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The older Shimano rear derailleurs (pre 7700) use a torsion spring inside the parallelogram. But with the release of the 7700 group they switched to a tension spring which they continued using ever since.

The 7700 was considered a complete redesign of their drivetrain components. I am wondering why they made this specific design decision. What were the shortcomings of the torsion spring that they hoped to overcome using a tension spring?

Here you can see the torsion spring inside a M900 rear derailleur: Shimano RD-M900 with torsion spring

And here is the "new" design with the tension spring insinde the parallelogram of a 7700 rear derailleur. Shimano RD-7700 with tension spring

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    A MTB derailleur from 1992 and road derailleur from 2002 are a bit odd choice for reference points. For what it's worth, I had a bike with 200GS RD in the 90s and it had a tension spring.
    – ojs
    Aug 21 '21 at 11:42
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    Dura Ace 7700 was introduced 1996 iirc
    – user430
    Aug 21 '21 at 12:10
  • Don't remember when you can look up. si.shimano.com/pdfs/ev/EV-RD-7700-1656A.pdf, disraeligears.co.uk/site/…, velobase.com/… etc say 2001 (yes, I got it wrong too)
    – ojs
    Aug 21 '21 at 15:42
  • Anyway, it looks like the earliest modern style Shimano rear derailleurs like Crane indeed had the torsion spring. I'm not that sure about that 200GS either, so it's a valid question.
    – ojs
    Aug 21 '21 at 15:58
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    This is conjecture and I +1'd the question, but I wonder if it has to do with the tension spring, being bigger and a design with more space available to take up, can do a better job of keeping the force closer to equal throughout the working range. Some Shimano RDs or RD/shifter combinations can struggle with the return force being strong enough at the small cog end. Aug 21 '21 at 17:49

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