By
Paul Abelson
Technical Editor
"To air or not to air, that is the question." Pardon the pun, but when spec’ing a tractor or trailer, should you specify a mechanical (spring) suspension, or upgrade to air? The suspension is among the major systems that define the characteristics of a truck, and thus determine how well suited it will be for its tasks. It affects ride quality, driver comfort, cargo protection, vehicle stability, maintenance costs and ultimately, resale value. Going through my files of press releases and articles, I found one from 1984 that stated that almost 25 percent of new tractors are being sold with air suspensions. A manufacturer’s sales literature from 1982 described applications where air suspensions might be advantageous. Among those mentioned were "electronics moving fleets, livestock haulers, furniture delivery and fresh fruit transport."
A
great deal has changed since 1984. Except for certain specialized
off-highway and vocational applications, virtually all tractors
and more than half the trailers made today have air suspensions.
My focus is not to sell you on the virtues of air suspensions—we
believe you already know them. Nor is it to list each of the
suspension manufacturers with a synopsis of the advantages of
their products—you can find that information at your favorite
truck or trailer dealer. What we will do is describe briefly
various types of suspensions available, what they do and how
they differ. Then we’ll give you a few tips on suspension
maintenance.
Suspension
Types
Any
compressible or deformable material that continuallyreturns
to its original shape can be used as a spring. Consider the
air and the bag that contains it to be a single entity and it
fits the definition. So do stacks of steel leaves, tapered one-piece
or two-leaf springs, rubber blocks, or steel rods. The rods,
fixed at one end, with the other attached to a wheel or axle
that twists the rod as it bounces or rebounds, is called a torsion
bar spring. Mechanical suspensions (all suspensions except air)
are broken down into four-spring, walking beam and, in the "other"
category, torsion bar and rubber block mounts.
Four-spring suspensions have each axle end of a tandem suspension attached to one of four-leaf springs. They properly position the axles, absorb road shock and (due to interleaf friction and the spring’s tendency to return to its original shape) damp the springs’ oscillations and their tendency to continue to bounce and rebound over and over.
Walking beams use a spring to support a pivot assembly, about which there is a beam on each side. Each end of a beam properly positions an axle. Bounce and rebound forces are centralized, while a great deal of movement is possible in the suspension. Walking beam suspensions are used in off-road applications and they are often seen on concrete and construction trucks. They offer great lateral stability, but tend to raise load height. Walking beams may have the beam pivot attached to a large multi-leaf spring or to rubber blocks that absorb shock.
The broadest differentiation is between air and spring suspensions, although technically, air bags are springs. Most air suspensions have a trailing arm swinging from a pivot point ahead of the axle. The arm has its pivot on the chassis and the axle clamped to the bar. This allows the axle to move up and down. As it does, it swings in an arc which brings the axle forward when it is down, and rearward as it moves up. Somewhere along the arm, an air bag/spring is mounted with its top fixed to the chassis. A torque tube is used to transfer engine torque or brake torque to the chassis. Depending on the design, the torque tube may also help properly position the axle. In theory, the airbag can be quickly emptied or filled with air, to react to any irregularities in the road while keeping the truck absolutely level. That’s theory, but in the real world, it doesn’t quite work that way. Here’s what does happen:
When a wheel hits a bump, the axle rises. As the volume inside the air spring decreases, air pressure increases, resisting the upward movement of the axle and reducing the upward force acting on the chassis. As the bump is cleared and the axle wants to fall back, or if the wheel falls into a pothole instead of hitting a bump, the volume inside the air spring increases, lowering the air pressure. The ride height control valve determines what the spring is doing. The valve can add air or exhaust it, as needed to control jounce and rebound. Shock absorbers, more properly called "dampers" in Europe, resist the tendency of the spring to oscillate. They also resist loading, especially during side sway.
Air suspensions can adapt to loads
better than any other type. Spring suspensions must be optimized
for their rated load. If carrying less, the springs are not
as flexible. That’s why air suspensions always ride more
smoothly.
Ride
Height
With
air suspensions, never alter ride height by adjusting the ride
height adjustment valve or its links. One "truckstop myth"
is that if you lower the suspension by adjusting the valve,
you’ll get a better ride. This isn’t so. If you’re
lucky, all you’ll do is bottom out a few times. If you’re
not, you could be headed for a rollover. Lowering ride height
also lowers the suspension’s roll center. Since the center
of gravity of a loaded trailer is above deck height and the
roll center is below, lowering roll center increases the rollover
leverage of the load. That increases the tendency to rollover—it
doesn’t decrease it.
During your regular pre-trip inspections, listen for air leaks, look for bent arms attached to the ride height adjustment valve, and check bushings for signs of wear. During PMs, unload the suspension (if you can) and check for free movement at all pivot points. Unloading the suspension will also allow grease to more evenly surround moving parts.
No matter if it’s mechanical
or air, a suspension selected with your operation in mind, properly
maintained according to its manufacturer’s recommendations,
will give you a good balance of stability and comfort, matched
with durability and reliability.
Design
Air
suspension designs vary. Air bags can be between the pivot and
the axle or behind the axle. Some suspensions have arms that
properly position the axle, but the air spring sits between
the axle and the chassis. Some positioning arms (more properly
called locating arms) are rigid, with all movement taken up
by the air bag. Other designs use flexible arms that act as
a spring leaf.
One of the newest suspension designs
is Meritor’s Highway Parallelogram. It uses multiple links
to create a leading arm/trailing arm arrangement that minimizes
the arc created by the axles’ movement. The air bags are
directly above each axle. It virtually eliminates a phenomenon
known as "dock walk," where the bounce and rebound
created during loading or unloading can force a trailer away
from the edge of the dock.
Maintenance
Whether
air or spring, all suspensions require some maintenance. No
matter what type of suspension you have, be sure all U-Bolts
are torqued to specification. That doesn’t mean hitting
each nut with an air wrench. Even with torque limiting devices,
air wrenches are notoriously inaccurate. Check manufacturers’
recommendations. Torque could range from 129 ft. lbs. for 1/2-20
U-Bolts to 710 ft. lbs. for a 7/8-14. While some manufacturers
allow lower grades, I recommend you always use grade 8 bolts.
In fact, it’s a good rule to use grade 8 or better for
every application on a truck. But that’s a subject for
a future article.
If you have a spring suspension, clean the springs with a wire brush and look for wear where the leaf ends contact the wear pads. The clunk you hear when spring suspensions bounce over railroad tracks or uneven roads is the unloading of the spring ends at the slipper plates, and the sudden re-loading as the spring end bounces between the plates. This can be minimized with the addition of steel or urethane shims. Spring eyes should be checked for bushing wear. Lube them as necessary.
You might want to consider Horton’s Roller Bushings. OOIDA member Mike Swiger told me he used to replace bushings every 60,000 to 70,000 miles. At a little more than 300,000, he switched to Roller Bushings and hasn’t had any problem since. His odometer now reads more than 700,000 miles. The bushings helped improve his ride, too. Upgrading bushings on steer-axle springs has cured many complaints about harsh ride (even with air suspensions).
If you have multi-leaf springs, keep them free of oil. Oil can severely alter inter-leaf friction, which can cause damping problems and alter deflection rates. When a spring is worn out, replace the springs on both sides. If one is bad, the other may follow soon. Also, you’ll want both operating at the same rates, or your side-to-side stability may be compromised. LL
