rollover

A rollover in Sydney, Australia on Christmas day, 2001.

A rollover is a type of vehicle accident in which a vehicle tips over onto its side or roof. The most common cause of a rollover is traveling too fast while turning.

Dynamics

A rollover in southern Italy.

Vehicles can roll over in several ways. These include excessive cornering speed, tripping, collision with another vehicle or object, or traversing a critical slope.

Rollovers caused by excessive cornering speed occur when cornering forces destabilize the vehicle. As a vehicle rounds a corner, three forces act on it: tire forces, inertial effects, and gravity. The cornering forces from the tire push the vehicle towards the center of the curve. This force acts at ground level. The force of inertia acts horizontally through the vehicle's center of mass in the direction opposite to the one it is turning. These two forces make the vehicle roll towards the outside of the curve. The force of the vehicle's weight acts downward through the center of mass in the opposite direction. When the tire and inertial forces are enough to overcome the force of gravity, the vehicle starts to turn over. Most passenger vehicles will slide or spin before this happens, but this is a common type of rollover for taller vehicles, including light trucks (SUVs, vans and pickup trucks), buses, and heavy trucks.

A vehicle may perform a complete rollover, rolling onto its side, roof, far side and finally ending up on its wheels again, or even going on to roll further.

Tripping rollovers occur when a vehicle is sliding sideways, and the tires strike a curb, dig into soft ground, suddenly regain traction, or a similar event occurs that results in a sudden lateral force. The physics are similar to cornering rollovers.

A collision with another vehicle or object can cause a rollover. These occur when the collision causes the vehicle to become unstable, such as when a narrow object causes one side of the vehicle to accelerate upwards, but not the other, causing the vehicle to rotate along its long axis. A side impact can accelerate a vehicle sideways. The tires resist the change, and the coupled forces rotate the vehicle. In 1983, crash tests showed that light trucks were prone to rolling over after colliding with certain early designs of guide rail.^[1]

A rollover can also occur as a vehicle crosses a ditch or slope. Slopes steeper than 33 percent (one vertical unit rise or fall per three horizontal units) are called 'critical slopes' and often contribute to rollovers.

A vehicle may roll over for other reasons, such as when hitting a large obstacle with one of its wheels or when manoeuvring over uneven terrain.

Competition

A skilled driver may stop a rollover by stopping a turn. Stunt drivers deliberately use ramps to launch a rollover. Vehicles with a high center of gravity are easily upset or "rolled." Short of a rollover, stunt drivers may also drive the car on two wheels for some time, but this requires precise planning and expert driver control. Specialized safety equipment is often utilized.

Rollover contest

The driver deliberately drives one side of their vehicle onto a ramp which causes their vehicle to roll over.^[2] The winner is the driver who guides their vehicle to the most rolls.^[2]

Vehicles

Rolled-over Ford Explorer (see Firestone and Ford tire controversy)

All vehicles are susceptible to rollovers to various extents. Generally, the higher the center of mass, the more sensitive the steering, and the higher the speed, the more likely a vehicle is to roll over. Civilian SUVs and full-sized vans are notorious for rolling over because they usually have a higher center of gravity, caused by a higher ground clearance. Manufacturers often post warnings on the driver's sun-visor. Among the vehicles which have received publicity for tendencies to roll over are the Ford Bronco II, Suzuki Samurai, Jeep CJ, Mitsubishi Pajero/Montero, and Isuzu Trooper.

Pickup trucks are also prone to rollover, especially those outfitted with long travel off road suspensions. This is due to the increased suspension height for increased clearance off road. In addition, 15-passenger vans, a segment dominated by the Ford Ford E-Series, are tall because of their heavy-duty suspensions necessary to carry large amounts of people which also make them prone to rollover. The rollover tendency is exacerbated when the vehicles are heavily loaded. It is recommended to not load anything on the roof of such vans, and to use drivers experienced or trained in safe operation of the vehicle. In such cases, being familiar with the vehicle's behavior loaded and unloaded, avoiding sudden swerving maneuvers, and reducing speed through tight turns can greatly decrease the rollover risk associated with these vehicles.

Military vehicles have a much wider wheel track than civilian SUVs, making them more difficult to roll over. However, IEDs in Iraq and Afghanistan cause roll overs not seen by civilian vehicles. The top turret gunner is particularly vulnerable. ^[3].

A tall passenger coach made US headlines when 14 passengers were killed in New York in 2011. The bus swerved, flipped on its side and hit a pole which split off the top of the vehicle.^[4]

Exit

After a rollover, the vehicle may end up lying on its side or roof, often blocking the doors and complicating the escape for the passengers. Large passenger vehicles such as buses, trams, and trolleybuses that have doors on one side only usually have one or more methods of using windows for escape in case of a rollover. Some have special windows with handles to pull so that windows can be used as an emergency exit. Some have tools fixed near the windows for breaking the glass and making an improvised exit. Some have emergency exits in their roofs. Some combine two or more of these escape methods.

Roll bars and cages

Rollover crashes are particularly deadly for the occupants of a vehicle when compared to frontal, side, or rear crashes, because in normal passenger vehicles, the roof is likely to collapse in towards the occupants and cause severe head injuries. The use of roll cages in vehicles would make them much safer, but in most passenger vehicles their use would cut cargo and passenger space so much that their use is not practical. The Jeep Wrangler, a vehicle which is short, narrow, and designed to be used on uneven terrain, is unusual in that it comes with a roll bar as standard equipment.

The decline in popularity of convertibles in the US was partly caused by concern about lack of protection in rollover accidents, because most convertibles have no protection beyond the windshield frame. Some convertibles provide rollover protection using two protruding curved bars behind the headrests. Some Mercedes convertibles have a retractable roll bar which deploys in case of an accident. Race cars almost always have roll cages, since racing is very likely to result in a rollover. In addition, the roll cage's chassis-stiffening effect is usually seen as a benefit to the car.

References

External links

• The Center for Auto Safety videos (Rollover videos at the bottom).

See also

• Dynamic rollover
• Roll over protection structures (ROPs)