33rd annual33rd annual special problems in special

Trailer Stability: A Look at the Stabilityof Both RVs and Utility Trailers

Mr. John DailyJackson Hole Scientific Investigations, Inc.

33rd annual33rd annual

Special Problems in Special Problems in Traffic Crash ReconstructionTraffic Crash Reconstruction

May 18 - 22, 2015May 18 - 22, 2015Clearwater Beach, FloridaClearwater Beach, Florida

Trailer Stability – A look at the Stability of Both RV and Utility

Trailers

IPTM Special Problems 2015 Clearwater Beach, FL

Presented by: John Daily, MSME Jackson Hole Scientific Investigations, Inc

Introduction

Many vehicles on our highways and byways are towing RV or Utility trailers…mostly without any problems.

Occasionally, vehicles towing trailers will have a single vehicle crash causing property damage, injury, or fatalities.

The Crash Reconstructionist will have the task of gathering and interpreting the evidence to determine what happened.

4/16/2015 Copywrite 2015 John Daily 2

What Are We Going To Do?

Part 1: Illustration of the problem

In some situations, things are obviously wrong!

In other cases, serious crashes have happened for reasons not so clear.

Part 2: Understanding Trailers, Hitches, & Towing

Part 3: Introduction to Stability

Part 4: Case Study


PART 1: THE PROBLEM


Some Trailer Set-ups are just WRONG!


Some Trailer Set-ups are just WRONG!

Nothing like putting a little leverage on your bumper mount…


Let’s Pull the Single-Wide with a Pickup!

Make sure the Pickup box is filled with cinder blocks first…

Well, the tow dolly wasn't quite up to the job…

Scott Skinner Photo Scott Skinner Photo


Some Crashes are Serious…

OSP Photo


Some Crashes are Fatal…


Why did this toy hauler trailer begin swaying, precipitating this rollover?

OSP Photo OSP Photo

OSP Photo

PART 2:

UNDERSTANDING - Trailers, Hitches, and Towing


Types of Trailers

-Most all utility and RV trailers are semi-trailers

This means that some of the trailer weight is carried by the trailer tires and some is supported by the hitch.

-Full trailers carry the trailer and cargo weight on the trailer tires only. The hitch supports none of the load.

Examples of full trailers are farm wagons and pup trailers. We will not be looking at full trailers in this presentation.


Types of Trailers

Trailers may also be classified by the type of hitch arrangement.

Conventional hitch (Bumper hitch) trailers attach to the tow vehicle on or just behind the rear bumper.

Fifth wheel or Gooseneck trailers attach to the tow vehicle (usually a light truck) with an attachment in the bed slightly ahead of the rear axle.


Conventional RV Travel Trailers


Conventional Single Axle Travel Trailer. The wheelbase of the trailer is measured from the coupler to the center of the axle.

Conventional Tandem Axle Travel Trailer. The wheelbase of the trailer is measured from the coupler to the center of the tandems.

Conventional Utility Trailers


Enclosed Conventional Tandem Axle Utility Trailer.

Open Conventional Tandem Axle Utility Trailer.

Fifth Wheel Trailers


Fifth wheel Tandem Axle Travel Trailer Enclosed Fifth Wheel Tandem Axle Utility Trailer.

Toy Haulers


Fifth wheel Tandem Axle Toy Hauler

Conventional Tandem Axle Toy Hauler

Gooseneck Trailers


Gooseneck Tandem Axle Flat Bed Trailer Gooseneck Tandem Axle Stock Trailer.

Boat Trailers


Conventional Tandem Axle Boat Trailer with surge brake coupler

Gooseneck Tri-Axle Boat Trailer.

Many boat trailers have the axle(s) toward the rear of the trailer because of the boat’s weight distribution.

Trailer Weights by Type


Table Ref. [6]

Trailer Hitches

There are four types of hitches in common use: Bumper hitches are integral to the bumper on many

pickup trucks and are for light duty towing

Frame mounted hitches are for heavier duty towing in any vehicle class, e.g. car, van, pickup. AKA Receiver hitch…

Fifth wheel hitches are used on larger travel trailers and some utility trailers

Gooseneck hitches are for larger trailers and are commonly used on utility and stock trailers


Frame (Receiver) Hitches


Diagrams Ref. [6]

Frame (Receiver) Hitch Installed


Draw Bars and Couplers


This simple draw bar slides into the frame hitch and is secured by a cross pin. Draw bars may have a rise or drop so the trailer may be leveled with the tow vehicle. Photos Ref. [6] The hitch ball is attached either to a

bumper mount or a draw bar. The shank must fit the hole in the draw bar closely or the ball could work loose.

