TRAILERS under 26,000 pounds GVWR increasingly are being used by the same customers who buy commercial trucks. Not a truck body, not a truck trailer, these light- and medium-duty trailers nevertheless are finding a home and filling a niche.
National Association of Trailer Manufacturers sent two of its own to The Work Truck Show to provide tips in selecting and operating light- and medium-duty trailers. Jack Klepinger, a former president of the association, provided an introduction to the product line, along with updates on regulatory activities. Richard Klein, NATM's consulting engineer, explained some of the technical concepts that must be considered to make sure that the combination of trailer and tow vehicle is safe and productive.
Klepinger began the presentation, introducing some basic information about light- and medium-duty trailers. Among his points:
-
A gooseneck trailer has greater capacity than an equally spec'ed bumper pull trailer because the fifthwheel trailer places more weight on the tow vehicle.
-
Load distribution plays a major role in the towability of small and medium-duty trailers. A trailer loaded heavily at the front can place too much weight on the tongue of the trailer and could exceed hitch capacity. Overloading the front of the trailer could put too much weight on the front axle of the trailer and cause sway in the tow vehicle. Too little weight on the tongue may cause trailer sway.
-
A good load tiedown system helps maintain proper load distribution.
-
There are two common types of axle suspensions — leaf springs and rubber torsion assemblies. Over the past several years, the rubber torsion assembly has become the most commonly used, due to its trouble-free performance.
-
Most trailer tires are either ST (special trailer) or LT (light truck) and can be either bias or radial. Their sidewall maximum load rating must either match or exceed the GAWR. If you replace a tire, it should be the same brand, type, size and construction. Even if different tire brands appear to have the same specs on the sidewall, there still could be some size and tread surface difference that could cause some undue sway.
-
Tires and wheels are often taken for granted. They should be frequently inspected for damage, and tires should be inspected for low pressure. “I can't stress enough the importance of checking the lug nut torque on each wheel, especially during the first 200 miles,” Klepinger said. “It is recommended that the torque be checked after the first 10, 25, and 50 miles of operation and periodically thereafter — a minimum of every three months or 3,000 miles.”
-
If you take a wheel off and put it back on, it is critical that you follow the manufacturer's recommended torque value and the tightening sequence.
Keeping the wheels on
Light- and medium-duty trailers got the attention of the National Highway Traffic Safety Administration (NHTSA) a few years ago. The agency began a recall based on several instances of wheels coming off recreational vehicles in 2003 and 2004. The Recreational Vehicle Industry Association and the NATM decided to take a proactive approach to address the problem with manufacturers.
In response, Klepinger helped form the Trailer Safety Industry Coalition (TSIC) consisting of manufacturers of all different types of trailers and the suppliers of tires, axles, and wheels. NTEA is also a member of TSIC.
“Even though only a few RV manufacturers were affected, NHTSA expressed concern that other types of trailers may have had similar issues,” Klepinger said. “Fortunately, this was not the case, but the coalition has put forth a concerted effort to make sure this problem has been properly addressed. NHTSA has been extremely pleased with our relationship.”
Basic brakes
Klepinger walked the audience through the different types of trailer brakes commonly found on light- and medium-duty trailers.
-
Electric. These should include a breakaway switch that includes a battery to engage the brakes should the trailer become disconnected from the tow vehicle. A brake controller must be mounted inside the vehicle to allow the driver to adjust and operate the trailer brakes.
-
Hydraulic brakes. Variations include vacuum over hydraulic, electric over hydraulic, and air-boost over hydraulic systems.
-
Surge brakes. The surge brake utilizes a single or dual acting brake cylinder. The brakes are actuated with a “surge” mechanism (coupler). The “surge” mechanism contains a hydraulic brake master cylinder connected through linkage. As the trailer overrides the mechanism, the master cylinder forces brake fluid to the cylinder in the brake thereby engaging the brakes. As the trailer decelerates slower than the tow vehicle, the brakes disengage. The surge brake system does not require a brake controller in the tow vehicle. Therefore, you have more flexibility in your choice of tow vehicles.
