A.D. Tupper & Associates Limited

Vehicle Launch Analysis

Introduction

When a vehicle or an object loses support from the ground it is launched into the air. It then travels horizontally at the same horizontal speed it had when launched, until it falls back to the ground. The fall may start as an initial upward, level or downward movement. This type of motion can be assessed mathematically to get the speed at start of the launch, and the process seems to be a particular favorite, and a seductive trap, for police officers and other non-engineer reconstruction analysts.

Terms

In order to clarify the terms, I set out the following definitions: 

1.       A fall occurs when a vehicle is traveling forward, and is no longer supported by the surface over which it is moving.

2.       A flip occurs when a sideways moving vehicle meets resistance, such as through striking the curb, and rises and moves through the air. It is thus contrasted with a rollover, which may occur without the wheels digging in.

3.       A vault occurs when a car moving forward or rearward, meets resistance, digs in and flips end over end.

Applicable Cases

Although relatively few cases involve significant flights of vehicles or objects, this type of analysis can be useful when applied properly to collisions involving:

  1. Pedestrian impact;
  2. Motorcycle/rider separation;
  3. Vehicle travel over an embankment or into a body of water;
  4. Travel over a ditch or a steep grade; and
  5. Vehicle rollovers.

Required Data

The basis of the launch analysis is that once a vehicle or object separates from the ground, it will follow a ballistic arc while in the air. The following information can be used to calculate the speed at which the vehicle left the ground: 

1.       Accurate location of vehicle centre of gravity (CG) at the point of launch.

 2.       Accurate location of vehicle CG at the first point of landing.

3.       Difference in elevation of the vehicle CG between the launch and landing points.

4.       The angle of launch.

Cautions!!!

It is especially important for analysts to understand the background and theory of a launch formulae, before relying upon the results of blind number plugging, in the manual formula, or a computer or a hank-held calculator. People without a proper physics background can be mislead, by their misinterpretation and in turn can mislead others, such as the court.

In one case I examined, for example, the police officer had used in the equation a horizontal azimuth angle where a vertical launch angle was called for. He did not understand the derivation or purpose of the formula, and did not comprehend his fatal flaw, even after the fact was pointed out to him.

As typically used, the relevant equations make several assumptions that can influence the results adversely. The equations ignore the effects of wind and air resistance. In most cases, these affects will be negligible but they may be significant for high-speed misadventures, or for objects that are not as dense as vehicles and people. Two factors that always have a significantly bearing on the truth of the results, however, are:

  1. The accuracy of the launch angle; and
  2. The horizontal travel distance of the object’s CG.

 If the vehicle left the ground while traveling over uneven terrain or because it flipped off a curb or guardrail, for example, the launch angle will be difficult to evaluate with any reasonable degree of accuracy, unless it bruised a tree or punched a hole in a fence or wall. A range of angular values will thus have to be considered, because the assessment is extremely sensitive to the departure angle.

 Vehicle rotation and suspension movement can also affect the evaluation of the horizontal travel distance from launch to contact. These will be especially important if said distance is not several times the length of the vehicle.

Conclusion 

In summary, the family of launch formulae has the potential of producing an accurate evaluation of speed of a vehicle in a launch, call, flip, or vault. The analysis is very sensitive to such variables as the launch angle, which is exceedingly difficult to quantify, and is thus prone to bad answers, particularly when used by inexperienced or improperly trained analysts.

 All interested parties, including analysts, insurance representatives, police, crown, defense counsel, and the court, should be prepared to test the assumptions and examine the particular application of this process of speed determination.