Rear derailleur "A-Tension" adjustment

Quoted message said:

I've read various newsgroup postings hinting that it is possible to
adjust the "A-Tension" on Shimano rear derailleurs. Two questions...

One, I interpret "A-Tension" as meaning the spring pulling the
derailleur outboard,

No, the "A" spring is the one that winds up the cage where it attaches
to the pivot at the bottom of the parallelogram. The "B" spring is for
the pivot at the top of the parallelogram.

Both of these springs are for the purpose of applying tension to the
chain, to take up slack as you shift. Usually one or the other of them
will be adjustable, as a way to regulate the balance between them, and
provide good tracking for whatever size cluster is in use.

Shimano and older Campagnolo have the adjustment on the "B" (upper)
spring; newer Campagnolo units adjust the "A" (lower spring.)

The spring you're thinking of is the parallelogram return spring.

Quoted message said:

higher tension means stronger downshifts but more
hand force required on upshifts...is this correct?

That's true if you have a "low normal" derailer, such as one of
Shimano's "RapidRise" units.

For most derailers, it's the opposite.

Quoted message said:

Two, how do I adjust it?

Quoted message said:

What Shimano rear derailleurs (if any) allow
this adjustment? I have an older 8 speed Shimano 600 rear der that has
a flathead screw on the underside of the der body...is this an
A-Tension adjuster?

That is the adjustment you're thinking of, but it isn't "A-tension."

Quoted message said:

Also, looking at a modern 9-speed DA rear der, I
see no such screw, and given that the body seems to be riveted
together, I'm stumped about how one would make this adjustment on this
derailleur.

Most derailers don't have such an adjustment, because it is rarely needed.

If the derailer is sluggish on upshifts, it generally is a sign of
excessive friction in the cable system, not of any fault in the derailer.

See: http://sheldonbrown.com/cables

Sheldon "Up And Down" Brown
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| until I give you the signal. |
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Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
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Comments at bottom-

Sheldon Brown said:

Quoted message said:

I've read various newsgroup postings hinting that it is possible to
adjust the "A-Tension" on Shimano rear derailleurs. Two

questions...

Quoted message said:

Quoted message said:

One, I interpret "A-Tension" as meaning the spring pulling the
derailleur outboard,

No, the "A" spring is the one that winds up the cage where it

attaches

Quoted message said:

to the pivot at the bottom of the parallelogram. The "B" spring is

for

Quoted message said:

the pivot at the top of the parallelogram.

Both of these springs are for the purpose of applying tension to the
chain, to take up slack as you shift. Usually one or the other of

them

Quoted message said:

will be adjustable, as a way to regulate the balance between them,

and

Quoted message said:

provide good tracking for whatever size cluster is in use.

Shimano and older Campagnolo have the adjustment on the "B" (upper)
spring; newer Campagnolo units adjust the "A" (lower spring.)

Shimano calls the lower spring the "P-tension" spring. (I have no
freakin' clue as to why.) Refer to this diagram:
http://www.shimano-europe.com/cycling/phpimages/product/exp_view/Rd_6500.pdf
The tension on this is adjustable to a degree- there's two anchor holes
on the outer cage plate on most (all?) Shimano rear derailleurs.

The adjustment that the OP refered to- a slotted arrangement on the
bottom of the parallelogram- is to increase the tension on the
parallelogram spring. I believe this dates to the era where internal
cables were all the rage. Their greater friction required more tension
than exposed cables.

Jeff