UNDERSTANDING

TRAILER
AIR SUSPENSIONS

Trailer Commercial Vehicle Systems
A true innovator in the industry, Hendrickson is always on the brink of new and exciting products
to adapt to an ever-changing market. With goals of reliability, quality and durability, Hendrickson
has proven to be the favored choice in the trailer air suspension market.

ULTRAA-K® UTKNT

VANTRAAX® HKANT

INTRAAX® AAZ

HT™ SERIES HT230T

INTRAAX® AAL

INTRAAX® AANT
INTRAAX® AAT RCA

CONNEX™-ST

86 6 - R IDE A IR (743-3247)

www.hendrickson-intl.com

CONNEX™

UNDERSTANDING AIR SUSPENSIONS

Table of Contents
Overview Air Suspensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Understanding Ride. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Understanding Roll Stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Smart Spec’ing Tips. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
TIREMAAX® . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
User Friendly QUIK-DRAW®. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Controlling Ride Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Loading Dock Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Value-Added Features and Options . . . . . . . . . . . . . . . . . . . . . . . 18
Extended-Service Wheel-Ends. . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Brake Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Vehicle Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Aftermarket. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Trailer Suspensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Standard Terminology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

OVERVIEW

UNDERSTANDING

Air Suspensions
The popularity of air suspensions has grown in nearly
all segments of the trailer industry. In 1996, air
suspension acceptance accounted for 36 percent of the
industry. Today air suspension usage averages about
75 percent. Truckers operating vocational trailers realized
the advantages of air
suspensions early. As a
result, the migration to
air suspensions leveled
off in applications that
show high percentages
of acceptance such as in
flatbeds and drop decks.
Air suspensions continue
to gain popularity for
tankers and have become
the suspension of choice for
aluminum and combo
flatbeds. Many benefits
drive the steady movement to air suspensions.
Fleet managers experience
reduced maintenance costs and a higher resale value on
their equipment by spec’ing air suspensions. This higher
resale stems from reduced shock and vibration transmitted
to the trailer from the wheels and road. In simple terms,
there is less “wear and tear” on the equipment. Fleets can
also experience enhanced tire life and improved fuel economy.

Shippers look for better cargo
protection. Drivers like the
flexibility of air suspensions,
which allows them to haul a wide
variety of freight while maintaining
a smooth ride. Unlike
a steel spring, the air
spring’s stiffness adjusts
to the amount of weight
carried by the trailer.
Drivers continue to insist on a better ride. In today’s
business climate, recruiting and retaining good drivers
holds more importance than ever and driver satisfaction
remains a key to success.
As technology advances, air suspensions become lighter
and more price competitive. In addition, air suspensions
increase the resale or trade-in value of a trailer. As the
benefits of technological advances add up, more and
more fleets and owner operators continue to make air
suspensions standard.
Steadily gaining in market share, air suspensions spawned
an emerging advance in the form of system integration. In
1995, Hendrickson introduced INTRAAX®, the industry’s first
integrated trailer air suspension, axle and brake system.

INTRAAX® AAT

4

AIR SUSPENSIONS
Compared to most other suspension products that require
assembly of ordinary components, INTRAAX® integrated
suspension systems are truly optimized designs, with each
element tuned to work as part of a harmonious system.
Benefits include higher standards of ride control, braking
performance and overall product durability. In addition to
unmatched performance and minimal maintenance,
INTRAAX offers remarkable weight savings that translates
into increased payloads for fleets. In 1998, Hendrickson
introduced VANTRAAX®, a tandem air slider system designed
for dry vans and reefer trailers. The popularity of integrated
systems has grown over the last 18 years and has proven
popular with both OEMs and fleets.

Value-Added Features and Options

Haulers’ jobs are much easier
thanks to the extended
warranty coverage provided
by Hendrickson. Fleets
benefit from the dramatic
weight savings and
durability inherent in the
design of our integrated
systems.

QUIK-DRAW®

Hendrickson’s trailer suspensions are the finest integrated
air suspension systems available today. INTRAAX and
VANTRAAX deliver an extremely smooth ride whether the
trailer is loaded or unloaded. This softer ride helps to reduce
driver fatigue and minimizes road vibration to help diminish
damage to cargo and wear and tear on the trailer chassis.
Operators maximize load equalization across axles regardless
of axle spacing and cargo type. Hendrickson trailer suspensions
come standard with a variety of time and money saving
equipment. Features such as QUIK-DRAW® for easy pin
pulling on slider boxes, Cam Tube System™ and QUIK-ALIGN®
for easy axle alignment are all standard on Hendrickson
suspension systems. Hendrickson suspensions also have
an array of value-added options to help fleets customize
equipment to meet specific needs.