More on Hitch Balls

Hitch balls come in different sizes and ratings.

1 7/8” hitch balls are usually limited to a 2000 lb trailer and have a ¾” diameter shank.

2” hitch balls are rated to an 8000 lb trailer and usually have a 1” diameter shank.

2 5/16” hitch balls may be rated for trailer weights of up to 30,000 lb and have a shank diameter of 1 ¼ “


Shank rise is determined by the thickness of the drawbar. At least one thread should be visible below the nut and lock washer.

etrailer.com/faq-hitch-ball.aspx

Draw Bars and Couplers


Couplers fit over the ball when the locking lever is raised. After the ball is seated in the coupler, the lock lever is lowered to secure the two together. It is possible to get a “false lock” where the ball has not been seated properly and is resting on the bottom of the lock. This results in decoupling.

This coupler incorporates a surge brake, which is commonly used on boat trailers. It is a hydraulic brake system. Photos Ref. [6]

Draw Bars and Couplers…finding Proper Drop or Rise


Diagrams Ref. [6]

Weight Distributing Hitches


Sometimes the tongue weight on a trailer is more than the rear springs on the tow vehicle can safely handle. In such cases, a weight distributing hitch is used to transfer weight to the front axle. Diagrams Ref. [6]

Weight Distributing Hitches


Weight distributing hitches work by putting a torque on the drawbar by means of two symmetrical spring bars. This transfers weight (force) along the length of the vehicle frame to the front axle. The system is set up by first leveling the vehicle and trailer unhitched. The trailer is then hitched up and leveled by the chains attached to the toggles on the right. Photos Ref. [6]

Weight Distributing Hitches with Sway Control


Trailers may be subject to wind gusts or sudden steering inputs that may induce trailer sway (oscillatory instability). Weight distributing hitches don’t control sway. An added link connecting the vehicle hitch to the trailer may be added for sway control. Sway control by be done with a cam, friction brake system, or may have a hydraulic damper (shock absorber). Photos & Diagrams Ref. [6]

Friction brake system

Safety Chains


Diagram Ref. [6]

Fifth Wheel Hitches


Fifth wheel hitches are mounted in a pickup bed with the king pin location slightly ahead of the rear axle. Some are mounted on a roller system so the pin location may be moved to the rear for tight turns (cab-trailer clearance) and then back to the front for normal towing. Fifth wheel hitches are usually more

stable and can handle more trailer weight than conventional hitches. Photos Ref. [6]

Gooseneck Hitches


Gooseneck hitches couple together with a hitch ball in the bed of the pickup, again just ahead of the rear axle. The ball is attached to framework beneath the bed. Gooseneck hitches are generally stable configurations and will carry the heavy loads of large stock or utility trailers. These may be used on camper trailers as well.

Right Photo Ref. [6]

Pintle Hitches


Pintle Hitches are not often encountered with anything except certain heavy duty utility trailers or portable equipment, e.g. air compressors, generators. The pintle hitch is attached to the tow vehicle and the lunette eye is attached to the trailer.

Photos Ref. [6]

Trailer Wiring

One might think that trailer wiring has nothing to do with trailer stability.

Many trailers have electric brakes. If these brakes are not wired properly, the brakes will either come on too soon (trailer swing) or not at all (jackknife under hard braking).

We will explore trailer wiring standards next.


Trailer Wiring


Some “do it yourselfers” will attach the electric brake line to the brake light switch. Instant full on with trailer swing to follow! Diagrams Ref. [6]

Trailer Wiring


Diagrams Ref. [6]

Trailer Tires

The tires installed on the trailer make a large contribution to the stability of the system.

Trailer service tires, designated “ST”, have stiffer sidewalls and a higher cornering stiffness than do like sized “LT” designated tires.

The tire in the photo is a bias ply, indicated by the “D” after the aspect ratio.

Trailer service tires “ST” are limited to 65 mph. (Ref. [5])


PART 3:

Notes on Trailer Stability


Trailer Sway


Trailer sway is a sign of oscillatory instability. Left unchecked, it may increase in magnitude to the point where the system crashes. Sway may be transient, e.g. a gust of wind or hard steering input. This may then damp out.

Trailer Sway


The tow vehicle – trailer system may become unstable if this second order dynamic system begins to accumulate energy faster than it can dissipate that energy.

The energy input comes from the interaction of the tires with respect to the road.

Certain combinations of vehicle inertia and geometry may cause small deviations from equilibrium to cause a positive energy input.

