August 2009
Rollover 101
How rollovers happen and what you can do to avoid one
A vehicle rollover is among the worst things that can happen to you on the road. Although rollovers occur in only about 3
percent of all serious crashes, they account for about one-third of people killed while riding in a passenger vehicle. More
than 10,000 of the 32,100 vehicle occupants killed in 2006 were involved in a rollover. Most of those killed, incidentally,
were not wearing seat belts.
Rollovers needn't be so deadly. Rollover-avoidance technologies, better vehicle design, enhanced safety systems, stronger government regulations, and increased use of safety belts could cut the number killed and injured by half or more. This primer will explain how rollovers happen, and how the government's rollover tests work.
HOW ROLLOVERS HAPPEN
Given the right circumstances, any vehicle can roll over. However, taller, narrower vehicles such as SUVs, pickups, and vans are more susceptible than traditional cars are because they have a higher center of gravity and thus are more top-heavy. As a consequence, sideways forces that develop when a vehicle rounds a curve shifts the center of gravity to one side, which can have a dramatic effect on the vehicle's balance. The lateral forces increase with speed and also with rapid changes of direction--for example, when a driver makes too sharp a turn one way and then overcorrects the other way. Those transitions can set up a pendulum effect, with larger and larger swings and an eventual loss of control.
A single-vehicle rollover is usually not caused by the steering maneuver. Instead, the vehicle usually has to "trip" on something, such as when it swerves into a curb, pothole, or a soft roadside shoulder. The government has estimated that 95 percent of rollovers result from trips. Some observers say that number is too high. If a vehicle leans in such a way that a tire's sidewall deforms and the wheel rim strikes the pavement and provokes a tip-up, then the government counts that as a tripped rollover.
But Consumer Reports encountered that very phenomenon many times during its emergency-handling tests of SUVs in the 1980s and 1990s. We consider those tip-ups to be untripped, because the vehicle essentially fell over itself on a flat surface without encountering some obstruction. The distinction is important because the supposed rarity of untripped rollovers has served as an excuse for the government and the auto industry to play down rollover risk and put the blame on the driver and road conditions rather than on the vehicle.
GOOD GRIP, BUT NOT TOO MUCH
In any case, tire grip plays a paradoxical role in rollovers. Ideally, your vehicle would stay on course, gripping the road with all four wheels on the ground, no matter what. But too much tire grip can allow excessive sideways forces to build up until the vehicle flips over. Before that happens, you want the vehicle to gradually and predictably lose some lateral grip. Sliding is better than tipping over, but it too can put the vehicle at risk of hitting something during the slide, and then rolling anyway.
One way that automakers make an SUV less prone to roll over is to equip it with less-grippy tires. That can help prevent some rollovers but is obviously a less-than-ideal solution since tire traction keeps you on the road and affects stopping distances.
The tire-grip issue sheds light on another potential source of rollover: those sporty aftermarket SUV and pickup truck tires. If "performance" replacement tires provide more lateral grip than the original-equipment tires, they might increase the chance of rollover in an emergency turn. Our advice is to stick with tires that are nearly identical to those that came with the vehicle. The characteristics of those tires formed part of the basis for the vehicle's original safety potential.
Rollovers needn't be so deadly. Rollover-avoidance technologies, better vehicle design, enhanced safety systems, stronger government regulations, and increased use of safety belts could cut the number killed and injured by half or more. This primer will explain how rollovers happen, and how the government's rollover tests work.
HOW ROLLOVERS HAPPEN
Given the right circumstances, any vehicle can roll over. However, taller, narrower vehicles such as SUVs, pickups, and vans are more susceptible than traditional cars are because they have a higher center of gravity and thus are more top-heavy. As a consequence, sideways forces that develop when a vehicle rounds a curve shifts the center of gravity to one side, which can have a dramatic effect on the vehicle's balance. The lateral forces increase with speed and also with rapid changes of direction--for example, when a driver makes too sharp a turn one way and then overcorrects the other way. Those transitions can set up a pendulum effect, with larger and larger swings and an eventual loss of control.
A single-vehicle rollover is usually not caused by the steering maneuver. Instead, the vehicle usually has to "trip" on something, such as when it swerves into a curb, pothole, or a soft roadside shoulder. The government has estimated that 95 percent of rollovers result from trips. Some observers say that number is too high. If a vehicle leans in such a way that a tire's sidewall deforms and the wheel rim strikes the pavement and provokes a tip-up, then the government counts that as a tripped rollover.
But Consumer Reports encountered that very phenomenon many times during its emergency-handling tests of SUVs in the 1980s and 1990s. We consider those tip-ups to be untripped, because the vehicle essentially fell over itself on a flat surface without encountering some obstruction. The distinction is important because the supposed rarity of untripped rollovers has served as an excuse for the government and the auto industry to play down rollover risk and put the blame on the driver and road conditions rather than on the vehicle.
GOOD GRIP, BUT NOT TOO MUCH
In any case, tire grip plays a paradoxical role in rollovers. Ideally, your vehicle would stay on course, gripping the road with all four wheels on the ground, no matter what. But too much tire grip can allow excessive sideways forces to build up until the vehicle flips over. Before that happens, you want the vehicle to gradually and predictably lose some lateral grip. Sliding is better than tipping over, but it too can put the vehicle at risk of hitting something during the slide, and then rolling anyway.
One way that automakers make an SUV less prone to roll over is to equip it with less-grippy tires. That can help prevent some rollovers but is obviously a less-than-ideal solution since tire traction keeps you on the road and affects stopping distances.
The tire-grip issue sheds light on another potential source of rollover: those sporty aftermarket SUV and pickup truck tires. If "performance" replacement tires provide more lateral grip than the original-equipment tires, they might increase the chance of rollover in an emergency turn. Our advice is to stick with tires that are nearly identical to those that came with the vehicle. The characteristics of those tires formed part of the basis for the vehicle's original safety potential.
