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

["Kevin D. Allen" <overlander64@xxxxxxxxxxxxxx>]

Greetings Oliver!

Thanks for the input!  One of the reasons for my post was to insure that
I wasn't overlooking something obvious after all of the problems with
getting this vehicle setup.

<<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).>>

The car is a well-maintained, relatively low mileage (110,000 miles) and
has had only one previous owner who had it maintained by the local
Cadillac dealer at 3,000 mile intervals.

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

The transmission oil cooler was among the first additions while an
engine oil cooler is on the hold list pending the outcome of a few other
issues - - primarily wheels and tires.  The radiator is a new 4-row core
heavy duty unit, and the water pump is also a new heavy duty unit.

<<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.>>

The wheels and tires are essentially the same size as stock.  The
aftermarket wheels are 15", 8" wide, 5 bolt on 5" circle, 3.4" offset,
and a load rating of 3,600 pounds - - the stock wheels are 15" 6" wide,
5 bolt on 5" circle, 3.31" offset, and 2,600 pound load rating.  The
original tires were LR78 15 with the most recent replacements being
P23575R15XC Coopers.  It is the offset difference that is providing the
potential for tire body contact if the springs compress 2.3" without
trailer or 1.3" with trailer.

<<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.>>

The car has new Cargo Coil springs that are 30% stiffer than the
originals to compensate for the fact that the original owner removed the
level-ride system.  It also has a new set of Monroe air shocks in back
and a set of Monroe gas shocks in front.  I don't like to put more air
in the shocks than would have been the case with the original level ride
shocks which is why I am considering the AirLift/Firstone Air
Spring/ShockWave 9000 systems to supplement the Cargo Coils that are now
on the car.

<<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).>>

Eldorados came with standard level-ride rear suspensions that depended
upon the air shocks to maintain a level-ride under all conditions.
Unfortunately, my car's original owner chose to remove the system rather
than having the compressor rebuilt when it failed in 1990.

<<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).>>

The Cargo Coils are replacement coil springs and are 30% stiffer than
standard to compensate for the lack of the standard level-ride system,
and to allow for slightly grater load capacity and less rear end squat -
- replacements are no longer available from GM and the OEM sources that
I contacted only had standard rate springs available and none that would
compensate for a vehicle where the standard level-ride had been removed.

<<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.>>

The Cargo Coil springs are actually variable rate and based upon the
original GM specifications.  They have been stiffened to compensate for
the lack of level-ride.  I have driven the car about 1,000 miles since
the new Cargo Coils and Monroe Shocks were added, and it handles as well
or better than my very low mileage 1984 Eldorado Biarritz convertible
that has the factory touring package.

<<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.>>

The Eldorado has a factory rear sway bar that is incorporated into the
rear suspension geometry, and it along with the balance of the rear
suspension was treated to new bushings when the front end was rebuilt in
1998 (about 4,000 miles back).

<<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.>>

The car already has a shelf under the hood for the air compressor and
controls as it had standard level-ride when it left the factory.  I
haven't decided yet whether to add a new compressor or not.  I am
scheduled to go over the proposed installation with the AirLift dealer
on Wednesday.

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

I may need some as Murphy's Law seems to be always "looking over my
shoulder" with this car.  One of the clerks at a wheel specialty store
refused to believe me that the car had front wheel drive and a 500 cubic
V8 stock from the factory - - couldn't find any mention of it in his
application directory so had to call the manufacturer - - needless to
say this wasn't the shop that found wheels for this car.

<<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.>>

The capacity of both wheel and tire were of importance given the sheer
size of the tow vehicle let alone adding the trailer.  The car has a
shipping weight of 5,120 pounds and the original wheels had a 2,600
pound load rating.  The specifications on the new aftermarket wheels
actually exceeded the factory stock wheels with a load rating of 3,600
pounds.  The wheels for the '75 Eldorado were only used between 1968 and
1978 and were shared only with the 1966 to 1978 Oldsmobile Toronado.  GM
no longer stocks new wheels and the OEM and aftermarket manufacturers no
longer make any wheels to fit these early style GM front drives.  I was
almost surprised to find any aftermarket wheels at all - - and at the
quoted price of $140.00 each, they were only slightly more expensive
than some supposedly "guaranteed" salvage yard stock wheels from
California with a price $98.00 each including shipping with no returns -
- or much more reasonable than the "restored" dealer optional magnesium
wheels from '75-'78 at $425.00 each.


Kevin

Kevin D. Allen
1964 Overlander International/1999 GMC K2500 Suburban
1978 Argosy Minuet
1975 Cadillac Convertible (towcar in-training)
WBCCI/VAC #6359