Tips for understanding the dynamics of properly specifying a truck and trailer combination

Aug. 1, 2009
Operators increasingly are using trailers in conjunction with light commercial vehicles (Class 1-4)

Operators increasingly are using trailers in conjunction with light commercial vehicles (Class 1-4). These trailers are commonly used to transport relatively heavy loads and can become easily overloaded.

Dave Decker, manager of truck engineering for Wheels Inc., said in a presentation earlier this year at the Work Truck Show that it is critical that operators understand the dynamics of properly specifying a truck and trailer combination.

He said the main factors to consider for determining truck specifications when towing a trailer are: gross vehicle weight (GVW), loaded trailer weight (“probably the most important factor to consider”), gross combination weight (GCW), trailer tongue/king pin weight, trailer frontal area, and operating conditions.

Decker said the GVW consists of the base curb weight plus the passenger weight plus the cargo weight, and must not exceed the manufacturer's Gross Vehicle Weight Rating (GVWR).

A breakdown on loaded trailer weight classes: Class I Light Duty (2000 lb gross trailer weight); Class II Medium Duty (2001-3500 lb); Class III Heavy Duty (3501-5000 lb); Class IV Extra-Heavy Duty (5001-10,000 lb); and Class V Maximum Heavy Duty (10,001 lb and over).

He said the GCW consists of the GVW of the truck (curb weight, passengers, and load) plus the loaded trailer weight, and must not exceed the manufacturer's recommended Gross Combination Weight Rating (GCWR).

Decker said that the trailer tongue/king pin weight is the amount of the trailer's weight that presses down on the trailer hitch.

“Too much tongue load can cause suspension and drivetrain damage,” he said. “It can force the truck down in the back, causing the front wheels to lift to the point where steering response, and braking can be severely decreased. Too little tongue load can reduce rear-wheel traction and cause instability, which may result in tail wagging or jack-knifing.”

He said that for trailers up to 2000 lb, the tongue load should not exceed 200 lb. For conventional trailers over 2000 lb, the tongue load should not exceed a range of 10-15% of the loaded trailer weight. For fifth-wheel trailers, the king pin weight should not exceed a range of 15-25% of the loaded trailer weight. The addition of the tongue weight or king pin weight should not exceed the truck's GVWR and rear Gross Axle Weight Rating (GAWR) as listed on the truck's Safety Compliance Certification Label as found on the driver's door frame.

“One of the best ways to check it is to have a portable scale and take your tongue out of the trailer and put it on a jackstand that's the same height that the hitch is,” he said.

Decker said the trailer frontal area is the total area in square feet that a moving truck and trailer exposes to air resistance. In general, 60 square feet is the maximum exposed frontal area that most manufacturers limit their Class 1-4 trucks to.

He said that exceeding the manufacturer's recommendations will significantly reduce the truck's towing performance.

“Selecting a trailer with a low-drag, rounded front design will help optimize performance and fuel economy,” he said. “Many times you see trailers that have rounded corners or angled corners. This helps the performance of the total vehicle.”

He said operating conditions include: hilly or steep grades; high altitudes (“every 1000 feet of altitude affects an engine 3-4%, so decrease your GCVW 3-4% for every 1000 ft); extreme temperatures; unfinished roads; off-road; and short or long distances. (“If it's just around town, it might make sense to use a gas engine. Or if you get out on the highway, would it be better to have a diesel engine to get better fuel economy and longevity?”)

He gave this tip on how to select the proper truck to meet your towing requirements: consult manufacturer's trailer towing guides found on their fleet Web sites to determine the model of truck, cab style, engine, transmission, rear-axle ratio, two-wheel or four-wheel drive.

The recommended truck factory options for a trailer towing package: Class III/IV receiver-style hitch; engine oil cooler; selectable tow/haul transmission mode; transmission oil cooler; upgraded cooling fans; trailer wiring harness with four- or seven-way connector; and HD flasher.

