[VACList] Car suspensions- Air Springs, or ShockWave 9000, etc.?

["Oliver Filippi" <ofilippi@xxxxxxxxxxxx>]

Kevin,

In my opinion you are trying too many wrongs to make a right.

First, the Caddie convertible, if in good shape, should be a good tow
vehicle.  It has a big engine and transmission and a heavy frame
(convertibles have reinforced frames over sedans before thay went to
uni-bodies).

An engine oil cooler and a transmission oil cooler are important additions
to handle the extra load.

However, putting bigger wheels and tires to the point of interfering with
the normal spring travel of the suspension is DUMB (I don't mean to be
insulting. I think you got some bad advice.) Obviously, you need tires of
sufficient capacity to handle the load, but that should not be a problem
with the factory size or a slightly larger, lower profile tire.  What you
want to do is KEEP the same diameter (or revs per mile, as it says in the
tire catalog).  This way you keep the rear axle ratio.  If you go to a
larger diameter tire, the effective rear axle ratio goes down (smaller
numeric ratio) which reduces the torque to the rear wheels - exactly what
you do not want to do for towing. Also, your speedometer will still work
correctly.

Ride comfort comes from sufficient spring travel WITH CONTROL.  This means
the proper spring rate (stiffness) and adequate damping (control of
oscillations) which is the purpose of the shock absorbers.

Remember, shock absorbers are designed (and their mountings) to dampen
oscillations after hitting a bump or pothole (rebound and jounce).  They are
not designed to support the vehicle, (except for some modern vehicles where
the shock, and the mount, is especially designed by the auto manufacturer to
provide a load leveling function (recent models would include Mercedes
station wagons, and GM Tahoes/Yukons).

Therefore, if you want to increase the stiffness of the suspension you can
either change the springs or install air lifts.  Stiffer springs are
probably a bad idea, as it is very difficult for a car owner to get the
proper spring ration and other characteristics to provide a good compromise
of load and ride, especially under a varying load.  On the other hand, the
air lift allows you to increase load capacity and stiffness as required
simply by adjusting the amount of air pressure in the air lift.  Since an
air spring has very good spring characteristics (linear) it will provide
both comfort and extra load capacity.  Thus, I would shy away from the
"Cargo Coil" and stick with the stock factory springs, and add the Air
Lifts.  Note: the Air-Lift brand has been around for years and has been used
very successfully in buses and trucks (so long that air-lift is almost a
generic name for the suspension device).

Thus, I would stick with the factory springs and then supplement their load
capacity with the air lift. Additionally, I would install a set of stiffer,
adjustable shock absorbers such as Koni or Bilstein.  A stiffer shock does
not mean that your Caddie will "ride like a truck," it just means that the
behavior of the vehicle will be a lot better on an uneven road or during an
emergency maneuver.

You are absolutely correct in that you want to maintain the original ride
height, which is consistent with all of my suggestions.

Another thing you might consider is a rear sway bar.  Addco and Helwig are
good aftermarket sources.  Virtually all modern road cars, especially
European ones have had rear sway bars for years.  This is a spring bar which
connects the left and right ends (sides) of the axle together to prevent
lean (by transferring some of the load to the spring (and air lift) on the
opposite side of the vehicle.  It does not change the stiffness of the
suspension, but it does effectively make it stiffer from a side to side lean
standpoint which is most noticeable when going around a turn.  Since lean
also affects the front end steering geometry, the car is much easier to
control (and safer) with the rear sway bar.  The car already has a front
sway bar.

One other option on the air lift is an on board compressor which will allow
you to change the pressure in the air suspension while on the road, vs.
going to a gas station and crawling under the car with an air hose.  This
would allow you to fine tune the suspension for your style of driving and
comfort.

Good luck, and let me know how this works out.

Sincerely,

Oliver Filippi

P.S. OEM wheels are very good, often stronger than the fancy aftermarket
alloy variety.  If you need wheels, don't be afraid to go to a Cadillac
dealer (they may even have some optional wheels listed which are ordinarily
used for ambulances and hearses.  The other option is a good tire dealer,
just ask for Kelsey Hayes wheels, not an imitation.  Kelsey is a major
supplier to the automakers. Tire dealers these days love to sell fancy (and
profitable) wheels on a cosmetic rather than engineering basis.