Originally Posted by ctmullins

Please excuse the Lazarus thread, but this is something I've been trying to understand, with little available information.

Can anybody explain this, in technical terms? I understand the SunTour design perfectly well, with the slant parallelogram and the B-tension screw. But I cannot, for the life of me, figure out how a sprung upper pivot improves things. As the derailleur moves rearward, as it will when a Servo-Panta is shifted to smaller cogs, the lower pivot will move downward with respect to the freewheel axis. In other words, the radial distance from the freewheel axis to the lower pivot will increase. This seems to me to be the exact opposite of what would be desired. Obviously, the offset jockey pulley compensates for this, just as it does in the classic Campy designs, but they didn't have springs in their upper pivots, so it's got to be more than just that.

The best I can come up with is that two springs might increase chain tension in the smaller cogs, but the claimed benefit, according to what little actual information I've been able to find, is improved jockey pulley tracking to the freewheel cogs, not chain tension. But how?

Does anybody have any technical literature on exactly what Shimano means by "Servo-Pantograph"?