Many RV owners, as well as dealers and manufacturers, have come to realize that today's larger trailers require more than a pick-up truck to tow them safely. Although it varies with manufacturer, the maximum allowable trailer weight for most pick-ups is in the vicinity of 13,000 to 15,000 pounds. This critical rating information can be found in the Towing Guides published by truck manufacturers, where the Gross Combined Rating (GCWR) is also available.

Advertisement Midwest Products Click Ad to pop-up full view

For larger trailers to be towed safely and within the manufacturer's limitations, a medium duty truck is typically the solution. Medium duty trucks are those such as the International 4700 LP series, the General Motors Kodiak, and the Freightliner FL50/60 series. The Ford F-450/550 series trucks are not classified as medium duty trucks, however they fill a gap between pick-up trucks and medium duty trucks.

Properly equipped, these trucks can provide a safe yet comfortable towing package, while giving years of trouble free service. Although their price tag (as much as $100K) may seem high at first glance, when their service life and resale value are compared to a pick-up truck, they can actually be less expensive to own and operate.

Most RV owners who have a medium duty truck will attest to its ease of handling, towing performance, and positive investment. However, many will admit to a less than desirable ride quality for their trailer, with frequent complaints of damage to the trailer, as well as poor ride quality for truck passengers. This situation is normally due to an improperly configured and/ or maintained truck, and in most cases is easily resolved.

The root of the problem is that medium duty trucks are designed to accommodate much higher loads than we place on them with an RV, such as the hitch load of a 5th wheel trailer. Commercial truck dealerships are accustomed to selling vehicles that will be used in severe service, and may not have the experience to understand the needs of the RV operator. If the rear suspension of the truck cannot respond (move) in reaction to trailer motion, it cannot provide any dampening of shock or vibration! Many RV owners, in pursuit of a "cheap" medium duty truck have purchased a "bare bones" cab and chassis unit, to which they have added a body and their own custom features. Many of these folks end up very disappointed with the performance of their truck.

For quite a few years the RV industry has had several medium duty truck conversions available on the market that are built specifically for RV service. Companies such as Cabriolet, Stalick, Wiers, Fontaine, and Freightliner Sport Chassis, just to name a few, have built vehicles with suspension modifications to improve ride quality. Typically these modifications consist of changing the rear air suspension using things like "ping tanks" to increase the volume of the system, enabling it to respond to loads lighter than the factory suspension was designed for. Some modifiers replace the factory metal spring configuration with air suspension systems designed specifically for the RV loads. Some add ballast to the rear suspension to balance the weight distribution, adding stability as well as enhancing ride quality. Special tuned gas shock absorbers are also used, as are many other special components, all making the vehicle uniquely equipped for the RV application.

Even with the above mentioned conversions, ride quality may not be satisfactory, especially if the hitch load is low compared to the rear axle Gross Axle Weight Rating (GAWR). An excellent compliment to the modified medium duty truck is the addition of an air suspension hitch. There are several on the market, such as the Trailer Saver, providing additional dampening motion between the truck and the trailer. However, if the hitch is not designed with the proper geometry, it may create more problems than it solves. If the vertical motion of the hitch permits a change in distance between the truck and trailer, such as happens if a short arc is traveled by the attaching point, then the result may be an annoying, if not alarming, fore and aft "surging" motion, which will be uncomfortable for the passengers. Other factors, such as harmonic frequency relationships between the hitch and the vehicle suspension must be considered. As with most RV after market products, careful investigation before purchase/installation is important.

Having said all of the above, most owners can make an immediate improvement to ride quality without costing them a penny! Having weighed hundreds of medium duty trucks, we can tell you that most owners operate their rear tires at significantly too high an air pressure! Tires are an integral part of the suspension and must flex if they are to perform as designed. Most all common medium duty trucks have dual rear wheels with tires that are designed for axle loads in the vicinity of 15,000 pounds or greater, yet a typical 5th wheel hitch load for a 18,000 trailer is in the vicinity of 5,000 pounds. Add this hitch load to the unladen weight of the rear axle, and a common result is about 10,000 pounds.

This tire loading requires an adjustment to air pressure that is frequently about 2 of that indicated on the tire sidewall for maximum load. All tire manufacturers publish load/inflation tables that give the required inflation pressure to ensure proper tire performance and life, however, the loads encountered in RV use may be well below that shown on the chart. This is why Michelin has published tables that are specifically designed to deal with RV use. For example, a 235/80R22.5 Load Range G tire typically used on a medium duty truck, which has a dual axle rating of 17,640 pounds at 100 psi, would be operated at 55 psi for a 10,000 pound axle load. This pressure permits the tires to flex in coordination with the suspension, while also providing the proper "footprint" or "tire patch" to give safe braking, especially on wet roads. Tire inflation pressure is an important ingredient in suspension performance, and ALL decisions regarding what pressure to use MUST be based on knowledge of the ACTUAL loads being carried by the tires, and with reference to the appropriate tire manufacturers load/inflation tables.