Status of surge brakes
In 1993, the Department of Transportation was asked to decide the legality of surge brakes. The answer was no, and many states began enforcing this interpretation. NATM, along with other associations, formed a coalition to petition the Department of Transportation to make surge brakes legal. Significant testing was conducted to provide the data needed to show that surge brakes are safe and meet the combination vehicle brake performance regulations.
In February 2002, the Surge Brake Coalition submitted a petition for rulemaking to make surge brakes legal. The Federal Motor Carrier Safety Administration granted that petition. “The comment period has just closed, and we are anxiously awaiting its decision,” Klepinger said.
However, in September 2004, the DOT offered an enforcement policy notice that allows the use of surge brakes on small and medium-duty trailers that meet certain tow-vehicle-to-trailer weight ratios, which were developed as a result of the testing that the Surge Brake Coalition conducted.
Separate ratios apply for trailers above and below 12,000 pounds GVWR. If the GVWR of the trailer is less than 12,000 pounds, it can be equipped with surge brakes if its GVWR does not exceed 1.75 times that of the truck. For trailers in excess of 12,000 pounds GVWR, the trailer GVWR cannot be more than 1.25 times the GVWR of the truck. For example, a 15,000-lb GVWR trailer can be equipped with surge brakes when pulled with a truck that has at least a 12,000-lb-GVW rating (1.25 × 12,000 pounds).
“The bottom line is that right now, you can use surge brakes,” Klepinger said. “Hopefully the petition will be approved, and you will be able to use them all the time.”
A look at electrical
A 12-volt connector is required to power accessories such as interior lights and electric brakes. Three types of connectors are the most common in the industry, Klepinger said. Matching the trailer connector to the tow vehicle connector is not difficult. The trouble is when the trailer connector is wired differently from the tow vehicle connector.
“For the most part, the terminals are identified by function,” Klepinger said. “Unfortunately, trailer and truck manufacturers have never agreed on a common wire color scheme. Therefore, it is necessary that the terminals match according to function, rather than color of the wire.”
Klepinger summarized some of the federal regulations affecting commercial trucks and trailers — and how they apply to trailers with GVWR ratings of 26,000 pounds or less.
-
Underride guards. This regulation applies only to those trailers that are greater than 22 inches off the ground and have GVWR ratings above 10,000 pounds. The typical light- and medium-duty trailer has a deck height lower than 22 inches.
-
Commercial driver license. The regulations require a driver to have a CDL if the GCW rating exceeds 26,000 pounds and the GVW rating of the trailer exceeds 10,000 pounds. “The word ‘and’ is important, because you have to have both of those things. If you had a trailer with a GVWR of 10,000 pounds, a CDL is not required unless the GVWR of the truck is more than 26,000 pounds. Or if you had a trailer that was greater than 10,000 pounds GVWR and you had a GCWR of less than 26,000 pounds, a CDL would not be required. There are some state regulations that vary from this, however, so you have to make sure you are in compliance. Typically, though, this is the regulation.”
Classification of hitches
The Society of Automotive Engineers classifies the various levels of hitches in SAE J-684, Trailer Couplings, Hitches, and Safety Chains. The most recent edition was published in April 1997.
Class I — for trailers that do not exceed 2,000 pounds.
Class II — over 2,000 pounds, but not to exceed 3,500.
Class III — over 3,500 pounds, but not to exceed 5,000.
Class IV — over 5,000, but not to exceed 10,000.
“That's where J-684 stops,” Klein said. “But there are a lot of hitches out there called ‘Class V’. That's someone's marketing department deciding that the next step up from four is five, but these are not recognized by SAE at this point. However, we do have a task force that has been formed to expand J-684 for conventional hitches going up to 20,000 pounds.”
Pintle hooks are another hitch option for bumper pull trailers, typically for construction and other heavy-duty applications.
