Re:Sport vs. HD Bilstein

[albert jenab <jenab@xxxxxxx>]

At 06:46 AM 12/3/98 -0500, you wrote:
> <<<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.

They are trying to achieve an overall handling balance between an
underdamped system and an overdamped system for a very complicated
dynamical problem.  The parameters you want to optimize are different for
different applications.  For example, street cars optimized more toward
soft ride want a damping ratio of like .15 or so, but maybe up to .3 or so
if you're emphasizing handling.  Race cars might want more like .5, and
even beyond critical for cars with lots of down force.  Note that when you
change the springs, the damping necessary to achieve a target damping ratio
changes (see below).  And if you want to optimize your suspension more
towards handling, you want to increase the damping even if you don't swap
springs.  This is why adjustable shocks work well in a street/track car.
You can tighten up the damping to emphasize the handling side of the
damping ratio and then dial it back to get a soft ride on the way home.
The trade off is between "floaty" (low damping ratio/lots of wheel
motion/lousy roadholding) and "harsh" (higher damping ratio/less wheel
motion/better roadholding).

Read Milliken ("Race Car Vehicle Dynamics") chapter 22 or Smith ("Tune to
Win", "Engineer to Win", etc.) or any good book on suspensions.  

Then perhaps you will see why virtually every on-and off- road race
car/truck on the planet has adjustable shocks.  And why every major team in
every type of racing travels with their own shock dyno.  And why these
teams spend thousands of hours matching springs and shocks to tracks and
conditions.

Sorry, perhaps I meant to say Bilstein is very good at separating people
from their wallets when it comes to servicing a small market of enthusiasts
who don't know how to set up their suspensions.  It costs too much to
properly engineer a suspension for a bunch of hardcore E30 M3 drivers with
lowered suspensions so they just chop off the street HD shock rod so the
springs won't fall out at full extension and sell it to them as a "sport".
Don't get me wrong, it's not a bad shock.   But to call the same valving
"sport" that's in the HD is deceptive.  Sure the car will work but not as
well as if someone spent the time and effort to do it right.  On Bilstein's
behalf, it just takes way too much time and effort to do this for them to
make a profit.  This is also why there is a large tuner aftermarket.  And
why Koni sells a lot of adjustable shocks.

>
>One approach (Hartge/Dinan) is to use some pretty stiff springs with stock
>sway bars - we'll call this *stage one* :).  BMW Motorsport decided to go
with
>some pretty big sway bars (on some of their cars) with stock springs.  You
>take the pieces one at a time and decide what you want to do with it.

I thought we were talking about shocks.....

> 
> <<Example: there's a 45% difference in spring rates between the stock
springs
> and the ubiquitous Eibach Pro-Kit springs I recently put on a Miata, lowers
> the car about 1.25" all around.  Turns out Bilstein doesn't make a Sport
> for it and believe me, with the HD's it's way underdamped.  I am having a
> set of the HD's revalved to Sport specs for it.>>
>
>Well,  this isn't a miata list but if Bilstein doesn't make a sport shock for
>the car I would seriouly question if one is required - after I had found a
>quantitative answer as to what a "sport" shock was.  Also,  if Bilstein
>doesn't make a "sport" shock for the car I would argue that it isn't possible
>to have the shocks "revalved" to sport settings - how can you revalve to
>something that doesn't exist?  but in the end I really don't believe you when
>you say the car is *way underdamped* with Bilstein shocks. 

I see.  I suppose since Bilstein doesn't make a Sport for the Miata, then
every conceivable spring rate will be perfectly damped by the HD?
Hmmm....wonder if they run HD's in E production Miatas?  Do you know
anything about shock absorbers?  Do you know what a damping ratio is?  I
suggest you go read Milliken's book or one of Carrol Smith's books on how
to set up suspensions and see just how complicated this can get.

My "sport" setting target for the Miata is 30% greater damping from 0 to 10
in/sec than the HD.  How did I get to this number?  First I measured all
the motion ratios and guessed at the sprung/unsprung masses for each corner
of the car.  I dynoed the stock Miata shocks and springs.  Then I dynoed
the Eibachs and the HD's.  Then I noticed that the HD's were just slightly
more aggressive than the stock shocks but the Eibachs were 45% stiffer than
the stock springs.  Then I hypothesized that if I were to use the HD's, the
system might be a little twitchy.  So I assembled the system and installed
them as sure enough, it was.  So next step was to "guess" that bringing
rates up to about 30% over HDs might be about right.  (Of course the curves
look a little different but you can't have everything.)

Why is this?  Because 

critical damping  = 2*sqrt(km)

So when you raise the spring rate 45%, the critical damping goes up by 20%
(sqrt(1.45) ~ 1.2).  But then that would only provide a system with the
same damping characteristics as stock, and rule of thumb (see Milliken
again) is you want a damping ratio closer to .5 for a track set up, than
the stock .25 or so (which I calculated from the motion ratios and the dyno
runs) so I went with 30% over the HDs.

Here is my practical definition of underdamped in this case:  Body motion
is too great over undulating bumps.  It wallows.  The car takes a set in
cornering and braking too fast.  Weight transfer on side-side transitions
is too rapid, same on accel/deccel. Generally the car just feels
sloppy/floaty as speed increases and control inputs become sharper.  Mind
you, this is with Eibach pro-kit springs and sway bars.  The shock's job is
to control the springs (and body roll rates and dive/squat rates......).
When the springs are too stiff and the shocks too weak, the stored energy
in the springs gets released too fast, so when you brake for example, the
rear end comes up too fast, the nose dives too quickly.  Similarly, in a
corner the inside wheel unloads too quickly because the rebound rate is too
low.  This makes for a rapid weight transfer to the outside wheel and a
very twitchy car.  The tires feel planted, but the body is wiggling around
on top of them.

> 
><< BTW, "valving" on Bilsteins are really thin shim stacks with various
> thicknesses and preloads, one set on either side of the piston.  The fluid
> goes through an uncovered hole in the opposite side of the piston and then
> bends these shims to create the damping force.  Thus the "deflected disk"
> terminology.  So there really aren't any "valves", there are just these
> shims covering up holes in the piston.>> 
>
>That's good information, what you've discribed are valves - reed valves.

Thank you so much for pointing this out.  Now go read Milliken and write a
book report.
- -Al
95 M3

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