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We have found there is about an even split between the desire
for better ride quality and the desire for a lower stance.
Our street rod customers want a Cadillac ride and our sport
truck customers want to drag the ground. We intend to explain
how you can have both, within reason. We will begin with the
ride quality issues.
A good ride quality means different things to different people.
Someone who is accustomed to driving a new Lexus or Mercedes
will have an entirely different idea of ride quality from
the guy who drives a 10 year old pick-up. Technically, we
define good ride quality as the ability to minimize the effects
of road irregularities to the vehicle passengers. When the
vehicle encounters a pothole or bump in the road, it should
transverse the obstacle with as little body motion as possible.
A simple explanation, but more complicated to actually perform!
To understand how to achieve a good ride quality, it helps
to understand exactly what happens within the suspension.
When the wheel hits a bump, for example, it must ride up over
that bump. Ideally the suspension would absorb this bump with
no transfer of motion to the body. In the real world at least
some of this motion will be transferred through the spring
to the body. How much transfer takes place is affected by
several components of the suspension.
The spring, be it coil, leaf, torsion, or air, is what holds
the vehicle up and also controls the rate of compliance of
the suspension. Springs are typically rated by two specifications,
Spring rate and load capacity. Load capacity represents the
amount of weight that a spring will support at a given height.
Spring rate represents how much weight change it takes to
change the spring’s height by 1”. For example,
if a spring has a load capacity of 1000lbs and a spring rate
of 200 lbs/in, it will take a 200 lb weight change to make
the spring gain or lose 1”. The higher the spring rate,
the more load change it takes to change spring height. Most
traditional spring manufacturers list the spring rate of their
springs, not the load capacity. That is why you will find
references for 200lb, 450lb, 500lb springs, etc.
The load capacity of a traditional spring can’t change
at a given height unless the diameter or wire thickness is
physically altered. Airsprings, however, are rated by load
capacity at the industry standard of 100psi because changes
in air pressure greatly influence both load capacity and spring
rate.
The shock absorbers also control the rate of compliance
of the suspension. A proper shock must be matched to the vehicle
weight, the suspension geometry and the spring used in that
suspension. A leafspring, coilspring and an airspring all
have very different spring rate patterns and would require
different shock valving to optimize ride quality. Too soft
a shock may let the suspension bottom out on hard bumps. Too
stiff will result in poor a ride over small bumps. Since there
are a wide range of vehicle weights, spring types, and customer
preferences, adjustable shocks go a long way towards optimizing
ride quality for your car.
The tires and suspension bushings also greatly influence
ride quality. Most OEM suspensions use large soft rubber bushings
and tall tires to enhance ride quality and noise transmission.
Some street rodders like to use polyurethane bushings and
short sidewall tires. It is especially tough to get a decent
ride quality with such a combination.
Now that you have enough technical information to bore you
for awhile, we will talk about real world combinations. The
typical OEM vehicle will use a tall sidewall tire, large rubber
bushings, a soft spring rate spring and a progressively valved
shock absorber. This is all done to optimize ride quality.
Some of the higher end vehicle use electronically adjustable
shocks to optimize the ride and handling over a wide range
of road conditions. Thousands of hours and millions of dollars
are spent to optimize each oem application. When that oem
combination is changed, all of that research is voided to
some extent.
Now think about building your street rod. For most of you
the first priority is to use components that will fit or are
affordable. While the general guidelines concerning bushing
and tires can be followed on any or every car, not everyone
will spend the time and money necessary to repeatedly change
springs and shocks to get the best ride quality. This is exactly
where the air spring suspension shines! Assuming that a feasible
airspring is selected for the suspension, it can be tuned
to the parameters of the vehicle and the customer by changing
air pressure while on the road. Likewise, an adjustable shock
can be tuned to individual taste. In theory it is possible
to make a vehicle ride as good with traditional springs as
with an air suspension.
This would require the aforementioned spring and shock testing.
All of this tuning would need to be repeated when there are
significant changes made such as fuel load, passenger load,
or road surface changes. With an air suspension and an adjustable
shock, these changes can be made in a fraction of the time.
It is analogous to tuning a carburetor or tuning EFI. The
EFI is much more versatile.
The analogy to EFI is relevant in other ways, too. As with
anything adjustable, it is possible to adjust yourself into
a terrible ride quality. The mistake most people make is too
low of air pressure or too soft a shock [or shock adjustment].
