| |
I will now to toss another flaming projectile into the suspension debate...
Car: 1996 GTI VR6 (12/95 production date)
Stock shocks/struts:
F 1 HO 413 031 L
R 1 HO 513 031 S
Bilstein Replacements:
F F4 V36 4023 H202
R F4 B36 1524 H101
Problem: The stock shocks are unable to control the stock springs providing
an 'oingo-boingo' ride. Visually, the car sits too high in front.
Goal: Improve control without going way stiff. Lower front about 1" for
improved looks, without slamming or compromising ride.
Method: Cut stock springs as necessary and add Bilstein shocks/struts from
Shine Racing Service. There are other good shocks as well that I could have
used. Personally, I *don't* want adjustability, I don't want to dick around
fiddling with my suspension. Bilsteins are well respected, reliable and
available, ergo I'm using them.
How much to cut? (Insert sound of a can of worms being opened)
I would like to thank Uwe Ross. We have had a lengthy side conversation about
suspensions, and his insight, as well as the formula that I used were
invaluable. Uwe also sold me his now unused stock springs to serve as back
ups in this experiment, so if I did create a Frankenstein, I had 'spring
insurance'. Uwe has also produced a installation guide, which is in the
proceess of being finalized. I used a draft copy, and found it very helpful.
Thanks Uwe!
It should be noted that for my car the stock vs. Bilstein perch heights
matched within 5 millimeters. This is important to know in order to calculate
the final ride height.
First, Data:
Spring Rates: (from Uwe)
Shine VR6: 300(f) 200(r)
Shine 4-Cyl: 200(f) 120(r)
Stock: 148(f) 80(r) (1)
Corrado SLC: 175(f) 100(r) (2)
NoSpeed: 225-250(f) ??(r) (3)
(1) Front according to Bob @ SRS; rear is my (Uwe) best guess.
(2) Both are my best guess. I (Uwe) have a set of these.
(3) These numbers are "hearsay"; but supposedly according to NoSpeed.
Bob @ SRS says their springs never measure what they claim. IMO, they
don't make any sense; the "spread" between the rates should be much more
given the coil geometry. (Uwe)
Bilstein gas pressure data:
Bilsteins by design exert pressure against the installed springs esentially
"stretching" them, in a small way negating lowering. Therefore we must
calculate this to be accurate in our lowering. The stock front struts have no
gas pressure, the rear stock rear shocks do have gas pressure, though it is
not as much as the Bilsteins.
Here are my measurements. Using a bathroom scale rezeroed after shock/strut
is placed on it (to negate the weight of said damper) :
1/4 1/2 3/4 full
F 32 37 43 57
R 36 38 41 43
Stock Rear:
1/2: 12 full:16
For the sake of simplicity I used the halfway readings for my calculations.
The Formula from Uwe:
Measure the distance between two adjacent coils on the springs in question
with the springs fully loaded by the weight of the car (car sitting level).
Now figure out how much you want to lower the car. The number of coils (or
fraction of a coil) that you want to cut will then be the ratio: (amount you
want to lower) / (distance between coils)
With my addition: the gas charged correction factor is added because the
Bilsteins by nature will try to raise the car.
For my GTI VR6 the distance between coils were 1 3/8" (35mm) in the front and
1 1/4" (32mm) in the rear.
Front:
1.25"lowering / 1 3/8" equals .9 of a coil
Rear
.3"lowering / 1 1/4" equals .25 of a coil.
But I wanted only an 1" in the front and no lowering in the back? I had to
overcompensate because of the Bilsteins:
How much will the Bilsteins try to raise the car? Before we start we need to
make some assumptions as to what will be the new spring rate will be after
cutting.
First, how much does the rate increase with each coil cut? dividing 5.5 coils
into 150 lb (stock front rate) gives approx 28 lb increase with each coil.
This assumes linearity, which would not hold true if we were cutting say, 4
coils. I'm sure the rate at some point becomes sort of exponential in nature,
but since we aren't cutting much this is a close approximation.
For the rear, 5.5 coils into 80 lb comes out to about 15 lb.
So for the sake of argument the new spring rates will be approx 175-180 in the
front (assuming one coil cut) and 85 in the rear (assuming one quarter coil
cut)
Front: 37 lb (bilstein pressure) / 175 lb/in (spring rate) gives approx .21
(1/4")
Rear: 26 lb (bilstein - stock pressure) / 85 lb/in (spring rate) gives approx
.3 (1/3")
The calculation is thus:
.9 coil off the front lowers the car about 1.25", while the Bilstein raises
it .25"
.25 coil off the rear lowers the car about .3" to compensate for the Bilsteins
raising of .3"
**********************
So what happened?
What I did:
I played somewhat conservative: I left the rears uncut and the fronts got
chopped 3/4 of a coil. The result; front end lowered 7/8" and the rear end
is within an eighth of an inch of stock. (lowered very slightly) Bob @ SRS
had said to me: "Be careful cutting the rears, it'll drop really fast."
This pretty much bears out the formula, at least for my car. Don't forget,
these calculations were made with a number of guesstimates, and let's face it,
measurement accuracy for ride heights, etc. is not micrometer perfect.
How it rides:
I am very pleased. This is subjective however, and what is good to me may not
fit *your* priorities.
It's no slotcar, and frankly, no matter how much you "Spaxerize" a tall,
noseheavy GTI VR6, it ain't gonna be a slotcar. Buy a Miata (I've spent 1000
miles driving one) *That* is a slotcar. It's simply a matter of physics.
The GTI is improved, less nose dive under hard braking. The oingo boingo is
gone. It is stiffer, one feels the road texture as opposed to floating over
it. There is a greater sense of control. It is subtle, you would notice it
but for example, my wife does not. Body roll, well, yup, it rolls. This is
physics again. (I have not changed any swaybars) However now it is
controlled, when you hit a bump mid corner, the shocks absorb it. Before, the
car was launched off the bump.... For me body roll is not what drives me
nuts, it's the oingo boingo float. I have not pushed so far as to test any
extreme limits, as it is March in Minnesota and the roads are covered in sand.
I would be very curious to drive a car with the 300lb. springs. It obviously
would roll less, possibly a lot less(?). My question would be how much
stiffer riding it would be. Frankly I don't want anything stiffer than what
I've got now. Those that have the 300lb springs have reported that they are
happy with them and I don't doubt it for a second. Those that have just
changed to Bilsteins without touching the springs have also reported
favorably. You guys out on the east coast that got together, consider
yourselves lucky to be able to sample each others cars...
Cheers!
Jon Lane
'96 GTI VR6 "FrankenBilstein"
'57 Sunroof Sedan
--
For info on: how to subscribe & unsubscribe, the list's mailing errors,
list archive, etc. see: http://www.panix.com/~aqn/gti_vr6_list/
|
Fri Mar 13 15:39 CST 1998
>