INTRODUCTION: This tutorial will cover both the REC-TEC and the TRUCK*BRAKE sections of the program. It will assist you in getting familiar with the program(s) and show you how to use the program to solve very different and complex problems. This will be a step by step approach to REC-TEC accident reconstruction. It is not intended to teach accident reconstruction, but to assist the accident reconstructionist in solving problems with the program(s).
REC-TEC BASICS
REC-TEC takes a modular approach to accident reconstruction problem solving, just as reconstructionists have always done. The problem must be broken down into the solvable components.
1. Determination of the primary objective of the investigation;
2. Break the overall problem down into solvable components;
3. Combine the individual answers into a unified solution to the problem.
With these steps in mind, we will begin to solve problems using REC-TEC.
It is suggested that you work the problems on your computer as we go through them here in order to receive the maximum benefit from this workbook.
All problems will assume the following configuration unless otherwise specified:
1. Swerve and Swerve and Return versus Yaw versus Motion Analysis.
2. TIME / DISTANCE - Which flavor do I use?
Swerve and Swerve and Return versus Yaw versus Motion Analysis.
These sections of REC-TEC all handle lateral accelerations. So which one is the best for the particular situation? This is something that you as the reconstructionist must answer. In order to come to a solution, let us examine the properties of each section in detail.
Swerve and Swerve and Return: This is actually accessed in TIME DISTANCE - DECELERATION. If a lateral distance is used in the problem, the program will generate both SWERVE and SWERVE and RETURN (RECOVER) data. This is where 3 avoidance maneuvers can be animated on the screen at the same time, in real time - a deceleration, a swerve and a swerve and recover.
It may be that for your particular problem, a deceleration is not involved. This is STILL the section used to compare SWERVE and SWERVE and RETURN.
If your problem involves a lateral acceleration, it is CRITICAL that you understand what that involves; most reconstructionists don't. If a vehicle is accelerating in a lateral direction as a result of a change of direction maneuver, it is also decelerating as a result of the maneuver, even if it is maintaining a constant velocity. How can this be? Simple, it is accelerating along the Y AXIS and decelerating along the X AXIS. If the available road surface friction is a .7 and the vehicle goes into a maximum rate change of direction (yaw), then the vehicle has a maximum acceleration factor of .35 on the Y AXIS and a maximum deceleration factor of .35 assuming that the vehicle maintains a constant velocity. The total instantaneous acceleration (positive and negative) factor of the vehicle is .7. Therefore the vehicle, under these conditions, can only use one half of the available friction to accelerate along the Y AXIS. If you are only doing a SWERVE, The "drag factor" must be adjusted to reflect 2 times the lateral (Y AXIS) acceleration factor you require.
CRITICAL TURNAWAY is often confusing. In simple words, it is a SPEED or VELOCITY at which both the distance slide to stop and the critical distance to swerve (or swerve and return to a parallel path) is the same. In order to verify this, go to CHANGE DATA and use the speed as the initial velocity and leave the drag factor alone. You will then notice that the Distance Slide to Stop and the Distance to Swerve are the same. Likewise if you use the speed under SWERVE and RETURN then the Distance Slide to Stop will be the same as the Distance to SWERVE and RETURN.
The YAW section of the program uses the chord and middle ordinate (or radius) to determine the speed of the yaw. The MOTION ANALYSIS section uses the position of the vehicle relative to the direction of travel to decelerate using the principles of the FRICTION CIRCLE to determine the speed of the vehicle at the different positions involved in the maneuver.
TIME / DISTANCE - Which flavor do I use?
Time - Distance is both simple and complicated. It must be remembered that a deceleration is nothing more than a negative acceleration. They are very closely related to each other.
REC-TEC offers several flavors of Time Distance:
· SINGLE SURFACE DECELERATION
· SINGLE SURFACE ACCELERATION
· TIME DISTANCE - MULTIPLE EVENT
· TIME DISTANCE - MULTIPLE VEHICLE
· MOTION ANALYSIS
· V-TRAX II
Most of these handle time-distance in a direction OPPOSITE from the direction of the TIME ARROW. In accident reconstruction we normally work from final rest backwards or opposite the direction of the time arrow. In REC-TEC some of these sections of necessity do not conform to this general rule. The exceptions are: MOTION ANALYSIS and TIME DISTANCE - Multiple Event. V-TRAX II is basically an animation package and will not be discussed here.
If TIME DISTANCE Multiple Event works in the direction of the TIME ARROW, is it possible to use it to determine the speed at the start of a segmented skid that has different friction values as is common in multiple surface skids? The answer is NO and YES. No, not if we enter the decelerations. But aren't decelerations merely accelerations? We can enter these events as accelerations. IF THIS IS DONE, we are now working OPPOSITE the direction of the TIME ARROW as we are accelerating from the FINAL SPEED.
