Dura-Ace 8spd RD spring

Glen Hendry said:

There appears to be a screw adjustment at the back of the derail. which appears to increase the
load on the main derail. spring. What is the exact function of this adjustment ? Is it for
slightly worn deraileurs, or does it serve some other purpose ?

That's what Shimano calls the "B tension" adjuster. It regulates the vertical distance between the
jockey pulley and the sprockets.

It generally only needs to be tinkered with when you change your cluster to one of a different size.

See: sheldonbrown.comderailer adjustmentOpen ↗

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Glen Hendry said:

Hi all,

I have an old DA 8spd ensemble which has done some kays, and it seems to starting to shift not-so-
sharply.

I suspect that the levers have worn the most, but I am also wondering about the adjustable rear
deraileur.

There appears to be a screw adjustment at the back of the derail. which appears to increase the
load on the main derail. spring. What is the exact function of this adjustment ? Is it for
slightly worn deraileurs, or does it serve some other purpose ?

On modern derailleurs there are two pivot springs, top and bottom. When you shift to the small
front, the derailleur moves back, away from the rear sprockets. That lets the cog get under the
large cog better. When you shift to the big ring, the rear derailleur pulls up, closer to the
smaller cogs.

That screw changes the tension of the top spring, effectively changing the balance of the two
springs to tune that effect.

--
Andrew Muzi www.yellowjersey.org Open every day since 1 April, 1971

On Thu, 05 Feb 2004 01:38:44 -0600, A Muzi <[email hidden]>

Quoted message said:

Glen Hendry said:

Hi all,

I have an old DA 8spd ensemble which has done some kays, and it seems to starting to shift not-so-
sharply.

I suspect that the levers have worn the most, but I am also wondering about the adjustable rear
deraileur.

There appears to be a screw adjustment at the back of the derail. which appears to increase the
load on the main derail. spring. What is the exact function of this adjustment ? Is it for
slightly worn deraileurs, or does it serve some other purpose ?

On modern derailleurs there are two pivot springs, top and bottom. When you shift to the small
front, the derailleur moves back, away from the rear sprockets. That lets the cog get under the
large cog better. When you shift to the big ring, the rear derailleur pulls up, closer to the
smaller cogs.

That screw changes the tension of the top spring, effectively changing the balance of the two
springs to tune that effect.

Another possibility is that the poster is talking about the parallelogram spring adjustment cam, not
the B-tension adjustment screw at all.

The parallelogram spring adjustment cam is positioned inside the rear parallelogram link. Its axis
is parallel to the pivot pins. You adjust it with a flat bladed screwdriver from underneath the
parallelogram.

The parallelogram spring adjustment cam has three settings that adjust the strength of the spring
that closes the parallelogram (toward smaller cogs).

The stronger settings help overcome high cable drag, for example if your frame has steel cable
guides (so you can't use Shimano's plastic guide under the bb).

The weaker settings make operating the shift lever feel lighter, which some people prefer (I do).