Rollovers are statistically the fourth most common type of heavy truck crash on North American highways, but they are responsible for more truck driver deaths than any other type of crash.
They may represent only about 4% of all fatal crashes, but also account for 48% of truck-occupant deaths.
The most insidious factor in a rollover is that the driver is totally unaware it’s happening until remedial action becomes impossible.
We can cut to the chase here if you don’t want to bother with the scientific bafflegab and the physics lesson on what causes trucks to flip over on their sides: Slow the heck down when traversing a curve, probably to about half the recommended speed posted on the yellow advisory signs. There. Mission accomplished.
That’s probably what they told you at the last safety meeting. I’ll bet that stuck with you.
Since you’re still reading, I’ll make it worth your time. Let’s look at the forces acting on a tractor-trailer navigating a curve.
Centrifugal force
Sir Issac Newton’s first law of motion states, “A body remains … in motion at a constant speed in a straight line, unless it is acted upon by a force.”
In the case of truck in a curve, that force is called centrifugal force. According to Newton, if the truck were traveling in a straight path at a constant speed, it will continue to do so.
But as the vehicle enters a curve, its direction changes. That change in direction acts on the vehicle in such a way as to cause some percentage of the forward momentum of the vehicle to want to maintain its original path while the other percentage corresponds to the turn.
Imagine the letter “Y”. You have two sets of forces acting on the vehicle in different directions. This is why vehicles lean in a turn.
Motorists making sharp turns in cars feel their bodies forced to the side of the car opposite the direction of the turn. This force can be measured in Gs, or acceleration due to gravity. For the record, 1 G equals 9.8 meters per second squared (m/s²). But that’s not important.
What is important, as a reference, is how we feel G forces in a vehicle. In fairly aggressive turns, as above, occupants will feel about 0.35 G of sideways force — and that’s pretty uncomfortable.
A truck making a full brake application from 100 km/h will exert about 0.5 G on the driver in a forward direction.
A bobtail tractor, because of its relatively low center of gravity — think of all the weight between and below the frame rails — has a roll threshold of about 0.8 G.
Center of gravity
The center of gravity is the point in an object where its weight is evenly balanced in all directions. For people, it’s usually somewhere around your belly button.
For a loaded tractor-trailer, it’s somewhere higher than the floor of the trailer. The exact point is determined by the weight, density, and the height to which the cargo is loaded.
If a dry van trailer were evenly loaded front to back and floor to ceiling, you would expect the center of gravity to be about 10 feet high, maybe a little higher.
On a flatbed with a full load of roofing shingles or rebar (12-48 inches high), the center of gravity might be only five or six feet high.
The height of the center of gravity acts like a lever in this case. As with a long-handled wrench versus a short wrench, you can exert a lot more force with the longer wrench.
So, as the center of gravity is raised, so too is its influence on the vehicle’s propensity to tip when traveling through a curve.
Drivers can roughly estimate their trailer’s roll threshold with a bit of simple math.
- Estimate the height of the center of gravity (i.e. 10 feet)
- Estimate or measure the track width of the trailer axles (it’s roughly at the center of the brake drum, or the gap between the two tires in a dual set);
- Take half the track-width and divide that into the center of gravity height.
Track width: 7.5 feet x .50 = 3.75 feet
Track width (3.75 feet) divided by center of gravity (10 feet) = 0.375 G
But wait, there’s more.
Common practice says you should deduct another 1 G for tire and suspension compliance. They allow the trailer to lean to a certain degree and thus need to be accounted for. You also have to account for the flex in the trailer. If you’re following, the roll threshold is now at 0.275 G.
And finally, we have a phenomenon called “rearward progression of forces.” The forces are multiplied the farther away from the fulcrum (the trailer king pin) you get. That’s why wiggle-wagons wiggle.
“Going back to the stability of the tractor, with its 0.8 G roll threshold, the driver will feel quite safe going through a corner at 0.25 G,” says Brian Bullock, principal of Arizona-based Road Aware Safety Systems. “Meanwhile, the rear end of the trailer is leaning at 45 degrees and is completely beyond recovery. The trailer has already gone beyond the point of no return, yet the driver hasn’t felt anything in the seat of his pants. He has no idea that the truck is flipping over.”
