Re: <E30><E28><E24><02> Shock Damping Numbers (Bilstein)

[albert jenab <jenab@xxxxxxx>]

Hi Pete,
Great post.  I am amazed at the HD/Sport numbers.  The '02 ones look more
like what I'd expect.  As you point out, the speed at which the numbers are
given really hides most of what a shock is doing.  I've never understood
why Bilstein does this.  At 52 cm/sec = 20.5 in/sec, most shocks have
topped out in the amount of force they'll produce.  Say you've got 2" of
suspension travel at some point.  At 20 in/sec, you've bottomed out in 0.1
sec.  That's a very large pot hole or road hazard.  The blow-off valves
have usually engaged at this speed and the force/velocity curves at that
point for both rebound and compression are nearly horizontal.

The portion of the F/V curve which relates to how hard or soft the shock
feels and how it affects handling is in the 0-10 in/sec range (roughly).
My Penske book only goes out to 10 in/sec.  And the shape of that curve
really has an impact.  For example, a shock that went from 0 lbf at 0
in/sec in a straight line to the numbers Bilstein gives at 52 cm/sec would
feel a lot different from a shock that shot up to that same number at 4
in/sec.  You can get progressive curves and "digressive" curves (concave up
and concave down), all kinds of knees and bends, etc.  These lower speed
damping numbers affect the rate at which the car transfers weight under
turning, braking, and accelerating; how fast and the manner in which it
takes a "set" in the suspension for a given control input.  The high speed
numbers relate more to road irregularities and keeping the tire in contact
with the ground over small bumps. 

The whole black art of tuning shocks falls in this low-speed region, and I
would be amazed if the HD and Sport shocks curves looked identical here.
It would just be bad engineering.  I mean think about it.  Same damping for
a lowered E30 M3 with 17" wheels and R1's, as you have on a 325i with stock
springs and tires?   But I need to find some proof, so let me dig around
and see if I can't find some dyno curves for a car with identical HD/Sport
specs.
- -Al
95 M3

At 05:24 PM 12/2/98 -0500, you wrote:
<snip>

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