[200q20v] Suspension advice for a wagon

[Bernie Benz]

Taka,

I bugged you into a well thought out post.  Congrats.

> From: "TM" <t44tq at mindspring.com>
> Date: Thu, 10 Jan 2002 17:51:10 -0500
> To: "'Bernie Benz'" <b.m.benz at prodigy.net>
> Cc: "'200q20V mailing list'" <200q20v at audifans.com>
> Subject: RE: [200q20v] Suspension advice for a wagon
>
> Bernie,
> Being able to adjust the rebound damping will give you a little
> bit of tweakability in terms of the handling characteristics of the
> car.
If, by "handling characteristics" you mean suspension dynamics, controling
the natural frequency oscillations.of the spring/mass system of each wheel,
you would be correct but, contrary to popular belief, it will have nothing
to do with statics, roll, pitch, under/over steer, etc. of the car under any
conditions.
>
> Are you trying to say that there is only one "right" setting, and so
> there is no need for adjustment once you get that "right" setting?
Right, there is one optimum, called critical dampimg rate for a given spring
rate/unsprung weight combo.  Damping is energy absorption, composed of two
parts, the inherent system friction plus the added hydraulic damping of the
shock.
>
> If this is the case, then disregard my comment about adjustability
> for autox purposes. I simply do not know whether I am totally correct
> or totally incorrect- that was my understanding.
>
> As for you comment re: shock dynos, etc., it was my understanding that
> you need this equipment in order to properly tune a shock valving,
> although the real-world in-car feel is what should drive the tuning
> first and foremost. How else can you quantitatively measure the shock
> and spring settings?
>
> Per your statement, I can infer that the OEM shocks (when brand new)
> are ideally configured for the stock spring and wheel/tire package,
> along with any other suspension variables (bushings, etc.).
Your inference would be correct in the ideal world of zero tolerences, zero
wear effects, and no cost constraints, but inasmuch as an overdamped stock
suspension system is the lesser of the two eveils, vs. underdamped, shock
damping is selected accordingly, thus allowing for wear without going into
an underdamped condition.

> If this is
> true, you would want to setup the Koni/Bilstein/whatever shocks to
> replicate this setting exactly. However, there are many other variables-
> if you change your wheels/tires or springs, the shock valving (ideally)
> would need to be changed as well.
Obviously, you're onto it!
>
> My understanding of this is that the shock is there to control the
> spring
not just the spring, but the spring/mass resonent system.
> so that the tires are in maximum contact with the road at all times,
With a period here, you are correct, so far.  But this is only one side of
critical damping, preventing an underdamped condition, wheel bouncing
oscillations.  But most suspension systems are overdamped, and performance
tuners tend to aggervate the error by cranking up the damping.  The problem
with an over damped system is that it unnecessarily slows the time required
for the bumped suspension to return to normal ride height.  Therefore, if
the system is hit with a second bump before nominal ride height is
reestablished, the useable bump travel is reduced and the bump stops will be
engaged.  Ride suffers with no benefit.
> loaded evenly to distribute weight as the car designers had envisioned.
> If this general principle is incorrect, then I need to learn what are
> the basic goals of suspension design in passenger cars.
>
> Taka
>
Yes, "shock dynos"  are available to test the damping characteristics of a
given shock against factory specs, but they are of no use in selecting the
optimum damping for a specific but uncharacterized spring rate/unsprung
weight suspension system.  To directly do so would require the driving of
the suspension system on an instrumented shake table to characterize the
spring/mass system to be damped.  Not practical, nor within my means.

Practically, I would start with a known underdamped system, driven at a
selected speed over a test bump and observe the resultant wheel rebound
bounce, first by the seat of the pants, then visually, and finally
photographicly, increasing damping until overshoot, wheel bounce, is just
eliminated.  This would be close enough to critical, and not grossly
overdamped.

FYI, the Koni rebound adjustment adjusts the cracking pressure of the
rebound valving in the piston of the shock.  It does not restrict the flow
rate through this valving.

Bernie