Proper loading can make the difference between safe trailer towing and trailer whipping. In this U-Haul demonstration, a scale model car and trailer remain in place while the belt beneath them moves, simulating road travel. When the load is properly loaded, the trailer performs as designed. If the load is concentrated at the rear of the trailer as shown here, minor lateral forces can cause the trailer to whip violently.

U-Haul International team details diverse load of parameters influencing trailer dynamics

June 5, 2017
The U-Haul Engineering Services team has decades of experience investigating trailer accidents. So Jim Fait and Marco Garcia were an appropriate pair to review key elements of trailer crashes

The U-Haul Engineering Services team has decades of experience investigating trailer accidents. So Jim Fait and Marco Garcia were an appropriate pair to review key elements of trailer crashes, early identifiers as to the nature of the crash, and explain how causation analysis may relate to the trailer.

Fait is the director of engineering services for U-Haul International, Inc, while Garcia is the assistant director of engineering services.

In their presentation, “Trailer Dynamics,” Fait and Garcia discussed various trailer parameters affecting combination vehicle dynamics as well as attributes perceived to affect trailer behavior—including the load-heavier-in-front philosophy using the Marco Garcia Trailering Demonstrator (the viral video seen on Facebook). They analyzed the role that effective tongue length, trailer center-of-gravity location, speed, trailer-tire pressure, and accessory devices all have on trailer dynamics.

Fait and Garcia made three key points:

• Trailer loading, speed, driver inputs are all important to sway.

• Trailer design cannot eliminate sway. Design parameters help optimize between understeer and sway performance.

• Crash analysis involves many elements. The more elements identified, the better understanding of what occurred.

Fait said trailer terminology is defined by industry standards (SAE J3085 and others), federal laws (FMVSS and CFR), industry practice, and company internal terminology.

Some common terms:

Coupler. “Coupler is often called a hitch. Be sure communication is clear with whoever you are talking to.”

Hitch ball.

Ball mount.

Safety chain. “The purpose of the safety chain is to maintain connection between the tow vehicle and trailer, in the event the coupler comes off the ball or the ball comes off the hitch, as defined in SAE J684. Safety chains include a retainer on the ‘hook’ to prevent the chain from coming off during transit. The standard also defines strength of chain to meet industry standard.”

Tongue. “The trailer tongue is a structural member that extends forward from the front of the trailer body to the coupler. The tongue may include a surge brake component on braked trailers. A damaged tongue is often the result of a crash: a jackknife will typically bend the tongue sideways. A head-on collision will bend the tongue up. A broken-off tongue is a severe example.”

Tongue weight. “Trailer tongue weight is used to determine if the trailer is loaded properly—center of gravity. Tongue weight is measured with a scale placed under the coupler, while the trailer is at rest, or static. The trailer center of gravity can be determined by weight measurements and analysis.”

Trailer brakes (electric or surge). “The purpose of trailer brakes is to shorten stopping distance; to meet industry standards; to meet legal requirements; and to permit heavier trailers to be towed. It’s not the purpose of trailer brakes to prevent or counteract sway or to prevent trailer jackknife or ‘pushing.’ ”

He said “Elvis lip” is special company terminology designed to describe damage to the coupler lip when the ball is forcefully pried out of the coupler.

“It’s typically a rollover—not a simple separation of the ball and coupler,” he said. “The direction, sequence, angle—there are many things that can determine it. It’s consistent with reconstruction. Depending on your reconstruction, which way the vehicles were moving, and which way they rolled over, you can tell which one rolled over first.”

He said security chains are like safety chains, but the purpose is a secondary connection as a backup for tire straps.

Some dynamics terminology:

Damping ratio.

“It’s a measure of sway stability. In your engineering calculation, it’s a dimensionless term. A higher value is more stable. A zero or negative is unstable. Some of you have heard of the term critical speed. The critical speed is when the damping ratio is equal to zero.”

Understeer—the understeer gradient. “That’s a measure of handling stability, cornering. Fundamentally, it’s how the tow vehicle performs based on the vehicle loading condition versus the loading condition of the trailer. A higher value is more stable, but very high is less responsive. Zero or negative is unstable—‘oversteer.’ Oversteer is generally undesirable. When you have oversteer, the rear end of the vehicle wants to come around.”

Tongue weight and center of gravity.

“Trailer sway is usually the result of not loading ‘heavier in front.’ Most of you all know trailers should be loaded heavier in front. Tongue weight is a way to check the loading condition. A proper tongue weight means the trailer center of gravity is in the correct location, for the specific trailer design.”

He said parameters that affect sway (other than trailer design) are speed, driver inputs (steering and braking), and trailer loading condition.

“Too much trailer inertia can cause trailers to sway,” he said. “It means that with the center-of-gravity location, the weight is too far behind the hitch ball. That means that as you move the center of gravity farther and farther away from the hitch ball, the trailer has more swing inertia. That grows very rapidly. Tire forces are scrubbing to resist sway. As you move the center of gravity, the trailer tire forces really don’t change much. So your resisting forces are essentially constant, but your inertial forces are growing very rapidly as your moment of inertia increases due to the CG being moved rearward.”

Speed affects sway response, as shown with the damping ratio graph.

“A higher speed has a lower damping ratio,” he said. “The damping ratio also decreases as the trailer center of gravity is moved rearward. Proper loading is important. A zero damping ratio is ‘critical speed’—unstable. There is trailer ‘whipping’ and a likely loss of control when driving at the critical speed.

“The last 5 to 10 mph can make a lot of difference when approaching a combination critical speed. You can go from a relatively stable and controlled condition, and it gradually deteriorates.”

