rear der adjustment

Ben C wrote:
On 2008-03-16, Lou Holtman wrote:
Ben C wrote:
On 2008-03-16, Lou Holtman wrote:
[...]
I find that a very confusing description 'adjusting cable tension'. The
tension is determined by de spring in the derailleur. You adjust the
POSITION of the upper pulley with barrel adjuster.
If the derailleur is on its lower limit screw and you turn the barrel
you will be increasing tension in the cable.
Yeah by 0.001% or something. The main purpose is POSITIONING the upper
pulley, by in- or decreasing the length of the outer cable between two
cable stops.

Why the outer cable?

The outer cable has to be filled by the same amount of inner cable. That
is only possible if the RD moves thereby positioning the upper pulley.


The way I assume it works is like this: the shifter is clicked into the
position for third gear (for example). You set the (inner) cable length
to just the right length to position the derailleur nicely on third
gear.

Correct.


You should now hit the other gears correctly as the shifter changes the
cable length by the right amount for the distance between gears.


Exactly.

The tiny weeny in- or decrease of the tension, which you
can not feel in the cable, is a side effect. Why not call by the right
name instead of that voodoo bicycle tech talk nobody 'not in the
business' understands.

Anyway if you did rack up the tension with the derailleur on the lower
limit screw you wouldn't achieve anything useful.

Correct. That's what goes wrong often. Alway check if the RD doesn't
bottom out against the limit screw(s). Limit screws are just safety
devices an with a proper set up indexed system redundant.

It's like calling shifting to another gear in- or
decreasing cable tension.

Which it isn't. As you say it's the cable length you're setting here and
the tension is always the same and determined by the derailleur spring.

Correct


Lou

Which it isn't. As you say it's the cable length you're setting here and
the tension is always the same and determined by the derailleur spring.

Correct

I disagree. As the derailleur moves, its spring lengthens or shortens, and
the spring tension changes:

http://en.wikipedia.org/wiki/Hooke's_law

Barry wrote:
tension is determined by de spring in the derailleur. You adjust the
POSITION of the upper pulley with barrel adjuster.
If the derailleur is on its lower limit screw and you turn the barrel
you will be increasing tension in the cable.
Yeah by 0.001% or something. The main purpose is POSITIONING the upper
pulley, by in- or decreasing the length of the outer cable between two cable
stops. The tiny weeny in- or decrease of the tension, which you can not feel
in the cable, is a side effect.

Here's how I would describe the physics. You apply a torque to turn the
barrel, which transmits a force to the cable and pulls it a little harder -
this pull is the cable tension.

When turning the barrel you lengthen of shorten the distance of the
outet/inner cable between two cable stop. By doing so the RD has to move
because the length of the inner cable is fixed between the shifter and
the fixing point on the RD when the shifter is in a gear position.
Because the RD moves the tension alters a tiny bit, but it is a side
effect caused by the movement of the RD. The same happens when you shift
from one gear to another only you shorten or lengthen the length of the
inner cable between the shifter and the RD. The rear derailleur takes up
the slack and moves because it is pre loaded by the RD spring.

The other end of the cable transmits this
pull to the derailleur. As the derailleur moves, its spring lengthens, and
the spring force increases to balance the increased cable tension.

Look at it this way. If the derailleur spring was replaced by something like
a solid rod, the derailleur would hardly move,

No it moves the same. You only have to apply more torque when turning
the barrel or operate the shifter.

and cable tension would have to
be increased a lot for a tiny bit of movement. On the other hand, if the
spring was very weak, a small increase in cable tension would cause a large
movement.

No it doesn't. Only the torque would be smaller to turn the barrel or
the force to operate the shifter. The spring in the RD is a compromise
between the force needed to operate the shifter of the sensitivity for
sticky cables. The amount of movement is determent by the lever ratios
in the shifter and RD

If you find it clearer to refer to cable length instead of tension,
that's fine. But I think it's wrong to call the tension increase a "side
effect," as it's the increase in tension that causes everything else to
happen.

No it doesn't. See above.

