Editor’s Note: This article was originally published in Overland Journal’s Summer 2022 Issue.
Many of you likely remember the Ford Explorer rollover lawsuits of the early 2000s. We were inundated daily with news reports about the dangers of the Explorer and the risk to society that SUVs posed in general. Fast forward 20 years, and though we hear less about SUV hazards, most drivers agree that a different driving style is required to that of a sedan. Typically, overland vehicles are in the same category that was labeled as dangerous. Add in that we seem to be adding more and more gear to our vehicles, and there is a potential recipe for disaster.
During my career as a development and testing engineer, I’ve been exposed to and involved in some of the mechanical and structural testing used to determine rollover risks. Manufacturers spend incredible amounts of time and money testing vehicles and lobbying the government to control standards. (Look at heavy-duty trucks here in the United States—no crash testing required.) This guide will give you some practical advice and insight into how manufacturers determine loading. With a little bit of knowledge and an understanding of the physics in play, you make better decisions in loading and hopefully avoid a dangerous situation.
You’ll need to learn some terms and information to understand the physics behind roof loading. For you engineers and mathematicians out there, there are forces and variables that I am intentionally omitting. My goal is to focus on the big picture and primary concepts.
While researching vehicles, you may see two published specifications related to roof load: static and dynamic (you won’t always see both, as some manufacturers don’t test for both).
STATIC ROOF LOAD is the weight that the manufacturer has tested to be safe for the vehicle structure in a non-moving environment. This load is only important in that it tells you how much weight the vehicle can support while stationary—for example, a roof tent with occupants.
DYNAMIC ROOF LOAD is the weight that the manufacturer has tested to be safe for the vehicle structure in a moving on-road environment. The force of wind on road vehicle sway, stopping distances, acceleration, and collision avoidance have all been tested, and the manufacturer feels the vehicle responds predictably and safely to the stated weight. If the manufacturer provides a dynamic load, this is a good specification to use for an off-road environment as well. Proper off-road driving techniques combined with speed control should prove to be less dynamic than a high-speed corrective maneuver.
You may notice that the specifications are given as loads, but I keep referring to weight. This is because load is what the vehicle is experiencing. Weight is a measure that we can control as drivers and how we load a vehicle. However, note that wind resistance, load stability, and movement change the vehicle’s driving characteristics.
CENTER OF GRAVITY (CG) is often a misunderstood term. Also called the center of mass, it’s a virtual point that all the surrounding mass converges on and is equalized. Think of a sphere; the CG would be the exact middle. In a vehicle, CG is the point at which all forces rotate or react upon.
MOMENT is the force that causes something to rotate. Think about how momentum is the force exerted away from a point while rotating. It’s the same idea here in that the moment is the force required to rotate something around a point.
ARM is the distance from a standard datum to a CG. Visualize a teeter-totter; the arm would be the end of the board where you sit.
DATUM will be the location we use to measure from after we determine the CG. I usually like to use the distance from the front and rear hub centers.
STATIC STABILITY FACTOR (SSF) is a static calculation to determine the likelihood of rollover. The lower the number, the higher the risk. You’ll generally find the results to be 1.0 to 2.0. Remember, this is a static calculation, meaning that if you have a low number, you have a vehicle that will roll easily. SSF is used by the NHTSA to score rollover star ratings and has also been shown to be remarkably accurate over time with actual rollover versus predicted rollover data.
TRACK is the distance of the width of the vehicle measured from the center of the tire to the center of the tire on the same axle.
Let’s dig into a practical application of using this knowledge to see where one of my personal vehicles stacks up. I have a Land Rover Defender 90 with a small 2-inch suspension lift. I’ve verified my fluids are topped up and have a full tank of fuel prior to testing. If you choose to do this at home, you’ll need vehicle scales and a lift or jacks of some sort. The intention is to make weight calculations to provide data on how easily my vehicle will roll over.
We start by finding the center of gravity using some trigonometry. In a very simplified way, all we need to know is the wheelbase, hub height, and to measure the vehicle’s weight when flat, and again with one axle (front or rear) lifted 10 inches. The change in weight distribution will tell us the height of the CG.
1994 Defender 90 Base Weight
Wheelbase, 93.9 inches
Tire height (measured), 32.9 inches
Vehicle weight, 4,008 pounds
Front axle weight, 2,483 pounds
Rear-axle weight, 1,525 pounds
Track width, 62 inches
Raised front wheels, 10 inches
New Weight Distribution Per Axle
Front axle weight, 2,451 pounds
Rear-axle weight, 1,557 pounds
We want to find where the CG is between the wheels then compute the height of the CG. Use the formula below, or make it easy on yourself and use the online calculator atwww.s2sx.com/calc.
