3 Types of Rollover Crashes and Why They Matter to Victims

Natalie KirbySeptember 19, 2022 7 minutes

Rollover crashes are frightening and often fatal. Most people know a rollover accident when they see one, but part of our government's work to prevent these crashes involves defining and classifying what a rollover is. The National Highway Safety Administration (NHTSA), defines a rollover as any crash where a vehicle impacts the ground on its side or its roof. So a crash is a "rollover" if a vehicle tips over onto its side or if it truly rolls over.

Additionally, the NHTSA divides rollovers into two groups: tripped and untripped. Before I go further, I realize most people probably couldn't care less how the government classifies rollover accidents or the difference between those classifications. Believe me, reading through federal crash safety reports is only my job and not how I spend my weekends. However, you may notice that the federal government only views two causes of rollovers, and the title of the article state that there are three. Let's look at how the government classifies rollovers, why this system is lacking, and how a better system could save lives.

Tripped

Before I talk about the current rollover classification's inadequacy, let me first explain the system that's already in place.

The first kind of rollover is what NHTSA calls a tripped rollover. This is when a vehicle strikes an external object and then tips or rolls. These are the most common types of rollovers, accounting for about 95% of all rollover collisions.

The most common type of tripped rollover happens when a vehicle slides sideways, and the tires hit a curb, dig into soft ground, or a similar obstacle causes a sudden increase in lateral (sideways) force. For example, it is raining and you are driving in the right lane next to the grassy shoulder. Then your vehicle hydroplanes to the right, sideways off the road. The softened grass allows the right side tires to sink in just a bit. However, the car is still moving sideways while the grass is holding the right side tires firmly in pace. The grass "trips" your vehicle and, depending on the intensity of the sideways movement, the vehicle could simply land on the passenger side or go into a full roll.

Another type of tripped rollover is when one vehicle crashes into another vehicle or object. These collisions can cause the vehicle to become unstable, and subsequently roll over. Most commonly people imagine this as a side impact collision. An impacted vehicle's tires will resist a sudden sideways force (or a change in direction), and the coupled forces send the impacted vehicle into a rollover crash.

And there are many more senerios that are considered a tripped rollover. For instance, a vehicle may roll over if it hits a large obstacle with only one of its wheels, if maneuvering over uneven terrain, if strong winds cause high-sided vehicles such as trucks, buses, and vans to be blown over, or if a trailer jackknifes.

One look at how frequently these scenarios occur and it is easy to see why tripped rollovers are the most common type. So what are "untripped" rollovers?

Untripped

Untripped rollovers are caused by steering inputs, speed, and varying forces between the ground and the vehicle's wheels, like friction, inertia, gravity, etc. As such, these rollovers are usually because of a driver's collision avoidance maneuvers. When a driver swerves to avoid a collision, especially in a top-heavy vehicle like a truck, the vehicle's momentum can cause a driver to lose control and subsequently roll over.

There are a couple of forces a vehicle experiences while driving and when rounding a corner:

  • Momentum is how much motion an object has. Think of this as the speed of the vehicle.
  • Inertia is an object's resistance to a change in motion (or lack of motion). It is the "an object in motion will stay in motion unless acted upon by an equal or greater force."
  • Centripetal force is the force that keeps an object moving in a circular path. Imagine you have a yo-yo. When you spin the yo-yo around in a circle, the string you are holding and your wrist movements keep the toy spinning. The moment the centripetal force stops, say the yo-yo string breaks, the object would travel tangentially to its original line of motion. More simply, the circular moving object (in this case a vehicle) would no longer follow the circular motion and instead travel in a straight line. In a vehicle, pressing the gas pedal and rotating the steering wheel, which changes which direction the wheels are pointing, works the same way as you moving your wrist and the yo-yo's string.
  • Centrifugal force is slightly different from the centripetal force in that it is a "perceived" force. If the centripetal force stops, the centrifugal force the moving object is “feeling” would instantly disappear. People experience a centrifugal force when a vehicle rounds a corner and their bodies seem to sway outwards or away from the direction of the turn.

Physicists can be much more precise when defining all of these concepts. For now, it is enough to know that these forces alongside gravity and the friction of a vehicle's wheels on the ground, allow a vehicle to move forward and to complete turns. If one or more of these forces are unbalanced, a vehicle can roll over.

Some small-scale examples can help show these forces at work.

A Toy Car can Illustrate the Difference Between Tripped and Untripped Rollovers

You push a Hotwheel car forward. It rolls straight ahead with no turns because there is not a tiny driver in the toy car to turn the steering wheel. Momentum carries the Hotwheel forward until the friction with the ground slows the car down until it stops rolling.

