problem with the derailleur, but either way that's beyond my abilities so I'll have to either take it to a mechanic or just make do without the lowest gear. Thanks again for the tips

Locate the problem. Disconnect the shift cable at the rear derailleur and then turn the crank by hand and pushe the rear derailleur in by hand. If it goes all the way to the largest cog (or perhaps beyond because you backed off the L limit screw) then the problem isn't with the derailleur. If you can't push it all the way to or beyond the largest cog, the rd is the problem.

Too add to Dave's very good start of the assessment process (know what you are actually checking, one part of the system at a time) try this. With the cable detatched hold the inner cable just as it exits the shift pod (pull back the casing enough for your fingers to grab the inner). Not while tugging on the inner cycle the shifter through it's range. If your system has 9 "speeds" (rear cogs) there should be 8 clicks between the 9 index points. With the cable free and held by your fingers it should be easy to count the position/clicks. Since we are clueless as to what kind of shifter you have or even how many cogs are in the system we can't give much more specific advise. Andy

The 2 posts preceding mine are exactly the way I would evaluate the problem

I tried this technique and confirmed that the derailleur is capable of moving all the way to the lowest gear

I also tried this technique and confirmed that the shifter is able to progress through 8 clicks (yes, I have 9 rear cogs). My thinking is that as the tension on the wire builds as I shift down to lower gears, the tension grows enough that the shifter can't lock in the final click. Is this the kind of thing that could be fixed by cleaning the shifter?

Simply try pulling the cable tight with your hand and see if it goes into 1st when pedaling. IF it doesn't, it's not the shifter.
Cable tension doesn't really increase that much until you try forcing it against the limit screw.

It sounds like you have the L screw on the RDER screwed in too much preventing the RDER from moving that last little bit.
You might try screwing it out 1/4 turn at at time and testing. Keep track of how much so you can reset it to where it was in case that doesn't fix it.
I have a hunch it will.

If this is the case and all else is correct then your cable tension is too high and the lever's ratchet can't fully engage when the der is in the largest cog. Note the "if all else" clause. One easy mistake to make is to miss run the cable past the anchor bolt. If this happens you've changed the intended amount of der movement with each click and after a bunch of shifts the lever's indexing won't match up with that of the der's positioning under the cogs. Check that the cable is properly routed on the correct side of the bolt. Andyt

the lower gears are the big rear cogs.... right?

Hi, I already tried unscrewing the L screw completely out and also I confirmed that the derailleur can go all the way when I unscrewed the cable and pushed the derailleur to the lowest gear (biggest cog) with my hand while turning the pedals with my other hand. So I'm quite sure the L screw can't be the problem

Hi Andy, I want to make sure I completely understand. You're saying that if I fasten the cable to the anchor bolt improperly, it will mess up the indexing, right? I ran the cable straight down, which runs past the left (inner) side of the anchor bolt. I have a Shimano Sora derailleur (if that helps).

If we run with the idea that the cable tension is too high...This may be a dumb question, but I thought that the barrel adjuster adjusts the cable tension, so if that's true how do I loosen the cable tension without throwing off the indexing?

I'm still a newbie, so I don't know if that's the official definition, but that's how I've been using it. Lower = bigger.

Will it go all the way by pulling on the cable?
That isolates it to shifter or RDER.

I've had this happen a couple times when I haven't gotten all the slack out of the cable. As a result what you think is the highest position on the shift lever is actually the second-highest. After doing this a couple times, I discovered this process:

My process for adjusting the derailleur and cable tension on indexed shifting systems:

Detach the cable.
Verify that moving the derailleur by hand moves the chain across all the cogs. Adjust the "L" and "H" screws so this is as far as the derailleur moves.
Shift the shift lever into the "highest" gear. Verify that the cable is fully seated in the shift lever and all of the cable housing is fully seated in their stops.
Manually shift the derailleur so the chain is on the second-smallest cog. Don't turn the cranks further- let the derailleur hang
Connect the cable to the derailleur, pulling out all the slack.
Verify that turning the cranks allows the chain to shift to the smallest cog. Fine-tune with the barrel adjuster so shifting between the smallest and second-smallest cog is consistent in both directions. Work your way up the cluster, shifting between adjacent cogs. Fine-tune as you go. If everything is in good working order you should be able to reach the largest cog when the shifter gets to it "lowest" position.

You are correct. I tried your technique and found some resistance in the derailleur that I couldn't easily overcome by pulling on the cable. Not sure if this will make sense, but I'll try my best to describe: If I push the derailleur by hand to overcome the resistance, it feels like I'm pushing past some solid debris, like a pebble lodged in there. I can eventually get past it by pushing, but it feels like a physical obstacle more than just increasing tension on the cable. What could this be...is it possible to have debris of some kind lodged in there?

Maybe it is a pebble?
Try turning the bike upside down and moving the RDER back & forth. Maybe it'll fall out?
Maybe something is simply defective???