This will allow the airspring to collapse too rapidly over
a bump. Since an airspring is very progressive [spring rate
rises in compression] it feels too firm. Many times adding
air or using a firmer shock will actually result in a better
ride quality. Symptoms of low air pressure are bouncy ride
on rolling bumps and bottoming on potholes.
The best way to start tuning an air suspension is to inflate
the airsprings to their design height. Any competent air suspension
manufacturer will be able to tell you this dimension. This
is where the airspring is designed to work the best. You favorite
ride quality should occur within a ½” of that
dimension, regardless of the air pressure. After that is accomplished,
start playing with the shock adjustment.
Ahh, a question with many answers! Run whatever air pressure
it takes to get the airspring to its design height. A 32 Ford
may take 45psi. A 57 Chevy may take 75psi. A new F150 may
take 60 psi. Keep in mind that the weight of the vehicle is
only one factor in determining how much air pressure is required.
A much more important influence is the geometry of the suspension.
The relationship of the load point vs. the pivot point vs.
the position of the airspring will be a huge factor in determining
the selection of the airspring as well as what air pressure
is required. If you are building your air ride suspension
from scratch, these are just some of the factors that have
to be considered. While you are at it, you must account for
ground clearance, driveline angles, and balljoint travel limits.
If you have purchased a pre-designed system from a manufacturer,
these issues have been addressed [or should have been].
No Problem! Building a successful air ride suspension is
no different that any traditional suspension. It is actually
easier.
In a leaf, coil, or torsion suspension you have to some
educated guesswork as to where the vehicle ride height will
end up and how the ride quality will be. Everyone has their
favorite recipe for using coil springs out of a Malibu with
1 coil cut off, or leafs from an S10 with a leaf removed.
This will all work to a certain extent, but lets look at why
they work and what would make them NOT work well.
Lets say your buddy builds a 48 Ford with a Camaro front subframe
and S10 leafsprings in the rear. It rides GREAT! You want
to build one the same way. Only yours will be a convertible
with a big block. It now sits too low and rides like hell.
Now you can start the research all over to find a set of coils
and leafs that will be appropriate for your car.
With an air suspension it is a matter of adjusting the air
pressure to accommodate the extra weight in the front and
the lesser weight in the rear. The added benefit is that when
you add 3 friends in the rear of that convertible for cruising,
all it takes is an extra squirt of air to re-level the car
and restore the ride quality.
There are a few things to be aware of when building any
suspension, especially an air suspension.
- Ride height – The vehicle needs to be mocked up
at your intended highway ride height. This highway ride
height needs to reflect proper ground clearance [at least
4.5”] and adequate suspension travel [at least 3”
compression and 2” in rebound]. After that, start
looking for a place to install the airsprings at their intended
design height. This is VERY important! If you have an airspring
that is intended to ride at 9” tall and you try to
make it run at 7” tall, the best ride quality will
make the car ride 2” taller than you want. If you
deflate that airspring to that 7” height to make the
car sit right, the ride quality will suffer. Any competent
airspring supplier will be able to give you the compressed,
ride height and extended dimensions of their airsprings.
- Ground Clearance – A lot of customers want to “lay
the car on the ground” DON’T DO IT! You must
have at least an inch or so of ground clearance when the
air system is fully deflated. You also need at least 4.5”
of ground clearance at ride height to be able to negotiate
speed bumps and other road obstacles safely.
- Driveshaft and balljoint travel – Balljoints will
only travel so far before they bind. The driveshaft will
go only so far into the transmission tunnel before it hits.
It is your job to find these limits and take them into account.
- Airspring clearance – The only rule here is that
the airspring must never rub on anything at any time. Period.
It requires considerable thought and planning to properly
satisfy all of these details. The nice thing about an air
suspension is that you can inflate and deflate the vehicle
through its full range of travel to check out all of these
parameters. A conventional suspension makes it much tougher
to accomplish this.
After you have properly designed and installed your air
ride suspension, you will need a source of compressed air
and a way to control it. While it is possible, at least in
theory, to use an inflation valve or even a bottle of compressed
air to inflate your suspension, the most efficient method
is an on-board compressor with a reservoir tank and a control
valve. It is with the compressor system that you can upgrade,
customize, economize or thoroughly overkill without drastic
compromise.
