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Truck Conversions

Reprint of article by Bob StraderIn the issue of the NKK Connection, John Anderson, Executive Director, RV Safety Education, presented an article entitled Towing a Fifth Wheel with a Medium Duty Truck. John’s article focused on safety and truck characteristics needed tow our larger fifth wheels. He briefly mentioned a lighter class of truck, commonly referred to as “light medium duty” trucks that are designed to fill a gap between pickup trucks and medium duty trucks. These lighter trucks are also referred to as Class 4 and Class 5 trucks as opposed to the medium duty, which starts at Class 6.

Starting with the 1999 model year, Ford Motor Company introduced new models of the light-medium-duty Class 4 and 5 trucks (F450 and F550) to accommodate towing fifth wheel trailers in the 16,000 to 21,000 lb. ranges, with a Gross Combination Weight Rating (GCWR) of up to 30,000 lbs.

These trucks are manufactured as dual wheeled chassis models with cab, drive train, and suspension completed. They are delivered as an incomplete chassis and then the conversion facility manufactures and installs a body on which the fifth-wheel hitch is mounted and storage compartments are incorporated into the design. The conversion is then ready to be sold through Ford Motor Company’s dealer network.

Why would we want to upgrade to the higher-class truck? (Safety) As John Anderson so aptly stated in his article, “the maximum allowable fifth wheel weight for most pick-ups is in the vicinity of 13,000 to 15,000 pounds”. The Class 4 and 5 F550’s will safely tow and stop our larger fifth wheels that are manufactured with multiple slideouts and carry a Gross Vehicle Weight Rating (GVWR) of 16,000 to 20,000 lbs.

These trucks usually are equipped with the 7.3-liter PowerStroke diesel engine, four-speed automatic transmissions, like the Class 2 and 3 pickup models, the cab area is also identical. The differences are in the frame, suspension, brakes, driveshaft, axle, rims, and tire size. These components are more heavy duty. Examples would include 15” ABS brakes as opposed to 12” ABS. Axle’s accommodate up to 13,500 lbs. as opposed to 6,830 lbs. SRD, and 9,750 lbs. DRW, on the F350 models. Payloads are significantly increased with GVWR’s up to 17,500 lbs. with the Crew Cab model.

An important consideration is how well the truck rides especially when used as a “daily driver”. This is where I believe a major difference exists between the Class 6 medium duty trucks and the Class 4 & 5 light-medium duty trucks. The front axles are rated for up to 6,000 lbs., as opposed to 8,000 lb. The 6,000 lb. axle provides a softer ride. The F550 especially benefits from optional rear air ride suspension. Air ride suspensions are adjustable and the ride can be significantly softened for the non-towing application.

As John Anderson mentioned in his article, the addition of an air suspension hitch will provide additional dampening motion between the truck and the trailer. Some hitch manufacturers recommend this style of hitch on all dual pick-up through medium duty trucks. Some fifth wheel manufacturers have had to redesign their fifth wheel pin box areas to accommodate the heavier suspensions of this newer breed of the tow vehicle. The combination of rear air-ride and the air suspension hitch has made a significant difference in the ride and elimination of the “surging motion” common with standard suspended models.

Towing your fifth wheel safely is an important responsibility. Overloading the truck and fifth wheel can create a significant liability. Use of these newer model trucks helps add the needed margin of safety not available with the pick-up. In the September 2001 issue of Trailer Life, writer Ken Freund responded to a question about overloading a tow vehicle with this quote: “When you overload a vehicle, you are using yourself and your family as test pilots to find out what fails first. It also puts all other users of the road at risk. In all cases, our response will be to not exceed manufacturer’s ratings.”