A weight-distributing hitch is another option. These are used for trailers that have significant tongue weights. These hitches have bars that act similar to handles on a wheelbarrow. They lift weight off the rear wheels of the truck and put some of it onto the front axle of the tow vehicle and onto the trailer axles.
“Few people, however, weigh axles in their configured condition,” Klein said. “When you put the tongue weight on the vehicle, it unweighs the front axle. But the recommended practice for weight distribution is to put the front axle of the truck back to what it was before the trailer was attached — as a minimum. The rear axle of the truck should carry 50% of the tongue weight, and the trailer axles should carry the other 50%. You also can go as much as 33% on the front and 33% on the rear of the tow vehicle. That generally results in a very level configuration.”
One rule of thumb that truck dealers use is to measure the distance from the ground to the front and rear bumpers of the truck prior to hitching the trailer. Measure again with the trailer attached. When they put the trailer on, they want the distance to remain the same — just squatted down. General Motors, however, just came out with a different preference — a little lower at the rear. That means that the weight transferred to the truck should return the front bumper distance to what it was before the trailer was attached.
Be careful with load distributing hitches, Klein advised.
“Back in the 1970s, we load-leveled a Buick Riviera so much that we were able to lift the rear wheels of the car off the ground,” he said. “That's instant jackknife — even when you are going at slow speeds.”
Fifthwheel hitches are designed for heavier trailers. They place the trailer weight directly over the rear axle. That's good, Klein said, because it eliminates all rear overhang — one of the items that contribute to trailer sway.
Important accessories
Klein listed several components that are important to successful towing of light- and medium-duty trailers. They include:
-
Automatic (electronic) brake controller. These frequently need to be set up properly — even when installed at the truck dealer. The controller has a gain adjustment that regulates the amount of voltage going back to the trailer when the truck brakes are applied. They also have an offset adjustment that works with a pendulum. As the vehicle decelerates, the pendulum swings forward and moves a rheostat. This is what allows the voltage to go back to the trailer brakes. This is not the sort of product that you just get in and drive. Someone needs to set it up. Also, check the ability of the controller to manually activate the trailer brakes independent of the truck brakes.
-
Sway control system. This is not necessary, but it can be beneficial. There are many types of sway control systems. The most common utilizes a friction slider bar which can be adjusted to regulate the amount of friction the sway system will provide. These, too, need to be set up. “Don't just slap on a sway control system,” Klein said. “Sometimes the trailer has a fundamental problem that the sway control system is inadequate to solve. And do not believe that a sway control system eliminates sway. Whenever a trailer is attached to a truck, there will be at least some sway. It's just a question of how much.”
-
The towing package that can be ordered with the truck. These include items such as a lower rear axle ratio, transmission and oil coolers, heavy-duty suspension and electrical systems, and extended mirrors.
Words of caution
When selecting the powertrain of the truck, Klein suggested that these factors be considered: the aerodynamic drag of the frontal area of the trailer, the top speed the customer will want to go, how steep the hill that the vehicle will travel, and the desired acceleration rate.
Klein also offered these guidelines for matching the trailer and the truck:
-
Never exceed the hitch manufacturer's load ratings.
-
Hitch attachment must be consistent with the hitch manufacturer's recommendations.
-
Hitch attachment should be consistent with the towing vehicle manufacturer's frame and attachment recommendations.
“All the components must be compatible,” Klein said.
The proper hook-up
Klein's company used to do trailer towing research for the National Highway Traffic Safety Administration (NHTSA), which is part of the Department of Transportation of the United States government.
“This was under the regime of Joan Claybrook,” Klein said. “When she was the head of NHTSA, she put a lot of money into trailer towing research. Our company was fortunate to get most of that work.”
One of the keys to successful towing is to match the trailer with the right sized tow vehicle. Klein showed a generic graph in which the ratio of tow vehicle to trailer weight was plotted along the X axis and the percentage of trailer weight on the tow vehicle (hitch load) ranged from 0 to 25% along the Y axis.
