It took me a while to figure out why the Simplex spring-loaded upper pivot makes sense (I raced for years with a Campy SR, now use a Simplex SLJ on my daily bike):
Basically, the upper pivot allows the derailleur to adjust to chain length changes (front shifts) without changing the geometry of the jockey pulley to freewheel cog.
When shifting on the rear, it makes no difference. (On a single-chainring TT bike, it wouldn't offer any advantage over a Campy Record/NR/SR.)
When shifting on the front (with cross-over gears such as 52-42 or larger differences between chainrings), the spring-loaded pivot allows the Simplex derailleur to swing back and forth, but maintain the jockey to freewheel geometry. (It basically swings around the rear axle, thus maintaining its geometry.)
Thus, the Simplex shifts the same on all rings.
This is very different from a Campy Record/NR/SR, which only has one pivot - on the jockey wheel (the upper pivot only serves to allow removing the wheel by swinging the derailleur back, but it is fixed in operation, where the derailleur rests on the stop at all times).
On the Campagnolo, when you shift to the large ring, the extra chain needed up front rotates the derailleur cage. Because the upper pulley is above the pivot, it swings closer to or further away from the freewheel cogs, depending on the set-up (as it rotates back, it first moves closer to the cogs, then further away). Thus, this type of Campagnolo derailleur shifts differently on the large vs. the small chainring. Set-up always is a compromise to allow shifting on both chainrings, but if it is optimal on one ring, it won't be on the other (others).
One wonders whether locating the upper pulley in the pivot would have improved the shifting, as it would have eliminated this factor. (The Record/NR/SR had the upper pulley closer to the pivot than the Gran Sport that came before. Of course, the GS usually was used with 3-5 teeth difference in chainring sizes, where the issue is moot.)
There probably are other differences between the Campagnolo Record/NR/SR derailleur and the Simplex SLJ that influence how they shift.
What both have in common is the "incorrectly" oriented parallelogram,
which moves the jockey pulleys further from the freewheel center as
it goes to smaller cogs - the opposite of how it should be! _In that
respect_, earlier derailleurs like the Cyclo, JUY 543 etc. (constant
distance from freewheel center) and Nivex (follows contour of the
freewheel) were superior designs. Obviously, this is a bigger problem
with wide-range freewheels than with "corncobs."
--
Jan Heine, Seattle
Editor/Publisher
Vintage Bicycle Quarterly
http://www.mindspring.com/