July 2018 | VehicleServicePros.com 27
S A F - H o l l a n d
manufactures axle
and suspension systems,
fi ft h wheels, kingpins and landing gear for the
commercial market.
While a lower spring rate provides a more
comfortable ride, it is not ideal for vehicle
handling and stability, Heath says. Th is is especially
an issue for commercial vehicles hauling
heavy loads, since the weight of the load will push
the springs down even further. To correct this,
engineers specify a higher spring rate to give the
vehicle better handling, stability and capacity.
With traditional leaf springs, however, a higher
spring rate equates to a suspension that is quite
fi rm and creates a rough ride for the driver, the
cargo and the vehicle itself. Th is is particularly
apparent when the vehicle is not loaded.
Why air suspensions?
Air suspension can provide the best of both
worlds. Air suspensions use air springs - rubber
bellows fi lled with pressurized air - instead of
traditional coil or leaf springs, and the air pressure
in the suspension can be increased or decreased
depending on load.
“Air ride suspensions require a special air valve
called a height control valve that controls the
amount of air pressure in the air spring,” Heath
says. “When the vehicle is loaded with
more cargo, this valve senses movement
between the axle and
chassis. In this case, the axle
will ‘squat.’ When this movement is substantial,
the valve will send more air pressure to the air
springs to bring the axle back to the appropriate
height. Suspension engineers call this ride height.”
Th e same is also true when unloading the vehicle,
Heath confi rms. In this case, the axle will lift
the chassis and the valve will sense this movement
and will exhaust air from the air springs to
bring the axle back to ride height.
Th is results in better handling and stability
and a smoother ride, whether or not the truck
is loaded.
“Air suspensions provide improved cargo
protection, which can help reduce claims for
damaged freight,” says Steve Hampson, director
of marketing and business development at
Hendrickson International. “Th is can also expand
the types of cargo that can be transported.” He
notes that a smoother ride also protects components
such as lights and hinges, leading to reduced
maintenance costs.
Hendrickson is a manufacturer of medium
and heavy duty mechanical, elastomeric and air
suspensions, as well as other components, for the
commercial transportation industry.
Another benefi t to air suspensions is that they
keep the vehicle level and at the same height
whether loaded or unloaded.
“Keeping the vehicle level also reduces wear
on a vehicle’s tires,” says
Graham Brookes, director
of technology at air
spring manufacturer
Firestone Industrial
Products. If the vehicle
is not riding level, there
is a risk of tires wearing
unevenly, and therefore
requiring replacement
more quickly.
» Air suspensions
use air springs instead
of traditional coil
or leaf springs.
Image courtesy of Hendrickson
» The pressure in an
air suspension can be
increased or decreased
depending on load.
Image courtesy of Hendrickson
Air suspension ride
height measurement
procedures
Ensure your vehicle’s ride
height is up to spec.
The following steps are recommended by axle and suspension
manufacturer SAF-Holland for properly measuring
the ride height on trucks equipped with drive axle
air suspensions. The company recommends following
the specific height control valve manufacturer’s instructions
for ride height adjustment procedures, and the
suspension and vehicle manufacturers’ maintenance
manual instructions for safety and proper practice.
To check the current ride height of your suspension,
use the following steps:
• Determine the vehicle manufacturer’s correct suspension
ride height for the vehicle. Determine which axle the
height control valve is attached to on multi-axle vehicles.
• Drive the vehicle forward in a straight line for at least
two vehicle lengths to release any bushing windup.
• With the vehicle unloaded and/or trailer disconnected
on a level surface, chock the front tires to prevent
the vehicle from rolling forward or backward.
• Pressurize the air system with a constant supply
of air in excess of 70 psig (4.8 bars), and check
that all air springs properly inflate and raise
the suspension to the proper ride height.
• On the axle where the height control valve is
attached, measure the distance from the bottom
of the frame rail to the ground.
• Measure the distance from the center
of the wheel to the ground.
• Subtract the measurement found in Step 6 from
the measurement found in Step 5 to determine
the current ride height of your suspension.
• Example: The bottom of the frame rail to the ground
measures 30” (762mm) and the center of the wheel to the
ground measures 20” (508mm). Subtract the second measurement
(20”) from the first measurement (30”) and this
equals 10” (254mm) for current ride height (30” – 20” = 10”)
• Check the value against the specification in Step 1.
Note: If the measured ride height is not within +/- 0.25”
(6mm) of the manufacturer’s specification, it must be
adjusted using the height control valve. Refer to the height
control valve manufacturer’s instructions for adjustment
procedures of that equipment and use the suspension and
vehicle manufacturers’ maintenance manual instructions
for the proper procedure for making adjustments. These
sources of information typically contain diagnostic measures
to follow to troubleshoot the cause and effect of problems.
Information provided by: SAF-Holland
/VehicleServicePros.com