Thus, the amplitude of the oscillations increase.

Trailer Sway


As the trailer begins to sway, it will tend to rotate about its center of mass.

The hitch acts as a constraint, so there will be a force acting on the hitch from this oscillation.

This side force will try to move the vehicle hitch.

With a bumper pull configuration where the hitch is well behind the rear axle, there will be a relatively large torque acting on the tow vehicle, causing it to yaw in the opposite direction.

Trailer Sway


Fifth wheel and gooseneck trailers have the coupling point much nearer the neutral point of the tow vehicle.

The neutral point of the tow vehicle is the point when a lateral force on the tow vehicle will not cause any yaw motion.

The neutral point will vary depending on vehicle loading and tire inflation pressure but will be well forward of the rear axle.

Fifth wheel/Gooseneck Stability


A fifth wheel configuration with a ball mount enabled this small VW Beetle to tow a reasonable sized camping trailer. This would not be possible with a bumper hitch. The ball is mounted at the neutral steer point.

youtube.com/watch?v=SpwH9WeVEfU

Trailer Design Factors


The wheelbase of the trailer should be relatively long compared to its overall length.

The trailer should have a relatively low yaw moment of inertia about the hitch.

Towing speeds have an effect on stability.

The trailer should be equipped with stiffer sidewall tires, e.g. “ST” designation, with “LT” as a second choice.

Trailer Design Factors


For bumper pull trailers, the hitch load should be at least 10% of the trailer weight.

Increasing the hitch load may improve stability as long as too much weight is not taken off the front tires.

Fifth wheel and gooseneck trailers are more forgiving. Typical values for hitch loads are 20% to 25% of the trailer weight. Ref.[5] suggests hitch loads of 10% are acceptable.

Combination Factors


First of all, the tow vehicle must be rated to pull the trailer to which it is attached. This may be looked up for most vehicles.

For bumper pull trailers, it is best if the tow vehicle is heavier than the trailer. A recommended ratio of tow vehicle to trailer weight is 1:1 or greater.

Load distributing hitches may allow for a heavier trailer.

Combination Factors


For fifth wheel and gooseneck trailers, the trailer may be heavier than the tow vehicle, up to the rated value for the tow vehicle.

In any case, trailer sway may happen. If it damps out slowly, then loss of control may result.

Accordingly, we may calculate the System Damping Ratio, Dc , to determine if the combination will be stable.

The minimum recommended value for Dc is 0.15 for all hitch types.

Ten Inputs to Calculate Dc

1. L = Trailer wheelbase

2. Υ = Tire Cornering Stiffness, lb/radian

3. N = Number of trailer tires

4. U = speed in fps

5. Ig = Yaw moment of inertia about the CG

6. Ih = Moment of inertia about the hitch

7. Wt = Trailer Weight

8. Wv = Tow vehicle weight

9. H = Hitch load in %

10. Empirical constant, 3.7

These ten things represent the evidence to be gathered for the analysis.


PART 4:

Case Study


The Crash

A Chevy van was pulling a trailer loaded with a hot air balloon on a two lane highway.

As the van entered a bridge, it crossed the center line and hit a small pickup head-on, killing the driver and sole occupant of the pickup.

The van driver admitted to being DUI, but claimed the trailer was unstable and caused the loss of control. REALLY?


Let’s Gather Some Evidence

1. L = 9.67 feet

2. Υ = 7165 lb/radian

3. N = 2

4. U = 80 fps (55 mph)

5. Ig = To be calculated

6. Ih = To be calculated

7. Wt = 2180 lb

8. Wv = 5535 lb

9. H = 340 lb, 15.5%



Cornering stiffness data may be obtained from NHTSA after some searching or by contacting the tire manufacturer. To convert from radians to degrees, divide Υ by 57.29 . We will be using lb/radian for our subsequent calculation.

Calculating the Yaw Moment of Inertia about the CG

We may approximate the yaw moment of inertia around the CG of this utility trailer with a 10’x8’ bed by using the appropriate equation for the moment of inertia for a rectangular plate about its z-axis.

𝐼𝑔 =1

12

𝑊𝑡

𝑔(𝑏2 + ℎ2)

𝐼𝑔 =1

12

2180

32.2(102 + 82)

𝐼𝑔 = 925 𝑠𝑙𝑢𝑔/𝑓𝑡2


Ref. [7], Ch.9

Calculating the Yaw Moment of Inertia about the Hitch

We may approximate the yaw moment of inertia around the hitch of this trailer by using the Parallel Axis Theorem.