Why can't it be made simpler? It can. It is possible to use only the single surface ACCELERATION or DECELERATION to figure out the answers to all of these multiple surface or multiple event problems. Simply use the correct INITIAL and FINAL speeds. By using the more exotic sections, the reconstructionist is able to present a clearer picture of what occurs during complex maneuvers.
Careful attention to the objective and careful attention to the unique characteristics of the different sections will allow the reconstructionist to paint the most informative picture of the event. It is like using color pictures in place of black and white. Black and white can be spectacular, but sometimes there is no substitute for color.
Now a word about
screen resolution:
The setting for
screen resolution can make a huge difference in what can be done with the
program. Setting screen resolution can
be accomplished using START - CONTROL PANEL or, on most machines, simply go to
the lower right portion of the START taskbar (by the clock) and you will see an
ICON that looks like a small monitor.
Right clicking on this will bring up a panel (Windows 98+) that lets you
set the resolution with relative ease.
Through out the
Tutorial, most of the screens will be captured from a display using 800x600
resolution. On the next page there is a
graphic in 1600x1200 resolution on the top of the page and one in 800x600
directly below for comparison purposes.
You will note in the 1600x1200 that four panels can be shown at one
time. This is not possible with 800x600
as 3 of the panels would be cut.
This use of screen
resolution offers the flexibility to show more information in the same
space. Like the flavors of
Time-Distance, it offers you a choice.
PROBLEM 1: A vehicle skids to a stop in 67 feet. How fast was it going at the start of the skid and what was the time involved?
SOLUTION:
1. Select TIME DISTANCE from the REC-TEC MENU
2. Select DECELERATION - SINGLE SURFACE from the Time-Distance menu
3. Enter .6 for Mu or DRAG FACTOR (TEST)
4. Enter 0 for GRADE (TEST)
5. Enter 0 for GRADE (ACCIDENT)
6. Enter 100 for BRAKING
7. Enter 67 for DISTANCE
8. Enter 0 for FINAL SPEED
9. LATERAL DISTANCE. HUH? What is this all about?
Why don't we just take on the LATERAL DISTANCE problem a bit later?
The program shows a screen with the SOLUTION to our problem. As you will see when you run the problem, the program gives us all kinds of information including initial speed (and velocity), time, and rates of deceleration.
The program allows us to change almost every variable in the equations and generate tables based on the changing values of these variables.
The values presented for the EVENT are identical to the values for the STOP.
Initial Speed = _____________ M/H
Time (Event) = _____________ SECONDS
On the back of this page is one way (flavors remember) the problem can be run.
Now that you see how to do a simple problem, the step by step method will be dropped from future problems. A graphic showing the worked out problem will be shown along with the problem.
PROBLEM 2: A vehicle skids 67 feet to a final speed of 25 miles per hour. How fast was it going at the start of the skid and what was the time involved?
SOLUTION:
The solution presented shows a deceleration over a distance of 67 feet to a final speed of 25 miles per hour.
The values presented for the EVENT are different than the values for the STOP.
Initial Speed = _____________ M/H
Time in Skid = _____________ SECONDS
PROBLEM 3: A vehicle skids for 2 seconds using a .6 deceleration factor to a final speed of 25 miles per hour. Show data involving a lane change (12 foot lane). What was the initial velocity and what is the swerve and return distance?
SOLUTION:
The solution presented shows a deceleration over 2 seconds to a final speed of 25 miles per hour.
The values presented for the EVENT are different than the values for the STOP.
SWERVE-NO RECOVERY shows what would happen if the vehicle was placed in a maximum rate change of direction using a .6 coefficient of friction. This is EXACTLY the same as using a .6 G turn. This would allow the vehicle's center of mass (or any given point of reference) to pass 12 feet laterally from the initial path of travel. This would NOT have the vehicle headed on a parallel path, it would be headed in a different direction (like off the road and into the boonies?). If the vehicle is then brought back to a parallel path, the distance and time would be doubled as would the total lateral distance.
SWERVE AND RECOVERY shows what would happen if the vehicle was placed in a maximum rate change of direction using a .6 coefficient of friction with an immediate change to a maximum rate change of direction in the opposite direction, at the optimum point to accomplish the maneuver. This allows the vehicle's center of mass (or any given point of reference) to pass 12 feet laterally from the initial path of travel. This has the vehicle headed in a parallel path. This is a lane change maneuver.
Initial Speed = _______________ M/H
Swerve & Return Distance = ________________ FEET
PROBLEM 4: A test vehicle skids from 30 M/H to a final speed of zero (0) M/H in 47 feet. What is the drag factor based on this data?
