A pic will show it…
Imagine the S shape made by the chain… Stick a pin on arms between the wheels. The chain can go on one side of the pin.
The pin is slightly offset. With the chain on the correct side it moves freely.
The other way it just rubs on the pin.
The pin in this case is a tap of metal that is bent in from the derailleur arms.
When I post the pic it will become obvious.
No other way to get it right break the chain or dissemble the derailleur.
ok still need pic
If someone routed it improper, you should be able to lift the chain from the two derailer gear pulley wheels (jockey and idler) and start over.
Can’t imagine how someone could route the chain to cross the cage plate arm, but it would be possible. Also the pulley wheels can be loosened and removed if for some reason it is really fubar. There is usually a guard to keep the chain from jumping off held by the same bolts that connect the cage plate.
No need to break the chain unless you just got the urge.
Got finished with the infusions.
Sometimes ya’ get the good nurse – Sometimes ya’ don’t.
So here are some pics of my rear derailleur…
This is how mine is threaded. Taken from the opposite side from the cassette so you can see the path better.
The chain is surrounded on four sides by the derailleur wheel, the two arms that make up the derailleur, and the tab that is a bent hunk of metal that is part of the derailleur arms.
There is no way that I can see to get the chain on the other side of the tab without breaking the chain or dissembling the derailleur.
Breaking the chain is way easier/safer for a duffer like me. Besides it will give me the opportunity to clean that sucker.
Mine
Here is a capture from a Park video showing the correct way to thread the derailleur arm.
Correct
Thanks for all the info and links!
All the Best,
Jeff
i still can;t exactly tell what is going on
one thought it, someone took the back wheel off and didn;t get it back on right, with the cogs in the right place
it;s easy to mess that up. it would be pretty hard to install the chain wrong.
Some bikes have a single arm on the shifter. Easy to remove and reposition the chain.
My chain is surrounded on 4 sides. It is not possible to get the chain in the current location without threading it through there in the first place.
Here is another shot from the back side. You can’t see the problem from the front.
The chain is supposed to go above the tab so that it rides on the wheel not the tab.
Easy to do it wrong if not paying attention when installing the chain.
See that tab by the red arrow?
The chain must go on one side or the other.
On one side, the chain rubs on the tab.
On the other side it does not.
The chain goes around the cassette.
Over the first wheel.
Then on my bike - there is a tab in between the two derailleur wheels.
The chain must pass on one side or the other.
Take a look at the pic below. The correct way has the chain passing on the left side of the tab.
The chain does not rub on the tab.
The incorrect way has the chain passing on the right side of the tab.
The chain rubs on the tab.
See this video at 4:53 to see the chain routed and the tab of which I speak.
Not real easy to get a better pic in there.
After I get it corrected I’ll post a new pic.
All the Best,
Jeff
yes i see it now, that kind of thing would make me question any other work that was done on the bike!
also the chain looks worn, like they rode it that way… scraping and clicking!
breaking the chain - some chains technically require a new link or new pins, to re-join (shimano)
i recommend SRAM chains, they have a magic master link that does not require this
Yeah, I’m a bit spooked by this too. Didn’t notice it till I got it home.
I hope what happened was the chain broke (or?) and the bike got put up. Then new tires got installed (look nearly new to me) and the chain got put on incorrectly.
I’ll know more after I get feeling better (takes a day or two) and get the chain off there.
Thanks for the SRAM suggestion
Just looked. The current chain is marked: PC 951 and it has a magic link so it will be easy to remove.
wfe - are most chains basically the same as far as using them on a bike like mine?
Obviously I don’t need anything fancy.
The SRAM at $22 seems like a good deal to me. I’ll count the links/measure after I get it off the bike.
https://www.amazon.com/SRAM-9-Speed-Bicycle-PowerLink-Connector/dp/B07XYG3M38
All the Best,
Jeff
you do need to get the correct ‘speeds’ for the cassette
there is “6-7-8” speed, for 6, 7 or 8 cogs
then all the rest are unique, 9, 10, and 11 each have their own width
pc 951 is a sram
reuse if you can, it is prob not worn (“stretched”)
Just loosen that socket head cap screw that is holding the arm to the upper pulley wheel and loosen the cap screw in the idler pulley, then the arm can be moved to let the chain ride in the correct position, then reinstall the arm and tighten the screws. No chain break required.
You are correct that the chain is not threaded correctly. It should be inside the derailleur cage, not riding on the tab. If you are lucky, there will be a master link that you can open. That’s a lot easier with an $9 master link tool https://www.ebay.com/itm/353565699003 but if you remove the tension on the chain as shown at 1:09 in this video it is easier. Bicycle Chain Master Links: Tips & Tricks KMC Shimano SRAM - YouTube If your chain has no master link, then you need a chain tool for opening one of the pins or simply cut it if you are going to replace it.
The alternative is to unscrew the bolts that hold the jockey wheels in place, re-rout the chain, and reinstall the bolts. This is a bit difficult because it is tough to deal with loose parts and the chain. A new chain will make it a lot easier. The KMC chain with a master link is the OEM chain on a lot of bikes.
You can get a good price on eBay compared to going to your local bike shop. Just count the number of gears on the cassette and select the right chain.
KMC X8.93 6,7,8 speed chain $13 postpaid https://www.ebay.com/itm/284426075943
KMC X9.93 9 speed chain $19 postpaid https://www.ebay.com/itm/393386368206
Thanks all,
Got a chain tool on it’s way. The SRAM chain I have has a master link.
All the Best,
Jeff
the sram chains can be opened up without a tool though
look for their masterlink videos
it can be a little hard but don’t use a chain tool
you will just make the chain weak where you break it
Not mine! Tried everything except Devine intervention.
Almost got it when my snap ring pliers gave up the ghost.
That sucker is really on there.
Think master link pliers is my only hope.
All the Best,
Jeff
ok - you could still take that one bolt out of the derailleur cage and fix it!!
if you do break the chain, get another powerlink to put it back together
very easy
wle
Just thought I’d update.
Got in a Park Master Link tool.
8 seconds later the Master Link is out with no damage.
Wow is this chain in need of a clean and lube. It’s so gummed up with old lube it will almost stand up on it’s own.
Thanks everyone for all the help!
All the Best,
Jeff
it;s probably ok as is, put it in kerosene if you feel like you have to do something 
wle
sram chains do not have a ‘master link’ though
usually those are for single speed chains that do have a master link
you may have taken out one of the regular links
the powerlink [if it is a sram chain] would not really respond to a ‘master link’ tool
so i am a bit confused.
if you did take out one of the regular links, you should use a powerlink, to replace it
if you just jam the original pin back, it may be weak there…
wle
If the chain had a master link, then that would explain how it could get improperly routed such as it was.
Kennybobby (post #47 above) has the exact best solution to this problem. No chain break required. Seriously. Take it to a bike shop. They could fix this for you in about 30 seconds.