Fifth Wheels
Put one on a truck and you have a tractor. But there's more to it than that.
ED THOMAS
CONTRIBUTING EDITOR
Among the things that differentiate a tractor from a truck is the fifth wheel. It allows the tractor to pull a semitrailer while supporting its front end. The fifth wheel's latching mechanism combines with the trailer's kingpin to keep both vehicles together, so it is as basic to safe operation as the steering mechanism and brakes.
Like everything else on the vehicle, the fifth wheel must be sized right and installed correctly to get the job done. There are a variety of options and special equipment with which the fifth wheel can be set up.
The fifth wheel must not only be strong enough to support static weight and dynamic forces, but also must act as a suspension of sorts by swiveling up and down to allow movements between the two vehicles. All these factors lead to the following design and spec'ing considerations:
• Vertical capacity—The type and model of fifth wheel chosen starts with the weight of the loaded trailer your tractor will pull. With an 80,000-pound gross combination weight (GCW), the nose of the semitrailer will weigh about 25,000 pounds. The desired vertical capacity would be 40,000 pounds.
• "Drawbar" capacity—This is based largely on gross combination weight. For the usual 80,000-pound-GCW highway vehicle, the rated drawbar pull of our fifth wheel would be approximately 150,000 pounds. For heavier on- or off-highway vehicles, both vertical and drawbar capacities need to be hiked. Available vertical capacities range up to 165,000 pounds and drawbar capacities to 200,000 pounds.
• Full oscillation—Allows side-to-side tilting of the fifth wheel. The more the oscillation, the lower the required center of gravity on the trailer. A lowboy trailer is one example of a trailer that can use a fully oscillating fifth wheel to minimize frame stress off-highway, while a van or reefer needs only a conventional or semi-oscillating unit.
• Rigid or "no-tilt"—For use with frameless end-dumps, which need high stability while tipping (and whose upper couplers provide oscillation). "Convertible" non-tilting fifth wheels can be pinned for such use, then unpinned for use with other types of trailers.
• Secondary locking jaws—Sometimes used as an extra safety precaution with tankers hauling gasoline and other dangerous commodities.
• Left or right-side handle mounts—The jaw unlocking handle (or "pin") can be on the left side for convenience, or on the right to keep you safely out of street traffic.
• Manual or air-release mechanism—Fifth wheels are standard with a manual release; you reach under the trailer and grab the handle, then give it a good yank. This works fairly easily if the jaw mechanism is well lubed and used often. Air release is available where hooks and drops are frequent or to simply eliminate back strains that sometimes come from trying to pull open a stubborn mechanism.
• Electronic lock indicator—Identifies correct locking sequence and gives a green/red dash light to indicate whether a couple is good or not.
• Stationary or sliding mount—A sliding fifth wheel allows redistribution of weight between the tractor's drive axle(s) and the front axle. Usually, one inch of movement forward or rearward transfers 200 to 300 pounds to or from the front axle. This is often necessary to keep axle loadings legal and avoid fines for being overweight on the axles.
Operations with repetitive loads, such as tankers, use stationary fifth wheels to ensure precise axle loadings. This keeps drivers from misadjusting the slider and saves a bit of weight. To adjust the sliding fifth wheel, you set the trailer brakes, unlock the pins with a switch on the dash, then move the tractor forward or backwards. The fifth wheel, hooked to the trailer's kingpin, moves on its tracks as the tractor chassis shifts beneath it.
This maneuver can be very hard on the clutch and U-joints. An aftermarket powered slider mounts an air cylinder between the fifth wheel and the chassis; the cylinder pushes or pulls against the chassis, moving it closer or away from the frame. This eliminates stresses to the drivetrain.
• Fifth wheel height—Hooking onto a trailer is fairly easy if the top of the fifth wheel and the bottom of the trailer's nose (where the upper coupler plate is mounted) are about the same distance from the ground. Standard height above the pavement is 47 to 48 inches. If trailers are something other than standard, the fifth wheel's height needs to be altered as part of the spec'ing process.
This can be done by choosing the right combination of tractor suspension, wheel/tire size and fifth-wheel type. Low-profile fifth wheels allow more room for extra-high trailers. By comparison (and the exact numbers might vary with tractor make), a low-profile fifth-wheel's face is about 7 inches above the top of the frame rails; a "standard" fifth wheel is 8 inches above the rails, and a "high" fifth wheel is 9 inches above the rails.
Some low-mounted air-ride suspensions can bring the fifth wheel down too far, or can reduce the clearance between the tops of the tires and the bottom of the trailer. Sometimes miscalculations cause trailer crossmembers to rub on the tire treads. This is especially critical when low-profile tires are used, and/or when fenders are mounted above the tractor's drive wheels. Fenders need extra clearance.
If your tractor's tandem is on an air-ride suspension equipped with a dump valve, you've probably used it to lower the fifth wheel to get under a low trailer nose. Just don't forget to air up the bags before you get underway.
• Mounting—The fifth wheel's pivot point is generally positioned a foot or more forward of the center line of the single drive axle or the tandem bogie. This allows forward weight transfer to the steer axle. The fifth wheel must never be positioned behind the axle/bogie center line because weight will be taken off the front axle, seriously affecting steering and overall vehicle stability.
Exactly how far forward becomes critical with stationary fifth wheels. As with all factors in truck spec'ing and building, the ideal distance can be calculated; a truck salesman can show you how, but suffice it to say that every inch forward or rear adds weight to or removes it from the front axle.
Most truck builders mount the fifth wheel at the factory, while others leave it to their dealers. Some fleets want their shops to handle this job. The Technology & Maintenance Council of ATA (703-838-1763) has detailed recommendations, and TMC members have just updated them. Details of positioning, hardware and fasteners are in Recommended Practice 603C. If you're going to mount a fifth wheel yourself or have it done, this would be a worthwhile read.
• Swing clearance —There must be sufficient clearance between the trailer's nose and the rear of the tractor's cab to keep the trailer corners from banging any sheet metal. This includes exhaust stacks, headache racks, tool or battery boxes and cab side extenders. This may require altering the configuration or size of these components, or stretching out the wheelbase. Fifth-wheel manufacturers have charts covering this.
• Ramps and frame taper—A trailer's nose must often climb onto the fifth wheel during coupling. An extended ramp on the wheel or the top of the frame offers a more gentle angle to ease shock to all components. Also, make sure every tractor is built with a tapered frame rear (we've seen at least one that wasn't), or the rearmost crossmember can get snagged by the kingpin, especially during uncoupling when the tractor moves out from under the trailer. A truck's frame is box-like and can be a problem if a truck is converted to a tractor.
• "No-grease" plates—Composite plastic plates can be fitted to the top of the fifth wheel to provide the slippery surface needed during turns. As the term says, the unit then needs no greasing; this saves money in labor and materials and greatly reduces the mess associated with dirty grease (unless trailers are already messed up by dirty grease from other tractors). Water run-off regulation pending in California may make these, or environmentally friendly grease, a must-use item.
The face of the plate should have an indented area where the plastic material nestles; if you install one on an existing fifth-wheel, there may or may not be sufficient "grab" area on the king pin for the jaws to latch onto. If the kingpin slips out of the jaws, whoops! Your tractor's suddenly a truck.
