SUVs as Protection and Emergency Operations Vehicles

By Nathan Foster

SUVs have increasingly become a staple in the fleets of private security operations, law enforcement agencies, and military organizations.  The very characteristics that make SUVs a desirable alternative to sedans (increased cargo space, increased ground clearance, etc.) are attended by potentially negative consequences.  This article will seek to first define the problems with SUVs as protection, emergency operations, and convoy vehicles; and second to offer some very simple and effective solutions.

The Problems

Vehicle Design
We’ve all heard the nightly news reports, read the magazine articles, and had the coffee conversation with our mechanically inclined friends – SUVs are prone to rollovers.  First, SUVs tend to have narrow tracking widths (approximately the distance between the centers of the rear tires) and long heights of center of gravity (approximately the distance from the ground to where your center of gravity is when seated in the driver’s seat).  Just as we’ve learned by playing Jenga -- objects which are relatively tall, have their masses spread out above their centers of gravity, and have relatively small foot-prints tend to topple rather easily.  In fact, the propensity of an SUV to rollover can be found by the following equation:

In which

Pr = the propensity to roll d  = the distance of tracking width h  = the height of center of gravity

 

If P_r is greater than the coefficient of friction (the amount of “stickiness” between the tires and the road surface) then the vehicle will tend to slide during a loss of control.  If P_r is less than the coefficient of friction, the vehicle will tend to roll.  On average, the coefficient of friction for a dry, normal driving surface (such as asphalt or concrete) is around 0.7.

Driver Inputs
There are two types of vehicle rollovers.  A tripped rollover occurs when one or more tire sidewalls or wheels come into contact with a vertical surface and are tripped over.  It is also considered a tripped rollover when one or more tires on a side “catch” on an inconsistent driving surface.  This can be seen when a vehicle partially leaves a road surface and rolls over on the dirt.   Any vehicle can suffer from a tripped rollover – even multimillion dollar race cars can be tripped.  SUV’s are peculiar because they suffer untripped rollovers quite frequently.  An untripped rollover occurs when simply the lateral force placed on a vehicle when combined with the friction between the tires and the road surface cause the vehicle to roll.  Untripped rollovers are characterized by several things:

1. The vehicle is traveling at least 34 MPH.
2. High steering rates and high steering amplitudes.  The steering wheel is turned back and forth aggressively and rapidly.
3. Over-steer characterized by heavy side-to-side oscillations of the rear of the vehicle.  The rear end of the vehicle swings back and forth from one side to the other like a pendulum.
4. A high yaw rate and slip-angle.  There must be a large angle between the direction the tires are pointing and the direction the tires are moving.
5. The lateral acceleration capabilities of both front and rear axles are overloaded simultaneously.
6. The roll motion of the vehicle’s body causes the suspension to “bottom-out” on its bump-stops, overloading the tires resulting in a roll over.

For instance, while driving along in an SUV a child darts out in front of the driver, moving from the driver’s right to the driver’s left.  In response, the driver simultaneously decelerates and aggressively turns the steering wheel to the left.  Weight is transferred forward, off of the rear tires and weight is transferred to the right onto the “outside” tires.  The vehicle goes into an over-steer event; the rear end of the vehicle begins to swing to the right.  As many drivers have been trained to do, the driver aggressively counter steers to the right and turns into the direction of the skid.  All of the energy created by the weight of the vehicle being violently transferred to the right is suddenly snapped to the left.  On this, or perhaps one more similar side-to-side oscillation, both front and rear axels are overloaded and begin to lift on the inside of the slide; the suspension on the outside of the slide reaches its bump stops; the tires on the outside become completely overloaded and the vehicle violently rolls over.

We see over and over again that rollovers rarely occur in response to an over-steer event in one direction.  In fact, many SUV’s can withstand the lateral forces generated during rearward and forward 180º turns (J-turns and bootleggers, respectively) without rolling over.  There must be these side-to-side excitations of the vehicle – energy transferring one way, then immediately the other.