SURELOK®

QUIK-ALIGN®

UBL™ UNDER BEAM LIFT™

ZMD®

U

nderstanding Air Suspensions
addresses in detail the advantages of riding on
Hendrickson air suspensions. We believe fleet
managers and owner operators will quickly see
how choosing a Hendrickson system provides
an edge in today’s competitive market.

UNDERSTANDING AIR SUSPENSIONS

5

UNDERSTANDING

UNDERSTANDING

Ride

Some significant benefits associated

forces transmitted to the trailer and improve ride quality.
The travel of the suspension is closely tied to the natural
frequency. As the natural frequency drops, the amount of
suspension jounce or up-travel must be increased in order
to maintain acceptable ride quality.

with air suspensions include maintaining
consistent ride quality, enhancing driver
comfort and providing cargo protection
over a broad range of payloads.

How is ride quality
measured?
The industry measures ride
quality by the degree of
isolation the suspension provides
the vehicle from road inputs
without compromising vehicle control. How well a suspension
isolates or protects a trailer is reflected in the extent of road
forces and vibration reaching the vehicle and its cargo when
in service. Minimizing these forces and vibrations results in
less cargo damage and lower trailer maintenance costs.

What factors control ride quality?
Major influences on trailer cargo protection include the
suspension’s natural frequency and travel. In general,
suspensions with lower natural frequencies reduce the

5.00

Why do air suspensions work better?
Air suspensions are capable of achieving very low natural
frequencies and providing very high levels of trailer and
cargo protection. Suspension geometry and air spring
design work together to produce natural frequencies below
1.5 Hz. Recalling that lower natural frequencies create
lower cargo forces, typical mechanical suspensions have
natural frequencies that range from 2 to 5 Hz, depending
on payload.

Spring Comparison Air vs. Leaf Spring

Average Vibration Level
40,000
35,000

4.00

30,000

3.00

Axle Load (lbs)

Vibration

Natural frequency comprises the spring rate of the
suspension and the amount of weight the suspension
is supporting. The spring rate of the suspension is a
measure of the suspension's vertical stiffness or how
much the suspension deflects under a load. For a given
payload, the natural frequency of a suspension will be
increased if the spring rate or stiffness of the suspension
is increased, and it will be decreased if the suspension
spring rate is decreased.

2.00
1.00

25,000
20,000
15,000
10,000

0.00
Rough

Secondary

Highway

Simulated Road Conditions
Hendrickson
Air Suspension

Competitor
Air Suspension

Spring

5,000
0
-3.000

Jounce

-2.000

-1.000

Rebound

0.000

1.000

2.000

3.000

4.000

Axle Displacement (inch)
Spring
Hendrickson (Air Ride)

For all road conditions, Hendrickson air suspensions deliver superior ride quality and cargo protection.
6

5.000

AIR SUSPENSIONS
An air suspension changes its spring rate to
match the load it supports; a spring ride does
not. Thus, air suspensions deliver benefits that
a spring suspension cannot offer:
First, when the spring rate changes with the load,
the natural frequency of the suspension remains at
a constant, low level resulting in consistent cargo
protection regardless of payload.
Second, the suspension static height, therefore the
trailer static height, stays constant, which prevents
the loss of suspension jounce (up travel) when loading
the trailer. This is important when trying to maintain
required isolation levels for suspensions with low
natural frequencies.

Not all air suspensions are equal
Just because a trailer has an air suspension doesn’t
mean it delivers a good ride. The ride quality and vehicle
control a suspension provides is a result of the design of
the suspension as an integrated system. The suspension
system must work together with the trailer in its application
to give optimum system performance.
A Hendrickson air suspension is designed to achieve
optimum suspension system performance. Hendrickson
does this by designing a complete series of integrated
suspension systems to meet virtually any trailer application.

THE HENDRICKSON ADVANTAGE
Integrated suspension systems
Some air suspension designs are simply a compromise of
off-the-shelf components combined into workable suspension geometry. At Hendrickson, the core of our suspension
system performance is in the details of the development of
an integrated suspension system. Each suspension system
is designed to provide the optimum combination of ride
quality and vehicle control.
Suspension geometry, travel, load capacity, suspension
spring rate and damping levels are all interdependent
characteristics that the suspension engineer manages
when developing an integrated suspension design. No
single element alone can determine ride quality. Rather,
all of these factors must be carefully considered when
designing a suspension with superb ride characteristics.

Hendrickson controls the design details of the air spring
and shock absorber and their positions on the suspension.
Air springs are specified for load capacity and to provide
a spring rate and length matched to the suspension travel
requirements. Hendrickson shock absorbers meet specific
targets for damping levels based on suspension geometry,
application and actual fleet experience. Designing the
details into our suspension systems results in a superlative
combination of ride quality and vehicle control under
varying conditions using premium, serviceable components
and systems.

