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ridetech from Air Ride Technologies, some thoughts on ride
quality:
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!
How do
I get a good ride quality? 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.
No, really, how do I get a good ride
quality? 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 vehicles 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 many
people the first priority is to use
components that will fit or are affordable.
While the general guidelines concerning
bushings 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 but it 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 adjustable shocks, 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. nalogy 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 air pressure set too low or
shock set too soft [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 designed 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. Your
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.
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