Added Garcia, “The critical takeaway is that the laws of physics say that every trailer has a critical speed. Whether or not that speed is concurrent with state laws or not, every trailer at some point it will go unstable. Properly loaded and designed trailers will have a critical speed well above highway speed limits.”

Fait said driver inputs also can affect sway.

“Even a stable trailer can be made to sway,” he said. “Steering back and forth will cause sway. This is not ‘instability.’ It is ‘forced sway.’ A driver who overreacts to an initial sway motion by trying to ‘steer out of it’ will only make the sway worse. User instructions warn about this. Steer straight ahead, let off gas, do not brake.”

He said trailer design also affects sway. Parameters that improve sway stability: longer tongue length; axle farther rearward; center of gravity forward and cargo area location; and lower trailer yaw moment of inertia (MOI).

Fait said a sway-control device is typically a mechanical/frictional type accessory.

“It’s not intended to correct an ‘unstable’ trailer loading condition—the key word being unstable, meaning you are at your critical speed for that loading condition and your vehicle combination,” he said. “It’s actually defined in J3085: a device that attaches between the tow vehicle and trailer designed to resist minor trailer sway. These devices are not intended to eliminate excessive sway due to improper trailer loading or excessive speed.

“I’ll give you an example: You have a trailer loaded improperly and the critical speed is 65 mph and you’re approaching that speed. The sway-control device is actually going to help eliminate those minor sways as you get close to speed, but once you get to 65 mph, it’s still going to go unstable. So it’s not going to correct the incorrect trailer loading. It actually might mask it so you don’t get a warning that you’re coming up on a bad condition.”

He said industry standards define trailer performance and there are three primary tests: sway stability; understeer; and straight line braking.

Sway stability test (SAE J2664):

• Drive straight at constant speed.

• Input sudden steering pulse.

• Return steering to zero and hold steady.

• Maintain zero steering while trailer sway subsides.

• Speeds: 45, 55, 65 mph.

• Data used: speed, articulation angle, steering

• Results: damping ratio versus speed.

Understeer test (SAE J266):

• Constant radius or “circle test.”

• Driver follows the painted circle on track.

• Start from zero speed and gradually increase speed with pauses at intervals of about 5 mph.

• Continue to at least 35 mph (on a 200-foot circle) to exceed 0.30 of a G in lateral acceleration.

• Data used: speed, steering wheel angle.

• Results: understeer gradient.

Straight line braking test (SAE J2807 and state/federal laws):

• Drive straight ahead at constant speed.

• Release throttle and apply brake at maximum effort.

• Maintain braking until complete stop.

• 20 mph is common test requirement.

• Data used: speed, distance, travel path.

• Results: stopping distance.

• For stability, you must remain within the 12-foot-wide lane.

He said there are a variety of other trailer tests, often of secondary interest because the tow vehicle design determines performance as spelled out in SAE J2807 in terms of acceleration performance; gradability (hill climb) for powertrain capability and powertrain cooling; launch on grade; and parking brake.

Engineering analysis of a crash involves:

Sway and loss of control.

“Measure the center-of-gravity location of the trailer. Use engineering methods if the trailer is too damaged. If you can dial that in, it answers a lot of questions. Inspect the trailer for condition and decals: coupler, suspension, tires, brakes, structural. Does it have an Elvis lip? If it came apart and there’s an Elvis lip, you know it happened during the crash. If it doesn’t have an Elvis lip, then that was possibly a disconnect and the cause of sway and loss of control.

“With scene data, motions of the vehicle and the reconstruction are going to interact with what happened, where it happened, and why it happened relative to the trailer. The tow vehicle comes into play: hitch, steering, tires, brakes, defects. Maybe it had air loss and the tire could be the factor.

“High speed, loading condition, and driver inputs are all important and interact on a sliding scale to cause sway. So if you have improper loading, high speed and driver inputs are less of a factor. But if you determine the trailer is properly loaded, then it’s probably more of a factor of driver reaction or high speed.”

Classic sleeper—then sway.

“Falling asleep or inattention, then drifting off the road can often lead to sway when the driver reacts. Scene evidence is very important. Actually, it’s a ‘lack of evidence’ when drifting off the road, then sharp steering when the driver ‘wakes up.’ It can lead to multiple steering corrections. Witnesses will often see only the resulting sway. It can also be a sharp yaw, veering across the roadway, and then rollover.”

Jackknife/tongue bent.

“It’s when the tongue is bent to the side or broken off. Witness marks on the tongue, tow vehicle, hitch, or bumper can identify it. You measure to ‘align the witness marks’ on tow vehicle and the trailer. Is there any distortion? Is there consistent direction? Is the ball bent rearward?”

Coupler off ball alleged.

“There’s an immediate allegation that the coupler came off. Maybe it did. But perhaps it’s also forced off. So one of the first questions we have is, is there an Elvis lip present? If there is an Elvis lip, something happened in the crash. It’s typically been forced off during the crash, usually during a rollover event. If there is no Elvis lip, then the coupler probably came off the ball. Was the handwheel not tightened, was the latch flipped up? You’re looking for scene photographs of the coupler. You’re talking to witnesses.”

Trailer brakes inoperative.

“Test the brake operation during investigation if possible. Validate that it’s working or not. It’s either surge or electric. Was the break-a-way found tripped? Was it even connected? Look for damage to the brake system. Look for broken or leaking fluid lines. Check the condition at the wheel and brake drum area.”

Axle or other metal fracture.

“Could be a broken tongue, could be a broken axle. Could be any broken metal part. Reconstruction and metallurgy are both important. Do the reconstruction motions explain direction of forces that would break the part?” ♦