Fifth Wheels

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Fontaine’s 7000 Series fifth wheels have 55,000-lb vertical and 150,000-lb drawbar pull capacities. They feature the self-adjusting No-Slack II locking mechanism, a safety trigger and light release-handle pull force.

Any fifth wheel is an extremely important piece of equipment, since it is primarily a tractor-trailer coupling device. It has a locking mechanism, sometimes called jaws, which holds on to a kingpin that protrudes downward under the nose of a trailer. The lock can be released manually, by pulling a handle on the fifth wheel, or by air pressure applied through a valve inside the cab.

A fifth wheel must hold the tractor-trailer connection securely at all times, but it also must provide a pivot point, which allows a rig to make turns. Also, fifth wheels have a large, flat top plate, which spreads out loads and adds stability, and most top plates tilt fore and aft. This allows vertical articulation of the tractor-trailer, and eases coupling by letting the trailing edge of the plate slide under the leading edge of the trailer as the tractor is backing into position.

Fifth wheels are available in various load ratings and configurations for virtually any application. Here’s a rundown of the various types. For a full listing of specifications by manufacturer, visit

Types of fifth wheels
A sliding fifth wheel or ‘slider’ can be moved forward and rearward to affect axle weight distribution, overall vehicle length and ride quality. It’s mounted on a track that has locking pins to keep it in the desired position while the vehicle is moving.

A slider allows the use of different trailers with kingpins that are mounted closer to, or farther away from, the nose of the trailer. In addition, maneuverability can be improved in

ConMet’s Simplex 400’s compression locking system is designed to minimize high hitching and provide maximum kingpin bearing area. The cushioned system offers slack-free coupling by compensating for wear to the kingpin and jaw, and also aids in load dampening. When jaw adjustment is necessary, says ConMet, it can be accomplished in minutes.

tight locations when the slider is moved forward, shortening the turning radius of the combination. With the slider moved rearward, closer to the center of the rear axle or tandem, ride quality is improved.

A slider’s locating pins can be released manually or by air pressure. If it is to be moved frequently, the air option can increase driver productivity by allowing the slider to be released, moved and relocked from inside the cab.

A stationary fifth wheel is installed in a fixed position on the tractor frame and cannot slide fore or aft. An advantage of the stationary design is less weight and lower cost than the sliding type. However, it does not allow weight distribution changes, and it will not accommodate trailers with kingpins mounted too far rearward, since they may hit the tractor during tight turns.

Rigid fifth wheels are fixed to the tractor frame, and do not tilt fore and aft. They are used when articulation is provided by other means, such as an articulated upper coupler on a frameless dump trailer.

No-tilt convertible fifth wheels can tilt fore and aft, but this movement can be locked out for applications where additional stability is required.

Oscillating fifth wheels are used primarily on lowboy trailers and tankers, where the trailer’s center of gravity (CG) is equal to or below the fifth wheel mounting height. They tilt fore and aft and, to some degree, from side to side. This design permits articulation at virtually any angle, such as in off-highway applications. However, with this flexibility comes additional weight, higher cost and increased maintenance requirements. The reason an oscillating fifth wheel can only be used with a low-CG trailer is that the side-to-side tilt could otherwise pose a risk of rollover.

A compensating fifth wheel allows movement in a lateral arc, with limited swing distance to prevent vehicle rollover. It’s used primarily in non-North American markets to absorb shocks and prevent cracks in tanker rigs. It can only safely be used in applications where the trailer’s CG is low, again to reduce the risk of rollover.

Selection tips
The following general suggestions have been offered by fifth wheel manufacturers and veteran fleet managers:

  • Make sure the fifth wheel’s load capacity – both vertical and pulling – is sufficient for your operation;
  • Check for mounting restrictions. Check tire height, trailer height and suspension travel to avoid trailer/tire interference;
  • Consider maintenance/rebuild costs and degree of difficulty;
  • Try to standardize on one spec, fleetwide, so it will be familiar (and safer) for all technicians and drivers;
  • For drivers’ sakes, make sure release, if not air, is accessible and easy to pull;
  • Be wary of fifth wheels that can latch without the trailer kingpin fully seated. This dangerous condition is called a “high hitch”;
  • Ask about cold-weather operation when grease gets thick. The mechanism may not latch securely;
  • When spec’ing a slider, choose the shortest one that will work in your operation. Excess length takes up room, impedes maintenance access, weighs more and costs more;
  • Get input from your OEMs and fifth-wheel suppliers;
  • Do your own field testing before committing to a large order. Get input from your drivers and technicians. They’ll be the ones living with the product on a daily basis.