RE: Coupe (90) shocks

[Peter Henriksen <peterhe@microsoft.com>]

The dealer is ripping you off. Nothing new in that, really.

Below is some suspension information gathered by a friend of mine while
researching why the Neuspeed/Bilstein combo on this 95 Jetta GLS wasn't
cutting it. Bear in mind that the source here is Boge, but it's still
very useful information.

[begin mix of non-expert, personal opinions and expert opinions ]
Since I've been hearing so much conflicting opinion on what is and is
not advisable with regard to suspension tuning, I thought I'd go
straight to the source, so to speak, and wound up talking to [] at
Sachs-Boge of America in Atlanta. He and I spoke for nearly an hour, and
he provided many insights into suspension tuning, specific products and
the like. 

First, he said that the "old" philosophy behind performance suspension
tuning was "stiffness all around": stiff springs, stiff shocks and stiff
sway bars. The problem with this approach is that it tends to make for a
very uncomfortable ride and for sometimes-unpredictable handling on poor
roads.

Starting in the early '80s, Boge, in partnership with BMW, conducted
quite a bit of research into suspension tuning. What they found was
surprising to many of the people involved, especially since it ran
counter to the conventional wisdom of the day. In essence, they found
that too little compliance can be every bit as harmful to a
properly-sorted suspension as too much, especially for road-going cars.
At that time, Bilstein and Koni were the leading exponents of the
"stiffer is better" school, but within the past three or four years,
they have taken notice of Boge's work and altered their approach
somewhat. Even on the track, some compliance is advantageous to help
dissipate kinetic energy in transitional maneuvers. A completely rigid
suspension would be a disaster both on and off the track, as there'd be
only a very narrow range of slip-angles that would be controlled--beyond
that, the tires would simply lose traction and the car would skid.

The lessons of Boge's & BMW's research was to show that springs should
be carefully matched to the dampers (and vice versa!), not only in
simple rates, but also in terms of overall response (the "curve," if you
will). Too many people think that lower, stiffer springs are the ne plus
ultra of performance suspensions, but this approach often creates more
problems than it solves, especially when the damper response isn't
well-matched to the springs, and especially where roads are often
poorly-surfaced, as is often the case in the US. 

Putting larger wheels and tires on a car can exacerbate these problems,
as it increases unsprung weight and thereby changes the effective
characteristics of the springs & dampers. You may be able to rip around
corners at blinding speeds, but the car may become difficult to control
in transitional maneuvers, and will probably wear out key suspension
components earlier than it otherwise would. The tires themselves have
springing characteristics; the stiffer the tire (whether through shorter
sidewalls, higher pressures, or a harder compound), the higher the
effective spring rate of the suspension as a whole.

Keep in mind that the most critical function of the "shock absorber" is
to damp (i.e., control) the kinetic energy (or motion) of the chassis
and wheels. If there is too much compliance in the shoch & spring setup,
the car will tend to float or undulate and feel poorly controlled: the
body & suspension will feel "out of synch." Springs that overpower the
dampers can cause excessive rebound and serious directional instability,
especially at higher speeds. Dampers that are too stiff for the springs
can act, in effect, as springs themselves, and cause the tires to lose
contact with the road on irregular surfaces or during transitions. There
is a range of spring rate & damping combinations for specific
applications. The job of the automotive engineer is to choose the best
(in theory) for a given situation. There are necessary compromises, of
course, even for the track, but the compromises required are greatest
for "bread-and-butter" road-going cars.

[]'s recommendation for a "civilized," "real-world" setup for my car is
to tackle items in the following order: dampers first (find something
that will effectively control chassis motions while still providing
enough compliance to keep the tires in contact with the road to give an
acceptable ride); anti-roll bars, to control sway, or side-to-side
motions of the chassis, and finally, springs. He urged a conservative
approach with both sway-bars and springs, since the sway is, in effect,
a large transverse springs. Too little sway control and the car will
transfer weight from side-to-side and in extreme cases may actually roll
over. Too much, and the car will bump-steer, veer its line on
changing-camber or changing-radii surfaces, and will have, in effect, a
spring rate too high overall (as some of the side-to-side motions will
be translated into vertical motions by the suspension).

