all them the Rodney Dangerfields of the trucking world. When it’s time
to spec a new truck, fifth wheels get no respect. While hours are spent
going over the smallest of details for more glamorous components, such
as the engine and transmission, decisions about the utilitarian fifth
wheel can be glossed over. Yet fifth wheels do perform a critical job,
their design is deceptively complex, and there are more options to
consider than you might think. To help you make a more educated fifth
wheel spec’ing decision, we’ve enlisted the help of experts from
Holland Hitch of Canada Ltd. and Fontaine Fifth Wheel.
Stationary or sliding: This is your first consideration. If you
anticipate that the axle loading, kingpin setting and combination
length of the truck in question will remain constant, a stationary
fifth wheel will do. The style of mounting is the next consideration.
An angle-on-frame mount (low cost and less torsional rigidity) and a
plate mount (higher cost, more torsional rigidity) are the options.
If your operation requires different kingpin settings and varying
combination weights, the flexibility provided by a sliding fifth wheel
is worth the extra cost. It also has a higher resale value. With a
sliding fifth wheel, you will also need to consider the type of release
required. If your operation calls for frequent adjustments for weight
distribution or weight configuration, an air slide is the smart spec.
If few adjustments are required, the lower-cost manual release will do.
Your final decision with a sliding fifth wheel concerns the length of
the slide. Specify too short a length and you may not be able to shift
enough weight from your drives to your front axles to prevent an
overload situation. Too long a slide could end up interfering between
the tractor cab and the front of the trailer when turning, as well as
adding cost and weight.
Height: A critical spec as the overall unladen height combination of
13’6″ must be maintained. The fifth wheel height should match the upper
coupler plate level. If you’re running spring suspensions, keep in mind
that they can deflect up to two inches under load and your fifth wheel
height will have to allow for that degree of deflection. Remember that
lowering the fifth wheel height results in less forward and aft
articulation. Under severe applications, such as logging or
construction, this can result in damage to the fifth wheel as well as
the frame and trailer. The maximum height is determined by subtracting
the trailer height and unladen tractor frame height from the maximum
height of 13’6″.
Ratings & Capacities: The towed vehicle weight (TVW) to be pulled,
the maximum drawbar load expected, the vertical load to be carried, and
the type of operation the fifth wheel will be exposed to are the key
considerations in spec’ing the right rating and capacity. Vertical load
refers to the weight of the loaded trailer’s nose as it sits on the
fifth wheel. Drawbar capacity is a measure of the relative strength of
the fifth wheel in terms of what it must pull. Play it safe when
specifying a capacity rating; spec a rating that can handle the
heaviest weights you’re likely to haul.
Oscillation: This is the side-to-side tilting movement of the fifth
wheel that will change as the connection between the tractor and
trailer is altered while road bumps and curves are negotiated. The
standard over-the-road fifth wheel is semi-oscillating, which means it
can articulate about an axis perpendicular to the vehicle centreline.
It relieves stresses both forward and aft of the axis. A fully
oscillating fifth wheel is designed to provide both front-to-rear and
side-to-side oscillation between the tractor and semi-trailer. Spec
this one if the centre of gravity of the loaded trailer is at or below
the top of the fifth wheel. A rigid fifth wheel design is fixed in
location. The oscillation is provided by other means. There are also
elevating fifth wheels which allow you to convert a standard road
tractor for yard spotting, switching and hauling.
Weight: Weight adds to the initial cost of the fifth wheel and adds
down-the-road costs through reduced carrying capacity and higher fuel
bills. Yet purchasing a fifth wheel lighter than what your operation
requires can result in additional maintenance and downtime. There’s a
balance you will have to strike between cost and strength.