91 Avant Shock Recommendations

Bernie Benz b.benz at charter.net
Fri Nov 21 20:06:53 EST 2003

Hi Eric,  Comments interlaced below.
I also have Koni Yellows, w/ adjustable rebound damping on my 200Q-20V.  IMO
they are not worth the new price (mine were used), Boge Turbos are a better
match, and the piston rod is much beefer and therefore I assume has much
more bearing area to take the latteral cornering forces with minimum wear
over time, thus little loss of rebound damping over time.  My experience
bears this out.  Koni has incorporated adjustable rebound damping both to
offset their less than robust mechanical design, as well as an effective
marketing ploy.

> From: Eric_R_Kissell at whirlpool.com
> Guilty as charged, Bernie.  I was lazy with my terminology.  What follows
> below is probably no better...
> By 50% and 75% I am talking about the % of the 720 degrees of setting
> available by rotating the shaft of the Koni Sport (yellow) shock absorber.
> So, my descriptor of 50% suggest that I rotated the shaft 360 degrees
> (360/720 = 0.50, see how this works?) from the setting giving the minimum
> amount of damping. The setting of 75% that I want to try will be at 540
> degrees of shaft rotation (540/720 = 0.75) from the setting giving the
> minimum amount of damping.
I don't remember the piston rod adjustment range, (recalling maybe 3.5 half
turns?) close to what you claim.

> I did not actually measure the damping
> resulting from the settings so I cannot comment as to the linearity of the
> damping versus the setting. I suspect it is somewhat non-linear and that
> damping increases more rapidly for changes in the setting as the setting
> nears maximum damping.
FME, having overhauled many Koni strut inserts and coil over shocks (for
Lancias and one set of Audi 44 chassis rears), this rebound damping
adjustment is very linear.  It adjusts the piston rebound valving cracking
pressure by changing the linear spring force on the valve plate.
> My assessment of being underdamped is not due to a lack of understanding
> about a "resonant spring/mass system" as you carelessly assume,
I have a reputation for carelessness of this type, even if not intended.
> though I 
> have not worked specifically on a "resonant spring/mass system" in my
> current job for about 5 years. I suppose I could crank some calculations
> and measure some damping and spring rates to support my assessment, but I
> actually enjoy wrenching and testing theories too. My empirical assessment
> is that the car is underdamped because it does not settle down as fast as
> I would like once the mass of the car starts moving on the springs.
You are poorly describing a characteristic of an overdamped rebound system!

As you may well know, there are three possible damping conditions in a
simple reactive spring/mass system, related to suspension rebound as

Underdamped.  The condition in which a bump displacement from the SRH
(static ride height) returns, rebounds to SRH with excessive velocity and
therefore overshoots beyond SRH.  An oscillation of the unsprung weight at
the resonant suspension frequency.  One or more cycles of the tire bouncing
off the pavement, bad!

Critically Damped.  The condition in which a bump displacement from the SRH
returns to SRH with zero velocity in the shortest possible time.  Thus, SRH
is reestablished and the suspension is able to accept the next disturbance
in the shortest possible time.  The optimum condition.

Overdamped.  The condition in which a bump displacement from the SRH returns
to SRH with zero velocity but at a slower than critical rate.  An
unnecessary time delay during which the suspension is ill prepared for the
next disturbance. 
> Obviously you know more about my specific dampers and my specific
> "resonant spring/mass system" than I do.  However, I have my own
> first-hand perceptions about this particular "resonant spring/mass system"
> and IMHO it feels underdamped.  Since I am the one who has to feel the
> results of my "resonant spring/mass system" on my car each time I drive
> it, I am going to go ahead and try a setting with higher damping.
My knowlege or lack there of is always subject to question and of course,
you are free to try the full range of rebound damping.  I guarentee that you
will notice major differences, and your perception of "good" may not have a
sound technical basis, but your pants are in the driver's seat, sitting on
that thin wallet!

> I will report back my perceptions about my "resonant spring/mass system"
> with an increased amount of damping after I make and evaluate the change
> at the "75%" setting.
> My use of "stiffness" in describing dampers is clearly unforgivable. I do
> not know what I was thinking. Please replace the words "stiff" and
> "stiffness" in the previous message with more appropriate words to
> describe  "resonant spring/mass systems" with differing damping levels
> that either desirably or undesirably dissipate energy through damping
> losses versus transmitting energy through to the mass of the car either as
> a "shock load" for the overdamped case or as decaying oscillation of the
> car's mass in the underdamped case.
> Anyway, thanks for the feedback.  I will let you know if increasing
> damping improves my perception of my "resonant spring/mass system" that I
> perceive as underdamped yet you hypothesize is overdamped.
> As I recall, you have Koni Reds.  Where do you have those set?
> Thanks,
> Eric

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