Why did Shimano change their rear derailleur design going from a torsion spring to a tension spring inside the parallelogram?

Question

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:

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

Answer 1

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.