Rf = height from front hub center to ground
Rr = height from rear hub center to ground
wb = wheelbase
Wf = front axle weight
a = front axle weight/overall weight * wheelbase. This equals the CG location ahead of the rear axle.
b = wheelbase – a
W= overall weight
Center of Gravity
35.73 inches behind front hub, 23.45 inches above ground
Now that we know the CG calculation for my Defender, let’s see what adding some roof weight does. When installing a Hannibal aluminum roof rack (85 pounds), rooftop tent (110 pounds), and awning (35 pounds), I attempted to centralize the weight loading between the wheels. This roof load is 57 inches above the calculated CG and weighs 230 pounds.
1994 Defender 90 With Load
Wheelbase, 93.9 inches
Tire height (measured), 32.9 inches
Vehicle weight, 4,238 pounds
Front axle weight, 2,622 pounds
Rear-axle weight, 1,616 pounds
Raised front wheels, 10 inches
Front axle weight, 2,501 pounds
Rear-axle weight, 1,737 pounds
Let’s dive into the data and see how these numbers apply to our rollover risk. If we look at the CG location from front to back, it changed very little. It moved rearward .07 inches. In this installation, I couldn’t really move the load rearward as the tent opens forward. This also lets me know that as I load the vehicle for a trip, I have load capacity rearward of my empty CG calculation. The CG height of 23.45 inches with no roof load moved upward to 41.48 inches with the 230-pound load. That’s an 18-inch increase. So what does that mean as it relates to rollover risk? To find out, we need to calculate the SSF.
The static stability factor is a good way of looking at your vehicle’s inherent risk of rollover based on its CG and track width. Remember, the higher the number, the less likely instance of rollover. SUVs typically score 1.0-1.3 and sedans 1.3 and above. Try not to have a daily driver below 1.0, and specialized driver training is a must below 1.0.
T = track width
CG = height of the center of gravity
Center of Gravity
35.80 inches behind front hub, 41.48 inches above ground
My Defender’s SSF Scores
No roof load = 1.32 SSF
Loaded roof = .75 SSF
That equates to my likelihood of a rollover being increased by 56 percent by adding 230 pounds to the roof of the Defender. That’s a dramatic increase and doesn’t include the dynamic energy involved with driving. Statistical data does show that the SSF correlates remarkably close to actual crash data.
To make things worse, the higher the roof load is mounted from the CG, the more leverage that load has while driving off-camber. The force changes from a vertical force and adds a horizontal component as well. This means that moving the weight further from the center effectively increases it. This same concept applies to items mounted in the front and back of the vehicle. Adding a high-mounted spare tire on a back door or having a large winch hanging out front leverages its weight as it increases in distance from the center of gravity.
Roof Loading Specifications
I reached out to some manufacturers and requested roof loading specifications, which turned out to be more of a challenge than expected. Many manufacturers don’t test roof loading and use a simulated test value. When referencing the following samples, keep in mind what we’ve learned: roof loading specifications are just a baseline, and you can quickly nullify them simply by making minor changes.
Manufacturer Roof Loading Specifications
- 2020 Ford Bronco: 450 pounds (static load), 110 pounds (dynamic load). Dynamic only vehicles without 37-inch tires.
- 2020 Jeep Wrangler Rubicon: Static and dynamic loads not published. MFG states no additional weight allowed on hard top.
- 2020 Land Rover Defender 110: 661 pounds static, 370 pounds dynamic. Dynamic with manufacturer-recommended off-road tire and factory expedition rack.
- 2020 Toyota Land Cruiser (200-Series): 440 pounds static, 150 pounds dynamic.
- 2020 Toyota Four Runner: 265 pounds static, 120 pounds dynamic.
- 2020 Subaru Crosstrek: 700 pounds static, 176 pounds dynamic.
It doesn’t take much weight on top to quickly overload our vehicles. Anything that raises the CG will always affect the vehicle’s dynamics. It’s worth asking yourself what you really need to bolt to the roof of the vehicle. Do you need a rooftop tent? Why do you want a spare tire on the roof? Is a Hi-Lift jack something you know how to use, and can you accept the extra weight on the roof? My experience has been that the answer is usually a simple one—no.
Certain situations should be avoided if possible, such as storing fuel and water on the roof. Heavy liquids and items that can slosh should be loaded as low as possible. I always buy high-quality containers and keep them inside the vehicle. Behind the front seat is a great spot for fuel and water cans. Think about the weight before you load up. Put the heaviest items low and centered between the wheels.
If you decide to add weight higher up, learn to drive with it. You will need to drive slower, give smaller inputs, and more than anything, you’ll have to learn to plan ahead. You should develop a scan of the road to identify areas where problems may arise. Utilize the same scanning and focus techniques that motorcyclists adopt. Learn to read the road and adjust your driving style with the vehicle as it is. With proper planning, good driving habits, and loading the vehicle in a safe manner, you will have a much better chance of having a successful, enjoyable expedition.
Be an overlander, not an overloader.
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