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I'm trying to upgrade my cassette on a (commuter road bike) bike that uses a Shimano rear DX derailleur from ca 1990. What would the maximum tooth count be for this RD?

The model says rd-m650 but I'm not sure if it is the SS or SGS version? How can one tell?

It doesn't look like a short cage to my eyes but it's easy to trick ones eyes.

From what I found the it would be either 28T or 32T maximum. Do you think that a 34T would work? I understand that the extremes (smallest-smallest or biggest-biggest) won't work, but I don't use these gears anyways.

Thanks

rd-m650 front rd-m650 back

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  • Oh my! That is an impressively long cage!
    – Criggie ♦
    Commented May 21, 2018 at 8:13
  • It's about 86mm from cog to cog
    – Ola
    Commented May 21, 2018 at 9:23

2 Answers 2

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A quick detour into BING produces the manual from this link: RD-M650 and partially shown below: RD-M650

For your derailleur you're correct, 32T is the biggest cog you could have at the rear. It's possible a freewheel with a bigger cog (34T) would work, but you may have to block off that cog by adjusting the limit screws if the derailleur cannot handle it.

I don't recall these derailleurs being available in different cage lengths, they were long cage by default.

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  • I'm doing the same research on a XT RD from 1995 (bicycles.stackexchange.com/questions/54833/…). I could try a 34T on that and it worked. The length of the cages are same for this DX and that XT. I think I'll just try a 34T and see how it goes. I'll probably use this on a 1x setup...
    – Ola
    Commented May 21, 2018 at 8:51
  • If you have the 34T available then give it a try - you have nothing to lose.
    – KeithWM
    Commented May 21, 2018 at 11:01
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    Just my experience, but: It might work at first but somehow wear causes the top pulley to get closer and closer to the largest cog until it hits it even with the B-screw fully screwed in. Had this issue on a far newer Shimano RD-5700 with the specified maximum of 30T after about 20Mm of use. With the new replacement I have lots of clearance again. Springs shouldn’t get weaker with age/use but apparently they do. I’d get smaller chainrings before I try going out of the specification.
    – Michael
    Commented May 21, 2018 at 13:53
  • Thanks! I'll just try and see. Would love to get smaller rings but that means new crankset in this case.
    – Ola
    Commented May 22, 2018 at 8:52
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I just mounted a 34t cassette and it works fine.

The cassette was from another project that I'm working on with a similar cage length XT derailleur.

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