Truck News


Fifth Wheels

When it's time to spec a new truck, fifth wheels often don't get the attention they deserve. While hours are spent going over the smallest of details for more glamorous components, such as the engine ...

When it’s time to spec a new truck, fifth wheels often don’t get the attention they deserve. 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 are often 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 put together a list of the items that should be considered.

Stationary or sliding: This is your first consideration. If you anticipate that the axle load, kingpin setting and combination length of your truck 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 of 13’6″.

Ratings and 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.

Oscillation: This is the side-to-side tilting movement of the fifth wheel that changes as the connection between the tractor and trailer is altered as 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.

Weight: Weight adds to both the initial cost of the fifth wheel and the down-the-road costs through reduced carrying capacity and higher fuel bills. But buying a fifth wheel lighter than what your operation requires can turn into nothing but problems.

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