<MISC> Re:Sport vs. HD Bilstein

[Knute Ream <knute@xxxxxxxxx>]

ChrisBourk@domain.elided writes:
> [quoting Al Jenab]
> <<<Are you guys sure this is correct regarding "recent" HD vs. Sports?
> Shorter springs are generally (a lot) stiffer springs.  I would want more
> aggressive "valving" to go with them.  I can't believe the Bilstein
> engineers would be so stupid as to recommend the same damping for widely
> varying spring rates. <snip>>>>
>
>Al,
>
>Bilstein engineers aren't stupid.  They probably realize a thing or 2 some of
>the people on the digest don't about suspensions. One is that just because
you
>go to stiffer springs doesn't me you want stiffer shocks - or the other way
>round.  I've heard lot's of people say you need to get suspension components
>that are designed to work together - never hear those same people say what it
>exactly is they're working together to achieve - in a specific sense.

OK, time for a quick explanation of what "damping" means...

In electrical engineering, they refer this as an LC circuit, in which the
balance between capacitance and inductance results in a damping constant,
that determines how long the circuit will oscillate after being "twanged"
at a given frequency.  (Perfect damping can only occur at one specific
frequency unless there are "active" components in the circuit to alter the
capacitance or inductance)

Your car is physical as opposed to electrical, but it's the same math.  You
can easily observe the damping constant by leaning on car whose shocks are
pretty well shot; you'll push down on the fender, and when you let it go,
it will bounce up and down in progressively smaller cycles until it
eventually comes to rest.  This situation is called "underdamped", because
the oscillation persisted.  Overdamped means that your shocks are too
stiff, in which case the same test (leaning on the fender) will let the
fender rise more slowly than the perfectly damped situation, which would be
no overshoot.

Now what does that mean to us when we're driving?

A primary goal of the suspension is to keep the tires in consistent contact
with the roadway.  On anything less than a perfectly glassy, flat surface,
some suspension travel is necessary to keep the wheels on the ground and to
allow the wheels to react more quickly to changing terrain than the
momentum of the entire car (unsprung vs. sprung weight).  If the suspension
is over or underdamped, you will have more time that the tires are not in
full contact with the roadway (or uneven weight transfer), and your
cornering/maneuvering ability will suffer.

Now back to the original question of why to change the shocks when you
change the stiffness of the springs...

The damping constant will change as a result of three factors:  the
magnitude of the oscillating mass (wheel, half-shaft, etc.), spring
stiffness, and shock stiffness.  Change any of the three, and you will move
towards either being underdamped (shocks too soft) or overdamped (shocks
too stiff).  If you were fanatical, you'd change the shock settings if you
put on dramatically lighter or heavier wheels...  Also, because the
frequency (peak to peak travel time of a given suspension movement) is an
issue when tuning the damping, the type of roadways you drive on will also
determine where you want your optimal damping constant.

Is that specific enough?  ;-)

- -Knute

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