Next, you push a Hotwheel forward on a curved track. The car bumps into the track's walls and follows the curve around. Again the momentum carries the Hotwheel forward, but now the track is the "equal or greater force" that changes the toy car's direction and allows the Hotwheel to complete the turn.

Neither of those examples results in a rollover as there are no competing forces outweighing one another.

What would create a rollover? If you use a Hotwheel Rapid Launcher to shoot the toy down the curving track. Because of the rapid speed, the car's momentum greatly outweighs the force of the curved track wall, so the toy goes flying off the track.

Another way to think about this crash scenario, the Hotwheel fails to complete the turn because the curving track was not a great enough force to "change the motion" of the car. The initial speed of the car, the track's walls (which force a turn), and the toy's momentum work together to force the car into a rollover. That is a tripped rollover.

Alright, we've got that. So how would the toy car end up in an untripped rollover? Let's say you push a Hotwheel as hard as you can on a wide-open floor. There are no track walls or any other obstacles to impede the toy's trajectory. Nevertheless, a rollover is still possible. When pushing toy cars along as fast as they can, people often twist their wrist and/or hand slightly right before letting go of the toy car. That slight directional input combined with the high speed can create a rollover crash for the toy.

The Hotwheel is Much Like a Life-Sized Car

In real life, rollovers can result just as in the Hotwheel scenarios. As long as a car's alignment is correct, simply pressing the gas pedal moves the car forward in a straight line.

A life-sized vehicle has a driver inside it, so unlike the Hotwheel, a driver can use the steering wheel to complete a turn instead of relying on the track walls. However, just like the Hotwheel, when your vehicle's inertia is enough to overcome the force of gravity (assisted by the tires attempting to make a turn), the vehicle's momentum will continue straight not completing the turn. Again, just like the toy car, this can force a vehicle to roll over.

But, what if the crash isn't because of bad driving and/or external objects? What if a tire comes off a vehicle while driving? That isn't a tripped rollover, as the vehicle's tires did not hit an external object. It's not an untripped rollover either as the tire separation is not caused by steering inputs, speed, or forces.

It's fair to assume that tripped or untripped, most people read about the causes of these rollovers and assume that a driver somewhere either screwed up and hit an object that they shouldn't have or maneuvered their vehicle in a dangerous way. Most of the time they're correct, but there are also instances beyond a driver's ability to control that can cause these kinds of rollovers.

Simply looking at rollovers in terms of whether they are tripped or untripped allows us to avert our eyes from the group that can do the most to limit the severity of rollovers, automakers.

Vehicle Defect Rollovers

NHTSA does not distinguish between rollovers caused by poor vehicle design and/or defective products. Most likely, NTHSA simply categorizes these rollovers as "tripped" rollovers, but that lack of distinction completely disregards an automaker's role in some of these crashes.

The most common vehicle defects responsible for rollovers are:

  • Defective tires
  • Defective brakes
  • Steering wheel problems
  • Electrical system issues
  • Software defects
  • Defective suspension

Naturally, there are various ways and therefore reasons a vehicle could suffer from a defect. Some vehicle defects could be the result of bad design, could be the result of poor auto shop maintenance, and others could be because of an issue in the production process.

Still, other reasons might be software defects. An issue with the software could cause a vehicle’s programming to malfunction, and the programming itself could have complex computer errors. Furthermore, a vehicle could be damaged during transit or while on a car dealer’s retail lot.

Whatever the reason or the defect, an automaker is responsible for designing, manufacturing, and producing a vehicle that mitigates safety risks. At the very least, defects should be their own sub-category within the larger tripped/untripped framework. While it's difficult to train every single driver how to prevent rollovers in every conceivable real-world scenario, automakers have long had the ability to reduce the frequency with which their cars roll.

Why Would Vehicle Defects Need to Be Their Own Category?

We know rollover crashes are disproportionally deadly, and there are already calls to make rollovers more survivable by building better roofs. However, it is possible (and often beneficial) to attack issues from multiple angles. In addition to making vehicles safer during rollovers, we can also try to prevent rollovers in general. Don't get me wrong, rollovers caused by vehicle defects are not the largest and most common reason for rollovers, but preventing those we can is worth it due to the severity of rollovers.

Adding vehicle defects as a type or source of rollover crashes opens the door for NHTSA's scientists to study vehicle defects and hold automakers accountable. That saves lives.