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CAPITAL EQUIPMENT NEWS
DECEMBER 2015
35
DEFINING TRAILER SUSPENSIONS -
air or spring?
TRANSPORT
T
here are obviously many schools of
thought when it comes to trailer sus-
pension systems. Both have their par-
ticular applications and advantages but at
the end of the day the economies of scale
and applications will determine the outcome.
Leaf spring suspension is a simple form
of spring commonly used for suspensions
in wheeled vehicles. It is one of the oldest
forms of suspension, dating back to medi-
eval times. It takes the form of a slender,
arch-shaped length of steel of rectangular
cross section. The centre of the arc pro-
vides location for the axle, while tie holes
are provided at either end for attaching to
the trailer body. For very heavy loads, the
leaf spring can consist of several leaves. The
leaves that become progressively shorter
are stacked on top of one another. Such a
system is known as a multi-leaf spring.
The right type of spring to suit the appli-
cation is important. If the trailer’s springs
are too rigid, its wheels will often leave the
road when hitting bumps and disturb the
load. This also increases tyre wear. A good
suspension design keeps the wheels on the
road as much as possible. The ratings on
the spring packs are therefore paramount
to the application.
A leaf spring package consists of hangers,
u-bolts and a metal plate to which hang-
ers are fitted. Leaf springs require a cer-
tain amount of maintenance. They should
be checked for wear regularly. A worn or
cracked leaf spring can cause untold dam-
age to a trailer and its contents. Generally,
the leaf spring configuration on any trailer
has its place in the transport industry, espe-
cially for heavy loads and most rough terrain
applications, but keeping it well maintained
is the key.
The introduction of air suspensions in trucks
has over the years been extended to trail-
er suspensions. Although it has involved
re-configuring the trailer design, the ad-
vantages have been enormous. The most
significant has been the increased payload
as a result of a lighter construction for the
suspension system. In some cases the sav-
ing in tare weight can be as much as 300 kg
per rig.
Air suspension results in a lower vibration
level in the trailer under all conditions. It
not reduces damage, but also ensures that
there is uniform road contact by the wheels
at all times. This no doubt reduces tyre wear
as well.
The softer ride offered by air suspensions
protects the load from direct shock when
travelling on uneven road surfaces. This is
beneficial to transporters of delicate goods.
One of the major advantages is that all the
airbags on the trailer are interconnected,
thus ensuring that all axle loads are equal-
ised under all conditions.
The same size brake cylinder and slack ad-
juster lever lengths are used on all axles of
the trailer. The trailer with air suspension is
less sensitive to the angle it runs at due to
the differences between fifth wheel heights.
Long haul drivers using air suspension
trailers will generally find increased com-
fort and less fatigue at the end of a jour-
ney, as well experiencing less stress on the
prime mover and a reduction in fuel costs.
So, with less moving components required
for air suspensions, the maintenance factor
is greatly reduced, thus contributing to the
economies of owning such a trailer and re-
duced operating costs.
By Pierre Sanson