LETTER TO EDITOR
Oct 1, 2009 12:00 PM
Clarification needed on light duty trailer towing
The article by Rick Weber, appearing in the August 2009 issue of TBB summarizing Dave Decker's presentation at this year's Work Truck Show, offers an explanation of the dynamics of loads on trailer combinations and tips to avoid trailer overloading. The explanation and tips, attributed in Rick's article to Dave's presentation, contain a number of technical errors that should be brought to the attention of your readers.
There is no industry recognized “Class V” trailer weight class. The maximum recognized by the SAE is Class IV. The towing equipment companies have simply invented Class V for trailers that exceed 10,000 pounds GVWR. A sub-committee of the SAE Hitch Committee, responsible for Trailer Weight Classes, is charged with extending, or developing, a class rating for trailers up to 20,000 pounds GVWR.
The statement “too much tongue weight 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” is not the real issue with heavy tongue weights. The real problem is that the tow vehicle's yaw stability, as measured by “understeer gradient”, is severely decreased. This increases the propensity of the tow vehicle to jackknife in turning maneuvers. Specifically, recent full scale testing conducted by the SAE Tow Vehicle Trailer Rating Committee (and now published in SAE J2807), determined that the use of weight distributing hitch torque should be minimized. In fact they recommend that the Front Axle Load Restoration (FALR) not exceed 100% (100% means that the front axle weight is brought back, via weight distribution, to a weight equal to its “no trailer” condition).
The statement, “too little tongue weight can reduce rear-wheel traction and cause instability, which may result in tail wagging or jackknifing” again misses the problem. Too little tongue weight reduces the sway stability of the combination vehicle (CV) trailer. This is another performance measure also addressed in SAE J2807. Although this phenomenon is sometimes referred to as tail wagging, or more commonly “fishtailing”, it is a trailer stability issue. When the trailer sways excessively at highway speeds, the forces exerted on the rear of the tow vehicle can then cause a jackknife or rollover situation. This balance of tongue weight, between too much and too little, was discussed in detail in an RVIA Seminar I gave this last March in Elkhart, Indiana. Entitled “Current Issues in Trailer Design,” this seminar is available on video via the RVIA website.
The statement that one of the best ways to check tongue weight is to have a portable scale is only applicable for small trailers (less than 3,000 pounds). The best way, one that provides not only tongue weight but individual vehicle weights and axle weights, is to use a truck scale typically found in truck stops. A complete description of how this is accomplished can be found in most trailer owner's manuals as well as the new NATM trailer handbook on towing safety, to be published within the next few months.
Not all states require brakes on trailers with a loaded weight over 3000 pounds. There are several states (including Kansas, Kentucky, Oregon, Missouri, and Wyoming) have no trailer brake laws. As we are well aware, vehicles used in commercial operations, and subject to the Federal Motor Carrier Safety Administration (FMCSA)'s safety regulations, for all practicable purposes, require brakes on all wheels when the GVW of the trailer is 3000 pounds or higher.
I understood factory installed electric brake controllers (EBC), now offered on some new trucks, are not integrated with the truck's ABS system. Generally they are stand-alone acceleration-based devices, albeit pre-wired to the trailer wiring harness by the OEM. However, the Tekonsha built controller (developed by Ford) available in Ford's Super Duty trucks does use master cylinder pressure — from the CAN (Controller Area Network) bus — in lieu of an accelerometer. Thus the newer systems are getting some electronic “integration”.
The statement “In most states surge brakes are not considered adequate for heavier trailers or commercial operations” is false. Through the work and testing of the industry based Surge Brake Coalition, FMCSA amended 49 CFR 393.48 and 49 in 2005 to exempt surge brakes, on trailers up to 20,000 pounds GVWR, from the single-actuator regulations. As a result, they are now totally legal nationwide in all commercial operations, subject to a tow vehicle-to-trailer-weight ratio (different ratio for trailers greater than 12,000 pounds GVWR). Most states adopt these federal interstate regulations into their intrastate and non-commercial vehicle regulations, making surge brakes legal on all roads.
The statement, “there is a 10,000-lb GCW limit when crossing state lines” is not properly explained. A combination vehicle with a GCWR over 10,000 pounds is considered a commercial motor vehicle if it is being used in interstate commerce (for profit motives). In this case, the truck must meet CMV requirements such as having a DOT number and carrying equipment such as safety triangles and first aid kit. But this has nothing to do with the driver, unless he is driving a combination vehicle with a GVWR over 26,000 pounds In that case he is required to have a commercial driver's license (CDL).
The tips for better towing include one that asserts 60% of the cargo weight should be in the front half of the trailer and 40% in the rear. This is a very general rule of thumb, and one employed by U-Haul and NATM on one of their warning labels. However, there is no substitute for actually weighing the CV and determining that all the weights are within manufacturer's recommendations.
In summary, several material misstatements appear in the article that should be corrected, and several areas certainly deserve more explanation and greater clarity.
Very truly yours,
Richard H Klein, P E
(former NATM consulting engineer)
© 2013 Penton Media Inc.