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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, 2021 at 11:42
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    Dura Ace 7700 was introduced 1996 iirc
    – user430
    Aug 21, 2021 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, 2021 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, 2021 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, 2021 at 17:49

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Shimano made the change from a torsion spring to a tension spring in the parallelogram of their rear derailleurs for a few reasons. One reason is that the tension spring allows for more precise and consistent shifting performance. The tension spring also allows for easier adjustment of the derailleur, as the spring tension can be easily adjusted by turning a barrel adjuster. Another reason for the change is that the tension spring is generally more durable and reliable than the torsion spring, which can wear out over time.

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