Hendrickson Trailer Commercial Vehicle Systems
designs and manufactures truly integrated suspension
systems that are optimized for all trailer needs.

UNDERSTANDING AIR SUSPENSIONS

7

UNDERSTANDING

UNDERSTANDING

Roll Stability
A vehicle’s ability to resist rollover determines its roll stability.
A vehicle loses its ability to resist rollover when the tires on
one side lift off the ground. Many elements of a vehicle’s
design play a factor in determining its roll stability.
Roll stability for a trailer can largely be determined by
the height of the center of mass of the trailer and the axle
track. These factors can have the greatest impact on the roll
stability of the trailer. For example, changing the trailer’s axle
track from 71.5 to 77.5 inches can improve its roll stability
by eight percent. Reducing the height of the trailer’s center

of mass from 70 to 65 inches can also improve roll stability
by eight percent.
Chassis flexibility, the suspension, tire and axle deflections
are all factors that need to be considered when determining
a trailer’s roll stability. Hendrickson trailer suspensions
provide the cargo protection of an air suspension along
with ­roll stability performance equal to a steel leaf-spring
suspension.

THE HENDRICKSON ADVANTAGE

TRAILER ROLL ANGLE

Hendrickson air suspensions provide a soft ride for driver
The vertical stiffness of a leaf-spring suspension can be
comfort as well as cargo and equipment protection.
four to five times stiffer than a Hendrickson air suspension.
At the same time, a Hendrickson air suspension provides
While a soft ride seems to be in conflict with roll
roll stability equivalent to a leaf-spring suspension. This is
stability, in reality it is not. The answer lies in the
evident in the chart below, which shows that while air and
fundamental difference between the suspension types.
leaf-spring suspensions exhibit different degrees of roll,
Since air suspensions provide a considerably softer ride
both suspensions reach tire lift-off
than leaf-spring
at approximately the same
suspensions, they
Roll Stability
cornering speed.
must rely on other
Typical Leaf-Spring Suspension
means to achieve roll
Since both suspensions reach roll
stiffness. This other
HENDRICKSON Air Suspension
stability limit at approximately
means, commonly
Spring
the same cornering speed, both
referred to as “auxillash area
suspensions are considered to
iary roll stiffness,”
provide the same roll stability
comes from the axle
performance.
on Hendrickson trailer
air suspensions.
However, suspension
vertical stiffness can be
quite different between
Trailer
CORNERING SPEED
air and leaf-spring
Tire
Lift-off
suspensions.
Hendrickson air suspensions provide roll
stability equivalent to a leaf-spring suspension.

8

(roll stability limit)

AIR SUSPENSIONS
Hendrickson air suspensions resist trailer roll by
utilizing the axle in a manner similar to a torsion bar.
The axle tube provides the necessary trailer roll
resistance that is equal to or greater than what can
be provided by a leaf-spring suspension. At the same
time, the air suspension eliminates the spring lash
of a mechanical suspension resulting in a lower roll
angle for air suspensions. This allows Hendrickson
air suspensions to have roll stability characteristics
similar to spring, while providing a ride that is
significantly superior to a leaf-spring suspension.
What this brings to fleets and owner operators is the
ideal combination of ride softness and roll stability —
in other words, the ultimate in cargo protection
and comfort.

Low-speed cornering event
• Trailer roll resisted by suspension

High-speed cornering event
• Tire lift-off
• Approaching roll stability limit

UNDERSTANDING AIR SUSPENSIONS

9

UNDERSTANDING

SMART

Spec’ing Tips
The decisions fleets and owner operators make about the equipment they run
can have a dramatic impact on operations and profitability. Recognizing the
importance of wise spec’ing choices, Hendrickson lists eight tips outlining how
choosing a Hendrickson trailer air suspension can make a difference.

1 3
4
2
TIP #1

TIP #3

Look for a suspension brand with a good reputation.
A proven track record is a good indication of your
ownership experience and how you will be treated
after the sale. It can also help down the road when
the trailer is sold.

Select a product with a good performance record in the field.
A good warranty and solid support are important, but
experiencing little or no downtime is better. Hendrickson
suspension components are designed and tested for
strength and durability over the lifespan of your equipment.

As a market leader, Hendrickson Trailer Commercial
Vehicle Systems builds its reputation on air suspension
system excellence. Hendrickson’s brand equity and
reputation for superior engineering, service and support
is strong in the market, and will retain a premium on
trailer resale.

When service is required, Hendrickson offers
excellent technical support and the most extensive