He said some states require a separate braking system on trailers with a loaded weight of more than 1500 lb. All states require it on trailers with a loaded weight of 3000 lb or more. It provides trailer braking proportioned to the vehicle's braking, using braking input, vehicle speed, and ABS logic to balance the performance of the truck's brakes and electric trailer brakes. It normally includes message-center display warnings.

“Electric brakes are activated by an electrical connection to the tow vehicle's brake pedal,” he said. “When the brakes are applied, an electric current proportional to the rate of deceleration energizes a magnet inside each brake. The magnet moves an actuating lever to apply the brakes. When the driver takes his foot off the brake pedal or the vehicle starts to move again, the current to the magnet is cut off and the brakes are released. The controller is adjustable to compensate for varying trailer loads.

“With surge brakes, braking is automatic and requires no electrical connection between the tow vehicle and trailer (except for the lights). The surge coupler is mounted on the tongue of the trailer. Inside is a linkage connected to a hydraulic master cylinder. When the tow vehicle applies its brakes, the forward momentum of the trailer pushes on the surge coupler, causing it to slide rearward and apply pressure against the master cylinder piston rod.”

Brake Controllers

He said electric trailer brake controllers are now offered as factory-installed options on many new trucks and are integrated with the truck's ABS system.

“Electric trailer brakes are activated as soon as you apply the brake pedal,” he said. ”With surge brakes, the amount of pressure applied to the trailer brakes will be proportional to the force exerted by the trailer tongue against the hitch. The greater the rate of deceleration, the greater the hydraulic pressure that is applied to the trailer brakes. After the vehicle has stopped and starts to move again, the forward pull on the surge coupler relieves pressure on the master cylinder and releases the trailer brakes. Surge brakes cannot tell the difference between normal braking and backing up.

“In most states, surge brakes are not considered adequate for heavier trailers or commercial applications (3000 lb or greater loaded trailer weight).”

He said weight-carrying hitches are intended for lighter trailers because the entire trailer's hitch weight is carried on the ball and transferred to the rear axle of the tow vehicle.

On a Class III receiver hitch, the receiver hitch platform is bolted or welded to the truck's frame. It's normally used with slide in ball mount or pintle hitch required for trailers with a loaded weight of 3500 lb or more.

He said a Class IV receiver hitch with weight-distributing equipment typically uses a much heavier ball mount (adjustable in height), plus a pair of spring bars are connected from the ball mount to the trailer's a-frame. They can be adjusted upward with chains to lift some of the weight from the rear wheels and transfer it to other wheels of the truck and trailer.

A fifthwheel hitch is mounted in the pickup bed to put more of the trailer weight more closely to the truck's rear axle. The receiver centerline of the hitch should be mounted at least two inches forward from the rear axle of the truck. This will distribute the king pin weight of the trailer for optimum load-carrying and sway-control.

Decker said a lot of people don't know or pay attention to DOT regulations.

“In many cases, a trailer being pulled by a pickup truck will exceed that limit,” he said. “We see clients that have pickup trucks — could be an F-250 or Chevy 2500 — that's well within a 10,000-lb GVW, but they hook up to a trailer that's 5000, 6000, or 7000 lb, and take them across state lines, and they get a ticket.”

He said there is a 10,000-lb GCW when crossing state lines. A DOT number is required on the truck, along with an annual truck inspection, a driver qualification file on every driver that operates that vehicle (the driver needs to be carrying a medical card proving he can operate that vehicle), a driver log with hours of operation; and a daily truck walk-around inspection.

He gave tips for better towing: the load needs to be properly distributed for optimum handling and braking; keep the center of gravity low for better handling; approximately 60% of the allowable cargo weight should be on the front half of the trailer and 40% in the rear (within limits of tongue load or king pin limits); the load should be balanced from side to side for better handling and tire wear; the load should be properly secured to prevent shifting during cornering or braking; and cross safety chains under the trailer tongue prevent the trailer tongue from contacting the ground if separation occurs.

About the Author

Rick Weber | Associate Editor

Rick Weber has been an associate editor for Trailer/Body Builders since February 2000. A national award-winning sportswriter, he covered the Miami Dolphins for the Fort Myers News-Press following service with publications in California and Australia. He is a graduate of Penn State University.