Vehicle Loading
First, it must be recognized that cargo space and payload capacity are not the same thing.  SUVs tend to have very large cargo spaces (i.e. the spaces in the vehicle where drivers and passengers can place themselves and their cargo) but not necessarily a commensurate amount of payload capacity (i.e. the amount of weight that can be carried by the vehicle without overloading the tires or suspension system).  With the large amount of equipment carried by many law enforcement officers, private security teams, and military operators in working SUVs, it is very easy to overload the vehicle’s payload capacity – especially with under or improperly inflated tires.  If a vehicle’s tires or suspension system are overloaded, it is more likely to over-steer and loose control in an emergency response situation.  Also, items loaded above the center of gravity will exacerbate the problem  should the SUV wind up in the latter stages of a potential rollover event as described above.  Unfortunately, many aftermarket armoring packages used in high-threat areas add greatly to the problem.  In fact, we have seen armored SUVs being built for high-threat driving in which the armor packages alone were heavy enough to exceed the payload capacities without a single person or piece of equipment being placed in the vehicles.

The Solutions

Vehicle Loading
It would be overly simplistic to suggest that professional SUV drivers not overload their vehicles.  While in the ideal world this would be the case, often operational realities force us to use heavy armor packages and/or load in too much equipment or personnel into our vehicles.  So, the only common sense suggestions we can make are:

1. Load as little into the vehicle as is operationally acceptable.  If you must overload the vehicle’s tires and suspension system, try to go over the limits as little as possible.
2. Load wisely – as much as possible, load gear into the vehicle so that it is as low as possible and securely fasten it if operationally acceptable.  Ideally, we want as much weight as possible to be loaded into the vehicle so that it is below the center of gravity.
3. Check the tires – absolutely ensure that the vehicle’s tires are inflated to the manufacturer’s maximum recommended pressure for the given weight and make sure that tread-wear is within manufacturer specifications.
4. Have the alignment checked and the tires rotated frequently – we must absolutely give the tires every bit of advantage as is possible if we are knowingly overloading them.

Driver Inputs
Obviously, SUV drivers should try to avoid on-road emergency driving as much as possible.  In that effort, try to do the following:

1. Use situational awareness and always be aware of the conditions around you.
2. Drive as slowly as is operationally acceptable.
3. Leave as much space between your vehicle and the one ahead of you as is operationally acceptable.
4. Adjust your mirrors to accommodate for the width of the SUV and to avoid blind spots.
5. Avoid overly rapid, exaggerated, or jerky movements on the steering wheel.
6. If possible, do not attempt passing other vehicles or lane changes while traveling in a curve.
7. Use the SUV’s height to your advantage to maintain awareness of driving conditions well ahead of your position.

In the event of a loss of control, remember the following:

The SUV driver has to accept that in a loss of control situation, given the lateral acceleration handling limitations of an SUV, he might very well have to rapidly choose an alternate and less desirable path of travel for his vehicle to avoid aggressive counter steering.  Indeed the driver may have to choose a path of travel that limits the amount of damage incurred by his vehicle and potential injury to himself, his passengers, other motorists, and pedestrians but does not completely eliminate the potential for some damage -- he may well have to, "choose the lesser of two evils."

Once a loss of control has occurred (typically an over-steer situation), follow these steps:

1. Immediately decrease vehicle speed by aggressively applying the brakes.
2. Do Not attempt an aggressive counter steering maneuver; instead maintain a more neutral steering position.  At any cost, once you’ve experienced a loss of control in one direction, you must not use aggressive counter steering and create massive and rapid weight transfer in the other direction as this will almost certainly induce an untripped rollover.
3. Use ocular driving and look where you want to place the vehicle.  Remember, you’re looking for a safe path of travel that requires as little steering input as possible and that might or might not be your originally intended course.  If it is a new path of travel, it will more than likely be in the general direction of the over-steer occurrence.
4. Bring the vehicle to a complete stop; or, if other operational conditions call for it, continue with these techniques until the vehicle is back under control and continue forward with the march.

In conclusion, SUVs are here to stay in the world of professional security, law enforcement, and military operators.  There are certainly risks associated with their use given their propensity to suffer untripped rollovers; but with good training, common sense, operational forethought, and a cool head they can be used safely and effectively to accomplish the missions assigned.