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make the performance fall-off either side of the optimal cassette
Practically speaking, after installing a fair few so far, it always seemed to be the case. Shifting performance decreased on the upper or lower limits of the cassette irrespective of the amount of adjustment that was done.
Is this down to the actual parallelogram of the derailleur or the pre-load on the spring?
Amusingly enough, it was mostly triathletes that wanted them installed in conjunction with Oval rings. Correlation?
Chak-
Is this down to the actual parallelogram of the derailleur or the pre-load on the spring?
With SRAM, it's all about the parallelogram and cage geometry, because the B-pivot is fixed. With Shimano, the P spring fights the B spring, so the balance between the two has a slight effect on the position of the upper pulley relative to the sprocket.
in conjunction with Oval rings. Correlation?
Yes, just correlation. Oval rings have a negligible effect on the RD
gbj_tester
The claims about lower friction losses are true, but poor value for money.
Claims of faster shifts might be true for some cassette arrangements but not others, the geometry of a RD is always optimised for only one cassette, so changing it will change which cassette it is optimised for. If you're not as clever as Shimano it might also make the performance fall-off either side of the optimal cassette steeper than it was with the stock cage.