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I have an old mountain bike (21 speed) and I'm not sure what kind of rear derailleur i have. So far the performance is still okay but I was planning to replace it with my spare RD (SunRun Index) but according to the mechanic it's not compatible. Is there a way that I could still use my spare RD on my bike? if not then, what rear derailleur should i look for? (preferably shimano)

Still a newbie when it comes to bike parts😬 Rear Derailleur SunRun Index RD

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  • I find it best to have improvement goals in mind and work back to what parts/adjustments might be needed rather than the opposite. Traction, shifting and braking performance are usually pretty clear -- do you have any issues with those or improvements you'd like to see? Brake pads and tires can make a big difference and are relatively inexpensive to change. A lot of times, just having an experienced person spend a few minutes with the bike can identify trouble spots or easy improvements that you wouldn't notice. Also, it would help us if you could post photos of all parts of your bike.
    – Armand
    May 23 at 5:57
  • Lightly cleaning up the derailleur would reveal it's brand as stamped on the face. The style looks like a Falcon, but the lettering doesn't suggest that. May 23 at 18:59
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I think you will need a claw hanger adapter. The old derailleur has one that appears to be non-removable, and the new one has a 10mm bolt that expects to find a matching hole in the frame, which yours won't have.

You probably want something that looks like this:

enter image description here

There is a sunrace branded one, but it looks the same in the critical dimensions as a generic pictured above.

enter image description here

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  • do all 7-speed derailleurs have the same cable pull ratio?
    – njzk2
    May 23 at 18:05
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    @njzk2 pretty much yes - 7 speed was long enough ago that inter-brand proprietary settings weren't a thing.
    – Criggie
    May 23 at 22:24
  • good to know, thanks!
    – njzk2
    May 24 at 20:54
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Your derailleur looks deceptively similar to Shimano RD-TY20-GS and Shimano RD-TY15, but after taking a closer look at the embossed letters on the side, it turns out that it was manufactured by Lingxiang. It's probably model LXA-01

In case of Shimano derailleurs, the actual model name is stamped on the back [example]. Perhaps it's also true for this one.

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  • They're all very similar-looking, but OP's one has something embossed on the side, that doesn't appear to say Shimano. I'd expect it to be another brand.
    – Criggie
    May 23 at 22:26
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    @Criggie You are right. I have taken a closer look and it seems like the emboss says "LXIANG". That turns out to be an actual Chinese bike equipment manufacturer, with an identical rear derailleur on offer
    – macsakow
    May 31 at 14:49

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