Imagine the driver’s surprise when his tractor suddenly and unexpectedly lays over and is now skidding down the road on its side.
There are dozens of dash cam videos on social media that bear this out. If you see one shot from the rear, watch carefully and you’ll usually see the trailer wheels lift into the air while the tractor still has all the wheels on the ground.
How fast is too fast?
Bullock and his engineering partners at Road Aware Safety Systems have been studying rollover crashes for years and have developed an app for smartphones or tablets that can help drivers prevent rollovers by identifying unsafe speeds based on road geometry. He also provided most of the background information above.
“With curve speeds, it has less to do with the weight of the trailer and more to do with the height of the center of gravity,” he says.
The app allows the driver to input the type of trailer, the weight of the load and its estimated center of gravity.
“If it’s flatbed hauling rebar you can go through some curves at 0.4 or 0.5 G, but if it’s a tall load, where the center of gravity is halfway between the deck and the top of the vehicle, the tip-over threshold in G forces could be as low as 0.25 or 0.22. That’s where you really have to be careful,” he says.
The Road-Aware app uses 3D mapping, precise GPS geolocation, and the characteristics of the actual vehicle being driven, to alert drivers to potential rollovers before they even hit the curve.
Data on the characteristics of the actual curve, or in many instances, the roundabout, are already in the app. It calculates the safe speed to negotiate the turn and alerts the driver to slow down if they are going too fast.
“The margins on some curves are really small,” he told trucknews.com.
Bullock described a set of roundabouts on US Route 93 in Wickenburg, Ariz., near Phoenix. The safe speed to navigate those curves is about 15 mph (24 km/h). Many drivers, he says, go through there at about 20 mph (32 km/h).
The calculated tip-over threshold speed is 23 mph (37 km/h).
“At that point, you’re right on the edge,” Bullock says. “Most of the drivers we have coached with the app are unaware they are that close to the threshold. And the 2-mph margin can disappear if the center of gravity is higher than they estimated, or if there’s a soft tire on that side of the trailer or some issue with the suspension.”
In another documented incident, a driver pulling a livestock trailer rolled over on a typical interstate highway curve on I-15 in Utah. It was a 1,200-foot radius curve, so nothing out of the ordinary. The truck was traveling at 65 mph (105 km/h); again, nothing out of the ordinary.
Because of the very high center of gravity characteristics of livestock trailers, the Road-Aware app recommended a top speed of 57 mph (91 km/h) through that curve.
Expect the unexpected
Last August, four motorists died when a truck carrying a load of plastic pipe overturned in the eastbound lanes of I-70 in a construction zone in Wheat Ridge, Col., near Denver. Eastbound traffic had been diverted through an S-turn in the construction zone. The truck was traveling at the posted speed of 60 mph (96 km/h) reduced from 65 mph (104 km/h).
Bullock says the geometry of the curves at the site demanded a speed closer to 40 mph (64 km/h) for the high-profile load the truck was carrying. “In the driver’s defence, he was driving at the posted speed, but it was unsafe for that load.”
Improperly loaded or shifting cargo can upset a vehicle’s stability. Cargo could shift as the vehicle is negotiating a turn, causing a sudden change in the lateral balance of the load.
And with any dynamic load, such as livestock, liquid tanks or even dump trucks hauling slurry, the risk of a rollover is exponentially increased if speed isn’t reduced on curves.
Tips to prevent rollover crashes
American and Canadian crash data suggest rollovers most often occur on connector ramps between freeways, flyovers, and highway exit ramps. The second most common highway configuration for rollovers is winding two-lane roads, as found frequently in B.C. and Northwestern Ontario.
Be mindful of the locations and pay close attention to the speed advisory signs on curves.
As noted above, reduce your speed by one third to half of what those signs suggest.
Visually look at the load on or in the trailer to get a sense of the center of gravity. Then, remember that half of your axle track width is all that’s keeping that trailer upright in a curve.
And finally, leave an appropriate following distance to allow adequate reaction time in the event of a blocked lane or some other obstruction. You don’t want to make any sudden lane changes, especially with a tippy load.
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