Re: HD or sport?

[albert jenab <jenab@xxxxxxx>]

At 01:56 PM 12/1/98 -0500, Mike Klein <klein@domain.elided> wrote:
>Date: Tue, 1 Dec 1998 10:55:30 -0800 (PST)
>From: Mike Klein <klein@domain.elided>
>Subject: Re: HD or sport?
>
>> Date: Sun, 29 Nov 1998 19:08:21, -0500
>> From: NLNR36B@domain.elided (MR JASON R MCCOWAN)
>> Subject: Re: HD or sport?
>> 
>> >well, to get to the point, when i had the stock shocks off, i 
>> compared them
>> >to the bilsteins, and they were a bit longer than my stock ones, hum.
>> ..
>> >interesting, so then when i installed both of them on and when i 
>> took the
>> >car off the jack stands they raised my car!! should the shocks do 
>> that?? 
>> 
>> It's probably because the Bilstein Sports are stiffer, therefore not 
>> compressing as much with the car's weight on them.  
>> 
>> Later,
>> 
>> Jason McCowan
>> NCC
>
>"Stiffness" of a shock has nothing to do with height of the car.  There is no
>spring rate associated with a shock, even a high pressure gas shock.
Stiffness
>of a shock describes the damping factor, which has no effect on the height of
>the car or the shock's compress.  A stiff shock compresses exactly as much
as a
>soft shock with the same weight on it; it just takes more energy to get
there.
>
>However, as m6bigdog@domain.elided ably explained, high-pressure gas shocks (of
>which Bilsteins are one example) will add an offset to your height.  They
push
>their two halves apart with a certain constant force from the gas pressure,
>which raises the car a static amount until compensated by the suspension's
>springs.  This force doesn't affect spring rate because it is a constant
force,
>irrespective of position.  A spring's force is proportional to position.
>


Very good points.  However, there is a position-dependent force due to gas
pressure, it's just not real significant compared to the spring rate of the
(coil) springs.  The force is determined by % change in volume of the gas
reservoir as the floating piston, which separates the gas from the fluid,
moves to compensate for the fluid displaced due to rod volume.  But that
change is usually small relative to the change in displacement, at least in
automotive shocks.  But you will sometimes see 20 or more pounds of gas
offset force difference full extension compared to full compression.  It's
a trade off between large gas reservoir (hence large shock) for little gas
effect, and small reservoir (small shock) for large gas effect.  The
dynamic, as opposed to static, effects of this offset force in either case
are not trivial, requiring stiffer rebound damping to control, and it also
places a lower limit on compression forces.  If you look at a
force/velocity curve of a high pressure monotube, you'll see you cannot
generate any rebound forces until this offset force is overcome, and
compression forces cannot go below this offset.  This is not bad, just
something you need to know.

BTW, the gas pressure "offset" force is pretty easy to calculate.  It's
just the rod cross-sectional area times the gas pressure.  Bilsteins are at
about 360 psi, so with a common rod diameter of 14 mm this works out to an
offset force of around 85 lbs at each corner.  You then have to take this
force back through the suspension motion ratios to figure change in ride
height.

There are designs of certain types of monotube shocks in which the entire
spring force for the suspension is generated by the gas pressure in the
reservoir; i.e. no external mechanical spring.  You get this by making the
gas reservoir very small so that the movement of the rod causes large
movement of the floating piston which in turn causes changes in gas volume,
and thus a spring rate.  This can be designed in for a specific spring rate
and/or manipulated with changes in static gas pressure.  But this approach
has numerous drawbacks (like a ride height that depends on how hot the
shocks are, very high pressures, etc. etc.).

Finally, the gas pressure in a monotube determines an upper limit as well
as a lower limit to the total  forces a shock can generate in compression,
as the floating piston has to "hold still" while the piston on the rod is
forced through the fluid.  Beyond this the gas gets forced around the
floating piston into the fluid and bad things happen.  Anyway this upper
limit is just the piston area times the gas pressure.  So a 46 mm piston at
360 psi can put out a total of say 927 lbs of force in compression, of
which only 927-85 = 842 lbs. can be due to damping; i.e.
velocity-dependent.  This is a lot of force, but not too far outside what a
car might see in an extreme situation (say running over a curb at high
speed).  So typically people engineer blow-off valves and so forth to lower
the max (viscous) damping forces to well under this breakdown threshold.
- -Al
95 M3

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