Tips for understanding the dynamics of properly specifying a truck and trailer combination
Aug 1, 2009 12:00 PM, BY RICK WEBER
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?”)