Lou

Barry wrote:
Which it isn't. As you say it's the cable length you're setting here and
the tension is always the same and determined by the derailleur spring.
Correct

I disagree. As the derailleur moves, its spring lengthens or shortens, and
the spring tension changes:

http://en.wikipedia.org/wiki/Hooke's_law




What makes the RD move?

Lou

On 2008-03-16, Barry wrote:
Which it isn't. As you say it's the cable length you're setting here and
the tension is always the same and determined by the derailleur spring.

Correct

I disagree. As the derailleur moves, its spring lengthens or shortens, and
the spring tension changes:

http://en.wikipedia.org/wiki/Hooke's_law

Yes I think you're right.

Although it may be more complicated if there are also leverage changes
caused by the shape of the parallelogram.

At the end of the day it's the position of the derailleur you care
about, which corresponds to cable length. The spring is only there so
you can change up.

What makes the RD move?

Like anything else, the rear derailleur moves because a force acts on it. In
this case, there are two opposing forces, coming from the cable (tension) and
the spring. When the two forces are equal, the RD stays put. If the forces
are unequal, the RD moves.

Barry wrote:
What makes the RD move?

Like anything else, the rear derailleur moves because a force acts on it. In
this case, there are two opposing forces, coming from the cable (tension) and
the spring. When the two forces are equal, the RD stays put. If the forces
are unequal, the RD moves.




OK, lets rephrase my question: What determines the position of the RD.
That's is the only thing that is important for the adjustment and
whether it shift or not.

Lou

Lou Holtman wrote:
Barry wrote:
What makes the RD move?

Like anything else, the rear derailleur moves because a force acts on
it. In this case, there are two opposing forces, coming from the
cable (tension) and the spring. When the two forces are equal, the
RD stays put. If the forces are unequal, the RD moves.



OK, lets rephrase my question: What determines the position of the RD.
That's is the only thing that is important for the adjustment and
whether it shift or not.

Lou

dude, this tread is already painfully retarded - now you're just
prolonging it. cable length. end of story. now let's go home.

Ben C wrote:
On 2008-03-16, Barry wrote:
Which it isn't. As you say it's the cable length you're setting here and
the tension is always the same and determined by the derailleur spring.
Correct
I disagree. As the derailleur moves, its spring lengthens or shortens, and
the spring tension changes:

http://en.wikipedia.org/wiki/Hooke's_law

Yes I think you're right

Of course that is right.
..

Although it may be more complicated if there are also leverage changes
caused by the shape of the parallelogram.

At the end of the day it's the position of the derailleur you care
about, which corresponds to cable length. The spring is only there so
you can change up.

That is what I meant. Adjusting the RD is posioning it in- or decreasing
the cable tension. In a certain position of the RD corresponding to a
shifter position/click I can change the RD spring by a stronger one or a
weaker on. The RD stays in place. I changed the tension but not the
position of the RD.

Lou

"Ben C" wrote...
If the limit screw isn't what's limiting it then the cable may be too
tight.

"Jim F" wrote:
Bingo. That, plus I opened it up half a turn.

Dave wrote:
The only thing the high and low limit screws do is limit how far a
derailleur can move in either direction. That adjustment can be made
without a cable being attached. After that how well the indexing works
is determined by cable tension.

Lou Holtman wrote:
I find that a very confusing description 'adjusting cable tension'.
The tension is determined by de spring in the derailleur. You adjust
the POSITION of the upper pulley with barrel adjuster.

Ben C wrote:
If the derailleur is on its lower limit screw and you turn the barrel
you will be increasing tension in the cable.

Lou Holtman wrote:
Yeah by 0.001% or something. The main purpose is POSITIONING the upper
pulley, by in- or decreasing the length of the outer cable between two
cable stops. The tiny weeny in- or decrease of the tension, which you
can not feel in the cable, is a side effect. Why not call by the right
name instead of that voodoo bicycle tech talk nobody 'not in the
business' understands. It's like calling shifting to another gear in- or
decreasing cable tension.

Well phrased, Lou. Thanks
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971