Sport utility vehicle (SUV) rollover has been the subject of news documentaries and magazines for decades. The physics involved in a large number of these rollovers, however, have not been publicly discussed.

Stable passenger vehicles will not roll over until they hit something or are tripped by an obstruction such as a curb or dirt furrow. When steered sharply, stable vehicles will slide on pavement. Many SUVs roll over on the road in response to a driver’s steering inputs.When an SUV driver steers sharply to avoid an accident, their vehicle may roll over long before they lose steering control. Some of the most unstable SUVs can roll over at very low sideslip angles, in other words the vehicle’s back end has just begun to come around. Untripped rollover accident patterns are often overlooked because it is assumed that the driver spun and lost control of the vehicle.

Yaw marks (not skid marks) that end at the point of maximum separation are classic untripped rollover indicators. These tire markings are easily identified by the stripes, or striations (Figure 2), which are caused by the tire treads as they slide on the pavement. At the end of the yaw marks, the vehicle is at about a forty-five degree roll angle and becomes completely airborne. This is the defined roll point.

Figure 1
Police photos and measurements of yaw marks are extremely helpful to the accident reconstructionist. Figure 1 shows evidence of a typical untripped rollover. The tire mark on the right was caused by the vehicle’s left front tire, and the one on the left by its left rear tire.

Notice that these tire marks are only separated by a few feet. If the vehicle was completely sideways, the marks would be about nine feet apart (wheelbase of the vehicle). Had it not rolled over, this vehicle could still be under the control of the driver and the tire marks would continue.

Figure 2

Documentation of yaw marks and vehicle-to-ground contact points during the rollover, are used to calculate the vehicle’s orientation and motion. The vehicle’s sideslip angle at the point of roll is determined by mapping the separation between the yaw marks. The total rollover distance is used to calculate the vehicle’s speed before it rolled. The longer the roll distance, the faster the vehicle was going at the roll point. Scratch marks on the vehicle and on the pavement are used to determine positions during the roll sequence.

For example, notice in Figure 1 the vehicle shape is actually outlined by the white scrapes downstream of the yaw marks (next to the police car).

Figure 3 is part of a rollover diagram with vehicle positions. Once the speed and vehicle positions are determined, the rates at which it is yawing (turning) and rolling can be calculated. These calculations are important when evaluating the cause of the rollover and the effect of variables such as driver input, vehicle design and environmental factors.

Figure 3

In Figure 3, the vehicle position at ninety degrees of roll (laying down on the driver’s side) was defined by the scratch marks. Using this vehicle position and its position at the roll point, the rate at which the vehicle yawing and rolling is calculated. The results are then compared to rates of other typical untripped rollovers. When vehicles roll over as a result of tripping or collision with other vehicles, the yaw and roll rates may be extremely high.

SUV untripped rollover accidents involve unique dynamics and accident patterns. Unlike stable passenger vehicles, most SUVs may roll over simply as a result of hard steering on typical roadways. Reconstruction and careful study of the accident evidence can be used to determine whether a vehicle’s design and lack of stability caused it to roll over.