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Currently repairing a bike that I got off the road for free years ago. Around three years ago, a stuck link caused a chain jam while pedaling at high speed, causing the metal around the b-knuckle on the rear derailleur to shear, rendering it irreparable.

Currently have gotten into biking again and am trying to repair this bike by first cannibalizing a derailleur from another broken bike I have. I got the shifting to work relatively well, except the current derailleur is bolt-on and the parallelogram gets caught on the derailleur hanger when up or downshifting between gears 4 and 5 (see picture).

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The derailleur hanger, for some reason, is not removable unlike the others I have seen online while researching. Thus, I want to buy a derailleur that mounts onto the dropout. I've already found the tooth capacity I need [(48-28)+(28-12)=36], but now I am not sure on the mounting method to use. If the dropout looks like this, should I buy a hanger or direct-mount derailleur?

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For reference, I would have bought one to match the still functioning front derailleur, but the model, a Shimano Exage 300 LX, seems to be out of production (and used ones are a bit too expensive, I'm looking for something <$40). Any recommendations on compatible derailleurs to buy?

Also another issue I've noticed is sometimes the chain "skips" on a single gear under load (not between gears) What could be causing this? I checked every link individually and none are stuck. I've searched online and only found a solution where the guy said he replaced all the chainrings and cassette without mentioning what the problem might be. I don't want to replace anything outright if I don't have to, any ideas?

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The derailleur in the first photo doesn't appear to be mounted correctly. See how there's a big gap between the axle and dropout slot? I think you should just go with a regular, bolt-on derailleur. Non-replaceable hangers are usually found on steel frames (check yours with a magnet) and are perfectly fine to use. With steel, you can just bend it back into shape if broken, or even cut it off and weld on a new one if i remember correctly. Any 7 speed (it might even be 6/7/8 compatible) derailleur should be the right one.

The chain skipping could be caused by worn out drivetrain components like you mentioned, or it could be due to the wonky derailleur setup. I would wait until the correct parts are installed and it shifts fine before sorting that problem out.

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  • If we take the second picture (imgur.com/BumRwWJ), when I mount the bolt-on derailleur in hole 1, you get the wonky result you mentioned. The derailleur shifts if you give it a tap with the foot, but on the whole, completely unideal. If I mount the screw in hole 2, you get this (imgur.com/NsxwnmX), which is worse (complete blockage compared to a rivet getting caught on the side). This is why I want to invest in a derailleur to mount off of the hanger, as I'm afraid any bolt-on will have this problem with the hanger.
    – two_hearted_river
    Commented Jul 2, 2020 at 22:46
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    Those holes are not for the derailleur hanger bolt and in the wrong place. They are there for fender and rack mounting.
    – mattnz
    Commented Jul 2, 2020 at 22:55
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    ” The derailleur in the first photo doesn't appear to be mounted correctly.” if anything, this is an understatement.
    – Weiwen Ng
    Commented Jul 2, 2020 at 23:52
  • Some towns have shops or cooperatives with bins of (mostly older) used parts for sale at cheap prices. Someplace like that would likely get you a rear derailleur at a good price. (For me $20 or less is a "good price").
    – Armand
    Commented Jul 3, 2020 at 0:35
  • You can get a new RD-M360 for $20....
    – mattnz
    Commented Jul 3, 2020 at 1:01

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