“When you have a tow vehicle that is much heavier than the trailer, you can have a wide range of hitch loads (say, from about 5% to 22% in this example) without having stability problems,” Klein said. “But as the trailer gets heavier relative to the tow vehicle, you begin to limit the range of allowable hitch weights that can be put on there. If you have too much hitch weight, the tow vehicle will have less yaw stability — wanting to jackknife and oversteer. If the hitch weight is too little, you will have excessive trailer sway. There is a point, which I call the ‘coffin corner’. This is the point where the tow-vehicle-to-trailer weight ratio is such that there is no hitch weight that will allow you to have acceptable combination vehicle handling and stability.” Klein noted that graphs such as these are vehicle specific, and can only be derived through full scale testing.
What goes wrong
Klein listed five factors (in order) as the most common reasons for loss-of-control accidents when towing a light- and medium-duty trailer:
-
Improper trailer loading — particularly from front to rear.
-
Improper tow vehicle selection for the size and weight of the trailer.
-
Excessive speed.
-
Improper steering and braking. “If you try to compensate for trailer sway with the steering wheel, you are going to lose it. That's because your steering reaction will be out of phase with the trailer sway motion. When the trailer goes left, you will be going the other way, and you will amplify that oscillation,” Klein said. “The recommended procedure is to hold the steering wheel as straight as you can and make small, trim-like adjustments to the vehicle. And don't brake the tow vehicle excessively. This reduces tow vehicle stability. If possible, reach down and apply the trailer brakes so that you pull the combination apart. This will increase stability. Be particularly careful going downhill, because that's when accidents are more prone to happen.”
-
Improper coupling. This occurs primarily with hitch balls that are not the proper size, or if the coupler is not properly secured, allowing the trailer to become decoupled from the towing vehicle. “Safety chains and emergency breakaway brakes are designed for this possibility, however if a large heavy trailer comes loose at 60 mph, it's likely to come apart,” Klein said.
Tough driving conditions
Klein also warned drivers of trucks pulling light- and medium-duty trailers to be cautious of a variety of challenging road conditions, including crosswinds, slippery roads, shoulder drop-offs, passing vehicles (the same effect as a crosswind), going downhill, and mechanical problems such as flat tires.
All of these factors can cause the trailer to drift from the track of the tow vehicle and start swaying.
“If you are going over 60 mph and have an excessive trailer to tow vehicle articulation angle (sometimes as small as six degrees), you have lost it — the combination may be unrecoverable,” Klein said.
Klein said that 90% of the loss-of-control accidents he sees occur on downgrades. He advises that drivers anticipate the downgrade as they crest the hill and slow down early.
Basic ratings for spec'ing smaller trailers
-
Tire load rating. This must be greater than or equal to the axle weight rating. In some cases, a tow vehicle manufacturer will publish a lower inflation pressure for the tires in order to get a better ride. But in trailer towing, it is recommended that you fill the tires to the maximum pressure shown on the sidewall of the tire.
-
Gross axle weight rating (GAWR). The weight that the axle system is designed to carry. As is the case with trucks, the GAWR of the trailer is the weakest link in the chain — wheels, tires, axle, and suspension.
-
Gross vehicle weight rating (GVWR). This rating, determined by the trailer manufacturer, may be greater than the GAWR because some of the load that the trailer carries is distributed to the tow vehicle. The GVW rating is based on an equally distributed load — both from side to side and front to rear. As a rule of thumb, the GVWR of a bumper pull trailer is 10-15% greater than the GAWR. Gooseneck and fifthwheel trailers have GVWRs that are 25% greater than the GAWR.
-
The gross combination weight rating (GCWR) of the tow vehicle is something that is published only by the tow vehicle manufacturer in the product literature or the owner's manual. It is not something that is stamped somewhere on the vehicle.
-
Maximum trailer capacity. This is the maximum weight of the trailer that a vehicle can tow. This can be a bogus number. If you fill the tow vehicle to the limit and then try to tow the maximum trailer weight, you may be overloaded. Towing capacity typically is determined by powertrain constraints. Vehicle handling and other concerns may further restrict the rating.