Date: Fri, 18 Jun 1999 08:49:27 -0700
Reply-To: AL_KNOLL@HP-ROSEVILLE-OM2.OM.HP.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Al Knoll <AL_KNOLL@HP-ROSEVILLE-OM2.OM.HP.COM>
Subject: Re: Fizzix, Springs, ride height etc.
In-Reply-To: <19990617235624.36E7985@palrel1.hp.com>
Item Subject: cc:Mail Text
James, et. al.
1. Are you the fellow who had a few SA grilles avec lights for sale
before your escape to Aspen? My memory fails me on the supplier.
2. On the matter of spring rates. The unsprung weight includes those
things not really firmly attached to the chassis. Wheels, lug nuts,
tires, some percentage of the driveshafts, swing arms, outer cvs and
so on. The sinking crow has considerably more sprung weight than the
standard and for a similar ride height will require a "stronger"
spring. If my fizzix doesn't fail me now, it's F=K*x Where F is the
resisting force, K is the "spring constant" and x is the deflection of
the spring (in our case, compression). Now for F=K(x)*x or a
progressively wound spring the spring constant is a function of the
compression distance.
3. In practice to support a 1000KG weight, the spring must have an F
of 1000KG. How much x is given up depends on K or K(x). With a
spring constant of 1000KG the compression is 1M (MKS) With a K of
10000KG the compression is 0.1M or 100cm. With a K of 100000KG the
compression is 10cm. More like what the reality of Westie springs.
4. All well and good for static operation and measuring ride height
when stopped. For dynamic operation such as actually driving the
vehicle, the F required depends on the motion of the sprung weight as
dictated by acceleration due to high speed slalom events, berm
gelandespringen, curbs, dead animals or gravity warps. Once again
F=MA, where M is the percentage of the mass of your conveyance
supported by each spring, A is the acceleration you applied by
turning, hitting a bump or braking severely or just bouncing along on
those oversize tires. It's the same F as in 3, above. The more A you
have the more F you'll need to avoid the dreaded bottoming out. The
more F you need the more K you need.
5. Bottoming out is the condition where the spring no longer
compresses as the coils touch and the spring becomes a fixed cylinder
which transmits any additional A directly to the frame and spring
mounts resulting in higher elective dental restoration bills for the
unfortunate occupants and structure damage to the beloved conveyance.
So don't do that. Put in a resisting thingy that resists as a
function of A. Like a shock absorber. The gnomes at VW did that for
us thank goodness. We just have to make it right for our use and
replace the deteriorating parts now and then.
6. The second function of the acceleration damper is killing off
harmonic transients or keeping the wheels on the ground so to speak as
the M dangles merrily on the spring. Too much damping factor or too
stiff shocks and you approach the fixed cylinder state. Too little
and you have the bottoming and boinging effect. All dependent on K
and M and damping. Change M significantly and you are out of the
ideal operating range of the damped spring system. Perhaps not at the
gross limits but not ideal.
7. The unsprung weight has an effect but not nearly what one would
think. It's effect has to do with transmitting motion to the sprung
weight via A. If the balance is such that the unsprung weight is very
small compared to the sprung weight then the only effect has to do
with keeping the wheels in contact with the road at some relatively
constant ground pressure. The damper helps here. The spring pushes
down from the dynamic inertia of the sprung weight to try to keep the
wheel on the ground when the sprung weight goes up.
Now there's more of course with a compound spring connected masses
system (the tires have their own K and M and damping factor) between
the ground force and the sprung mass. The ideal answer is " probably,
for a limited range of motion, mass, forces etc. I can make you a
compensator device that will guarrantee an absolute smooth ride and
ride height using parts from a Citroen DS-19/21, but you wouldn't like
the maintenance cost". Or sense all that stuff in real time and
adjust the damping and resisting force in real time using a computed
ride map of preset parameters that are selected based on the last
several samples and applied for the next sample time group.
Simple, eh?
Now back to our regularly scheduled programming...
al NOT ON THE SAFARI due to motor problems and gnashing and snarling
therewith.
