Dave Mayer said:Someone just gave me a bizzarre late-80's Campagnolo rear derailleur. With a little research, I've
figured out that it is a Chorus A-B unit. It is light and looks very well made. I stripped it
apart and put it back together; I found that it has the unique feature that you can adjust the
slant of the rear parallelogram. Set in the A position, the slant of the cage matches a close
ratio freewheel, and in the B position, the slant of the cage tracks a bigger ratio freewheel. It
has no spring in the upper pivot...
I've checked the geometry of the derailleur. The cage is identical to modern Campy units. Set
in the B position, the parallelogram slant is the same as modern Campy road units. The only
difference is the where the cable terminator sits relative to the derailleur cage. I can
compensate for this difference by mounting the cable on the top (opposite) side of the
mounting bolt.
Once this is done, the overall geometry of the derailleur should be the same as a modern Campy
unit, and it should index with Ergopower shifters. Right?
Alas, no.
Quoted message said:Here is my question - which I cannot find a satisfactory answer on the web: Will this actually
work? I have a modern Athena unit sitting in a box. Should I waste my time fiddling with the
Chorus unit?
That unit will definitely not work with Ergo. It was intended to work with Campagnolo's original
"Synchro" indexing system.
The key issue is how far the cable moves for each shift. For indexing, it's better to have the cable
travel a long-ish distance per shift, because this reduces the effects of cable stretch and friction
on the indexing.
Back in the '80s, when Campagnolo had to respond to the challenge of Shimano's S.I.S., the company
was still committed very strongly to supporting their older products with their newer designs. The
"Synchro" system was intended to be compatible with existing Campagnolo rear derailers, even going
back to the '70s Nuovo Record.
Despite Campagnolo's good intentions, the short cable throw, high cable tension associated with this
made reliable indexing impossible. They tried it for a year or two, then finally bowed to the
inevitable and completely redesigned their indexing system when Ergo was introduced. Current
Campagnolo stuff uses much longer cable travel, and works great as a result.
Your old Synchro Chorus derailer is certainly a lovely piece of jewelry, but as a functioning
derailer, it's way inferior to a $20 Shimano tourney or any other modern derailer.
If I may digress, back in this era, Campagnolo also had the bright idea of making brake levers that
could be set up either with traditional exposed cables or with "aero" (under the handlebar tape)
table routing. Seemed like a good idea at the time, but the resulting overcompication obviated any
advantage this might have had.
To further digress, Shimano's indexing went through a similar progression...the original Dura-Ace
S.I.S. from 1984 used a shorter cable travel (though not as short as Synchro) and Shimano too wound
up increasing the cable travel for subsequent models.
Sheldon "History" Brown +---------------------------------------------------------+
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