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Derailleur moves the chain but won't stay in gear
Hi, I have an older road bike. It's a 12 speed with the gear shift on the lower bar. The rear derailleur seems to work, that is it shift from the small sprocket to the largest, but it will not stay in anything beyond the the small one. I have replaced the cable and adjusted the high/low as in the video (thanks) but nothing is working, it will not stay in any other gears.

Is the derailleur just popping back down to the smallest gear as soon as you let go of the shifter? If so, there's a little bolt on the outside of the shifter that screws it into the frame. Try tightening that up a little. It should be tight enough that the shifter won't move on its own, but loose enough that you can shift easily. Not a bad idea to open it up and grease the parts also.

I agree with Dave… Sounds like you may have friction-shifters? When you shift to a lower gear (larger rear sprocket) your tension on the gear cable allows this. However, the cable then slips back down (higher in gear terms) a bit. The reason it may stay in high gear (smallest rear sprocket) is because of the correct ‘h’ screw setting. You could see if it stays in the second-highest (second-smallest) rear sprocket if you tighten the ‘h’ screw…
Can you keep it in a gear with constant (hand) tension on the shifter? If you can shift I reckon the answer sounds like your friction shifter(s)(if you have them?) need a little work…

I have a similar problem, but it is with the front derailleur on a mountain bike. The cable will move it to the largest ring, but it won't hold it in place. I have not done a thorough inspection yet. Any ideas what would cause this?

You should take a look at the front derailleur tutorial. The derailleur settings might be off. Another possibility is, that the shifter is broken (or dirty). There is a ratchet mechanism that "locks" the gear in place. Cleaning and lubing the shifter might remove that problem.

Thanks Joe. I saw another post were someone cleaned theirs with WD-40, re oiled and it worked. The said the parts can gum up when idle, and my bike has sat idle for a while. I just started riding again because my son started.
I had some 3-1 I tried w/o cleaning. No success yet, so I will try the cleaning next. The cables feel fine. The components are 1990 Shimano Exage 400 LX. I know that sounds old, but everything still looks good on the bike - Trek 850.
I don't see a ratcheting mechanism that click, but I will keep playing with it before I give in to taking it to a bike shop! The mechanical engineer in me does not want to be defeated.

Progress. I believe it is my shifter that is the issue. I sprayed it full of a degreaser (liquid wrench) and then added a lubricant. If I hold the shifter in for a little while, it is taking hold on the 2nd and 3rd ring now. So maybe it will work itself back into good working order. I was unable to pop it open like they have shown in the tutorials or youtube videos. Anyway, so far, so good!

Things are working better this morning. I can shift up to the large gear with no problem. On downshifting, it jumps from the large ring to the smallest ring, so an adjustment will probably be needed.

I am having this problem, but because of my work schedule I don't have time to get it fixed in person. Is there any chance I can find the answer online after work so that I can find time in my schedule to resolve the problem?

It's a 28 inch 7 speed with 6 week new shimano derailleur and jammed grip shift replacement from sitting idle for 2/3 years.

I haven't tried adjusting H/L yet.

I checked chain tension, but it popped off spontaneously last night so I might need a new chain or to take out a link.

Cassette is ridden about 6 months back in 2020, then stopped while I was working until October 2023. I used to ride almost exclusively on gears 3,4,5, and sometimes 6. Now only gears 1, 7, and sometimes 2 work. I thought about replacing the cassette but honestly I only rode this thing for six months approximately 900 miles.

It sounds similar to the problem mentioned about friction shifters, or maybe in need of a cleaning after almost 3 years idle.

I appreciate the feedback, it really means a lot to me. I am using this bike everyday for my work commute, so I need to find the answer to fix before attempting any wasteless repairs because my schedule prevents anything else.
I’m no bike mechanic but I’ve had problems like this with my MTB and if you put some of that chain oil on it, Its like throwing a bandaid on it if your just busy and can’t get into a mechanic. As it does help with the gear shifting
Try to re-adjust the derailleur from scratch first. Dial the barrel adjuster all the way in (clockwise). Undo the cable. Pull the cable just lightly snug to remove basic slack. Do not overtug it or else you will put too much pretension on the cable. Refix the cable in the derailleur. Slowly begin to dial the barrel adjuster out (counter-clockwise) and try to shift up. Keep doing this and making adjustments until shifter becomes smooth up and down.

Next check the b-screw. Ideally, it wants to be as low as possible for the best shifting, but with certain gear ranges and chainring sizes, you will need to dial it out to get the proper clearance.
If the front derailleur is moving the chain from the small chainring to the large chainring, there's nothing wrong with the derailleur. The front derailleur's only job is to allow the shift cable to pull it up from the small chainring to the large chainring (against the spring which has the job of pulling it back down to the small chainring when the cable tension is released by the shifter).

If the spring is pulling the front derailleur back into the small chainring without you commanding it to via the shifter, the problem is at the shifter, not the front derailleur.

For anyone asking further questions about their derailleurs, it helps (if you can) to provide details of what kind of shifter your bike has.

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