𝐼ℎ = 𝐼𝑔 +𝑊𝑡

𝑔(𝑟2)

r = distance from the CM to the hitch = 8.17 ft

𝐼ℎ = 925 +2180

32.2(8.172)

𝐼ℎ = 5444 𝑠𝑙𝑢𝑔/𝑓𝑡2


We Now Have Our Evidence

1. L = 9.67 feet

2. Υ = 7165 lb/radian

3. N = 2

4. U = 80 fps (55 mph)

5. Ig = 925 slug/ft2

6. Ih = 5444 slug/ft

7. Wt = 2180 lb

8. Wv = 5535 lb

9. H = 340 lb, 15.5%


Now that we have our evidence, lets do the analysis…How?


Calculating System Damping Ratio

𝐷𝑐 =𝐿3Υ𝑁

𝑈 4𝐼ℎ−

𝐼𝑔−𝐿2𝑊𝑡𝑔

𝐻−𝐻2

𝐼ℎ(3.7

𝑊𝑡

𝑊𝑉)

𝐷𝑐 =9.673 7165 2

80 4(5444)−

925−9.6722180

32.20.155−0.1552

5444(3.7

2180

5535)

𝐷𝑐 = 0.279


Trailer Alone Hook-up Factor Tow Vehicle Sensitivity

The Next Step

Now that we have calculated the system damping ratio and find it is well above the recommended level of 0.15, we may proceed with on-road testing.

The trailer was loaded as it was on the day of the crash and was hooked to an exemplar van.

The combination was then towed at steady speeds of 40mph, 45 mph, 50 mph & 55 mph.

There was no trailer sway at any of the test speeds.


The Next Step

Since the combination is stable under steady state conditions, we will proceed with pulse steer tests at the same speeds. A pulse steer is essentially a high performance lane change. The wheel is turned rapidly to the left and then to the right, which gives an input frequency of about two Hz, well above the natural frequency of the combination.

This will induce an initial oscillation, which is then videoed. We are interested in the number of cycles for the oscillations to damp to half amplitude.

This system damped in about 0.35 cycles, for a damping of 0.30

This is close to the calculated value. The next slide has a graph showing damping ratio and number of cycles to half magnitude.


Case Resolution

When the defendant and his lawyer were confronted with the results of the analysis and testing, he decided to plead guilty straight up to the charge of Aggravated Vehicular Homicide.

He was sentenced to not less than 10 nor more than 20 years to the Wyoming State Penitentiary. Case closed…


Discussion

In Part 1, we defined the problem. Some cases are obvious, others not so much. How do we go about our investigation?

In Part 2, we looked at trailers, hitches, and towing in some detail. Essentially, this tells us we need to inspect the tow vehicle to see if they were compatible and, if so, was the system set up correctly?

In Part 3, we looked at stability in general. Hitch load, trailer wheelbase, tires, brake systems and trailer loading are all items to examine.


Discussion

We also want to examine the tow vehicle. If a rear tire had a sudden loss of air, this could induce an instability. Were the tires inflated to their recommended pressure? Any problems with the hitch? Was the tow vehicle overloaded.

Finally in Part 3, we looked at the evidence to gather to do an analytical analysis of the system stability by calculating the system damping ratio.


Discussion

In Part 4, we examined a case study involving an aggravated vehicular homicide crash wherein trailer stability became an issue.

The system damping ratio calculation showed the combination of van and trailer should have been stable.

On-road testing confirmed the stability of the combination.

A guilty plea resulted.



Questions?


References: 1. “Determination of Trailer Stability Through Simple Analytical Methods and Test Procedures”, Klein and Szostak. SAE 790186, SAE Warrensburg, PA, 1979 2. “Stability of Car Trailer Systems with Special Regard to Trailer Design”, Collins and Wong, Journal of Dynamic Systems, Measurement and Control, Vol. 96, No.2, June 1974. ASME 3. SAE Standards - Chapter 36 on “Trailers” 4. “Development of Car/Trailer Handling and Braking Standards”, Klein and Szostak, NHTSA DOT HS-805 327, Washington, DC 5. “Sway Stability of 5th Wheel RV Trailers”, Klein, Broen, Schlabach, SAE 2012-01-0237. SAE, Warrensburg, PA 2012. 6. www.curtmfg.com Excellent diagrams and photos, good discussion 7. Fundamentals of Traffic Crash Reconstruction, Daily, Shigemura, Daily. IPTM, 2006 ISBN 978-1-884566-63-9 (Chapter 9)

http://www.curtmfg.com/

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