So now I got to repair a Broken rear derailleur
 

I've been riding strictly singlespeed for the best part of the last two decades, and one of the reasons is because I hated troubleshooting derailleurs. But now my son broke his 1x8 speed bike (actually his friend) and so I must try to troubleshoot and repair the mess.

HOW IT HAPPENED
His friend changed speed from standstill and then stepped on the pedal. After some cracking he rode the bike, but after this the highest speed the chain on the largest sprocket) wouldn't work well at all.

THE SITUATION NOW
- the chain would fall off the largest sprocket and onto the inside, between the sprocket and the spokes. Also, the derailleur is hitting the spokes.

WHAT I TRIED
I tried to play with the cable's tension by turning the tensioning nut. I achieved that the chain would now stay on the largest sprocket, for the most part (though it does fall inside from time to time when going from 2 to 1). HOWEVER, now speed 8 is f*cked - the chain never goes onto the smallest sprocket. It stays on the next-to-smallest.

the hanger is bent. You need to straighten or buy a new hanger

To solve the issue with the chain going behind the largest sprocket you need to adjust the limiter screw, not the cable tensioner. However, if the problem arose suddenly after the event you described, it is a sure sign something got bent - most probably the derailleur hanger, as it is the weakest part (by design). Look up some tutorials on youtube, there are plenty of them.

Another vote for a bent der hanger.

Cuss at it some more and louder;)

Thanks. I vaguely remember that this issue would require some special tool.

Ideally you'd use a derailleur hanger alignment tool, and for modern, higher end drivetrains it's pretty much necessary to get good shift performance, but on a kid's 1x8 you will likely get acceptable results using an adjustable wrench or similar to bend it visibly straight and adjust it. Check that the derailleur tracks parallel to the cassette. Or bring it to a shop--should be about 10-15 minutes labor. If it's really bent be aware it may break bending it back.

Or take advantage of the expertise at your local co-op, if you are lucky enough to have one local to you.

Just make sure there's not a twisted spot in the chain too.

Thanks. I was going through the options in my head, and I decided that the one with a high enough chance of success in spite of my really poor eyes (I could never be able to eyeball if something is parallel to something else with sufficient accuracy) is to get a new derailleur hanger. I have identified the type I need, and if I could replace it, I assume I wouldn't need any special tool - the derailleur hanger should be straight out of the box.

The only remaining question, and you'll excuse me if it sounds obtuse, is this: if I recall correctly from back in the days when I had to deal with deraileurs (20 years ago, as I mentioned), if I want to remove and then reattach a rear derailleur, I do NOT need to detach, or otherwise mess with, the shifter cable. Am I correct? If I am wrong, what is the simplest procedure to follow so I have the least amount of adjusting to do?

Sadly, in this north-European country the bikeshops are both very expensive AND very slow - I was told I'd have to leave the bike for 3-4 days. Which, during these really excellent sunny days, seems like cruel punishment.

No, detaching the shifter cable isn't necessary. But since you already tried to adjust it, some re-adjusting will likely be needed after the hanger replacement.

You are right, I tried to play with the cable tension.


Thanks for your help.

That's how I ended up buying a Park DAG. The price of the tool is pretty much in line with shop cost + transport to the shop + aggravation of waiting hours/days.

Hey guys, I fixed it! I finally got the part (from Hong Kong - may they have courage and persistence, the brave souls) and replaced the bent derailleur hanger with the new one. I didn't detach the cable from the derailleur during the operation, which made things somewhat unpleasant, but I had my son help as a third hand.

After playing a bit with the cable tension, I have the machine working perfectly, except the switching seems slower than before. We can reach all speeds, including 1 and 8, just the response to the click-shifter seems delayed somewhat. Still, I'm over the moon that I managed to repair this derailleur issue.


Thanks all for your help!