Linear-rate springs are not necessarily a bad thing, he pointed out (and
I've read this elsewhere as well). First off, all springs are, to an
extent, "progressive," since as the spring is compressed, its resistance
to further compression increases. A well-made linear-rate spring should
do this on a relatively smooth curve, in effect. Materials choice and
manufacturing technique are critical here. 

One reason why many "performance" springs are progressive-rate is
because they'd be unacceptably harsh if they had their highest rate all
through their travel. If the spring isn't carefully designed, and
carefully matched to the dampers, the car may feel alternately floaty
and jiggly, as the springs move through their "soft" range and "bounce
off" their harder one. In some instances, this transition can be quite
abrupt. Some drivers won't mind this, but others will. It's hard, []
said, to design a really good progressive-rate spring, especially when
it's intended to work with many different dampers. A custom-designed
package would be ideal for any given application, but this is by far the
most expensive route to go, and is usually prohibitively expensive for
car-makers (as well as for most individuals).

For my specific car, [] felt that the Boge ProGas, with the stock
springs, should provide a good ride-handling combination. "Conservative"
sway-bar upgrades would be the next order of business, with springs a
"last resort." The ProGas has two (or, if you will, three) ranges: at
the top of its travel is a "load zone"; in the middle, a "comfort zone,"
with another "load zone" at the bottom. Damping in the comfort zone is
about 5-10% greater than stock (for the A3 VWs, it would be toward the
higher end of the range); in the load zone, it increases to about 10-20%
greater. The goal is to restrict the spring motions at the extremes of
the travel range, but to avoid excessive harshness in normal driving.
This is analogous to the functioning of a progressive-rate spring,
except that it's much easier to achieve a smooth transition using
dampers than it is using springs. With stock springs, the dampers should
normally be in the comfort zone.

The TurboGas is a constant-rate damper, equivalent to the ProGas in the
latter's comfort zone. The stock dampers on most of the upper-end VWs
and Audis have been TurboGas-derived (with valving for the specific car
and springs employed). Note too, that Boge's gas dampers are the most
widely used OE among all European makers, including Saab, BMW, Alfa
Romeo, and Fiat. The ProGas came out of work that Boge did on the A2
Jetta/Golf and Corrado 16v cars. [] was surprised when VW chose to bring
the A3 cars to the US with only hydraulic shocks, since the equivalent
cars in Europe (2.0-liter) all come with Boge gas-pressure units as
standard. He assumes it was a cost-cutting measure.

After learning these details, I noticed (from an ad in the paper), that
Carter VW had a 1991 Jetta GLI 16v for sale, so I called and asked to
drive it. They were happy to give me the keys and let me take it out for
a spin. It looked to be in excellent condition, and had only about 44k
mi. on the clock. I drove up to Golden Gardens and then along the bad
surfaces of north Ballard. This was an eye-opener, since even though the
ride was fairly comfortable (on 50-series Pirellis), overall chassis
control was quite good, and roll-control was also excellent. The car
took bad surfaces with aplomb, something that I couldn't say for my car
with its current setup when I took it over the same route immediately
afterward. The suspension on the GLI is stock, with OE roll-bars
(beefier than base-model) linear-rate springs and Boge gas dampers. This
would seem to bear out what [] was saying: namely, that dampers and
sway-bars are a better route for "real world" handling improvement than
the "replace the springs first" route.

[] emphasized that, for specific applications, Bilstein and Koni are
fine, and he couldn't quibble with their quality of manufacture. He did
say that "the profit margins are better" on Bilstein, as opposed to
Boge, and questioned whether there's enough added value in the Bilsteins
to justify the hefty additional cost. The Neuspeed-Bilsteins are
custom-valved for Neuspeed's springs, but [] pointed out that they are
one of the most expensive aftermarket damper lines currently sold.
[snip]

[end]

- peter henriksen, peterhe@microsoft.com, issaquah, wa
  91 200qw
  94 acura legend gs
  80 mazda 626

>----------
>From: 	tasos@avs.com[SMTP:tasos@avs.com]
[snip]

>The Boge turbo gas could be an alternative but I am getting suspicious about
>the fact
>that the dealer wants $800 (!!!!!!) for 4 stock Boges and I can get
>the turbo gas for 1/3 of that price. Something's not right.
>
>tasos