Any stand alone air suspension really needs an on-board compressor
and control system. Here is why: Ride quality tuning is done
in very small air pressure increments. Because the airsprings
are quite small in volume, it is very hard to inflate or deflate
in small enough increments to zero in on a great ride quality.
In addition to that, when you add load in the form of fuel,
people or luggage you have to go looking for an airhose! You
can quickly see how convenient an on-board system can be.
The sky is the limit on compressor systems. The main difference
here is rise time and convenience. The faster you want to
vehicle to come up the more compressors, more pressure and
more reservoir tanks you will need. A typical street rodder
who is not concerned with rise time will probably use a single
compressor with a 2 gallon tank and a 2 way controller. If
you want that car up in 2 seconds it will take a pair of 150psi
compressors, a pair of 3 gallon tanks and a 4 way control
system. Obviously this extra equipment will cost more money.
There are several reasons to control the airsprings independently.
Probably the biggest is to eliminate air transfer when cornering.
In a “2 way” configuration the airsprings are
tee’d together from side to side. When you negotiate
a turn to the left, for example, the vehicle will tend to
lean to the right and place more load on the right hand suspension.
The loaded right hand airspring will then start to transfer
air to the unloaded left hand airspring, thereby worsening
the problem.
Another issue is the leaning vehicle.
IF everything is equal, meaning weight, frame level, and
bushing resistance, then the car will level. In the real world,
however, driver weight, gas tank weight, and uneven suspension
bushing resistance may combine to create an unlevel car. Then,
just as with a coilover, you have to “wedge” the
car level with a bit more air pressure on one side or one
corner. These are very common problems, especially for top-heavy
cars with polyurethane bushings and no swaybars.
A 4 way control system will also offer a faster rise time
because it will flow more air. 12-15 seconds may not seem
like a long time but if you are sitting there with your finger
on the switch it may seem like forever! The RidePro solenoid
system, for example, also offers a simpler plug-in installation
and a digital gauge option.
As with any serious suspension project, a proper air suspension
system requires serious planning and forethought. DO NOT be
afraid to ask questions! Ask several different sources …you
will soon get a consensus of good information. When you find
the right enthusiast that will give you the information that
you need, don’t be afraid to reward him with your business.
You never know when you may need more information!
Advantages of an air suspension
- You can optimize the load capacity and spring rate for
nearly any weight vehicle.
- You can RE-OPTIMIZE your suspension when the load changes,
such as extra fuel, people, luggage or a trailer.
- You do not have to know what the finished weight of the
vehicle will be to select the correct components…you
can tune the suspension to the exact finished vehicle weight.
- An air suspension is tunable for different driving styles
and ride quality characteristics.
- Your vehicle can be lowered dramatically for parking
and fairgrounds cruising, and then restored to highway ride
height for a safe, comfortable trip home.
- Because you can compensate for various loads to maintain
a consistent ride height, an air suspension can actually
offer BETTER tire wear and handling.
- Air leaks – First off, airsprings DO NOT LEAK.
Look elsewhere. The leaf will be at a fitting. Spray soapy
water on all connections and look for bubbles. Don’t
forget about the control panel or digital pressure sending
units.
- Bouncy ride – low air pressure, too soft of shocks,
poor shock placement. Increase the air pressure, install
a stiffer shock, or find a more effective location for the
shock.
- Harsh ride – This is the one that is most understood.
Typically the cause of a harsh ride is too low of air pressure
which leads to an airspring height that is too low, which
forces the airspring to operate in a range that is not optimum
for best ride quality. Every airspring has an intended ride
height. Achieving that ride height is the goal…the
air pressure required to get there is not important. Poor
ride quality can also be caused by damaged or improper shocks,
or by too little suspension travel. There really needs to
be a minimum of 3” of compression travel [at the wheel]
to allow a civilized ride quality.
- Leaning – Most cars are heavier on one side than
the other…especially with a driver. Cars without swaybars
and tall, heavy cars such as a sedan delivery are most susceptible
to leaning problems. The fix is simply to use a 4 way control
system that offers independent control over each airspring.
Then you can increase the air pressure to the appropriate
airspring to level the car. We have seen some cars require
as much as 20psi difference from one side to the other.
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