Check the REC-TEC Menu and go to the appropriate section.
SOLUTION:
The computed DRAG FACTOR is ______________
PROBLEM 5: An F-18 lands on a carrier with an approach speed of 150 M/H. It stops in 200 feet after engaging the #3 wire.
SOLUTION:
What was the average "G" force involved in the stop? _______________ G
PROBLEM 6: A passenger in a crash strikes the dashboard (2" of padding) of a vehicle. The car struck a tree at 30 M/H. The dash had come to a complete stop when the unrestrained passenger struck his head on the dash.
SOLUTION:
How many "G's" did the passenger’s head encounter? ______________ G
PROBLEM 7: A vehicle accelerates from a stop for 67 feet. The acceleration factor is a .2 (assumed acceleration factor unless otherwise specified). What was the speed at the end of 67 feet and what was the time involved?
SOLUTION:
The program shows a screen with the SOLUTION to our problem. As you will see when you run the problem, the program gives us all kinds of information including initial speed (and velocity), time, and rates of acceleration.
You will notice that it gives information under two headings, EVENT and 0 -> S2. In this problem they are the same, but if the acceleration had not been from a stop they would be different.
Final Speed = _______________ M/H
Time = _______________ SECONDS
PROBLEM 8: A vehicle accelerates for 67 feet using a .2 acceleration factor, from an initial speed of 10 miles per hour. What is the final speed and time involved?
SOLUTION:
The solution presented shows acceleration over a distance of 67 feet from an initial speed of 10 miles per hour. The values presented in EVENT and 0 -> S2 are now different.
Final Speed = ________________ M/H
Time = ________________ SECONDS
PROBLEM 9: A vehicle accelerates for 2 SECONDS to a FINAL SPEED of 50 miles per hour. What was the Initial Speed and Distance involved?
SOLUTION:
If you were watching the screen during theses changes, you saw REC-TEC doing on the fly calculations every time a new number was pressed. You see the effect of changing a variable immediately.
The solution presented shows an acceleration over 2 seconds to a final speed of 50 miles per hour. The values presented in EVENT and 0 -> S2 are different.
Initial Speed = ___________________ M/H
Distance = __________________ FEET
PROBLEM 10: Vehicle 1 skids from 60 miles per hour to a final speed of 15 miles per hour (striking vehicle 2). How fast was Vehicle #2 going when vehicle #1 was 1.5 seconds from impact?
Vehicle 2 accelerates from 7.5 miles per hour to a final speed of 30 miles per hour (striking vehicle 1). How far away from impact was Vehicle #1 when Vehicle #2 was 5 seconds from impact?
When vehicle #2 was 25 feet from impact, what was the speed of vehicle #1?
SOLUTION:
NOTE: Please see the discussion of how REC-TEC uses GRADE in "Effects of Grade on Kinetic Friction. "
This section allows some unique options such as CLOSURE, which can help investigate the sight triangle between the two units involved. Either vehicle can be studied for any time or distance prior to the event in question, be it a collision, stop, or any point under consideration. If it involves TIME-DISTANCE, this section may prove invaluable.
As was done in this problem, two vehicles can be taken into collision. Tables can then be used to study what happened prior to impact. With the ability to look at units in any combination of acceleration, deceleration, or constant velocity, any situation can be analyzed.
This section can be used to analyze vehicles, or vehicles and pedestrians in collision. This section can also be used to study the effects of different values for the same vehicle (side by side).
Vehicle #2 speed at 1.5 seconds from impact = ____________ M/H
Vehicle #1 distance 5 seconds from impact = _____________ FEET
Vehicle #1 speed when Vehicle #2 was 25 feet from impact = _________________ M/H
PROBLEM 11: Vehicle 1 skids from 60 miles per hour to a final speed of zero. The drag factor is .72.
Vehicle 2 skids from 60 miles per hour to a final speed of zero. The drag factor is .61.
Vehicle 2 is "TRAILING" 100 ft. behind Vehicle1.
Vehicle 2 has a 1.5 second perception reaction time (PRT) when Vehicle 1 brakes.
Once the problem is setup up, select Animation. Select "FROM" impact, and use the parallel format. Answer the remaining questions using the information listed above. Compute for TIME.
Do the vehicles hit? At what speed? See next page for SOLUTION.
PROBLEM 11A: How would the answer change if the PRT was 2 seconds and the trailing distance was 75 feet?
Do the vehicles hit? At what speed?
Vehicle 1______________________
Vehicle 2______________________
PROBLEM 11B: How would the answer change if the PRT was 1 second and the trailing distance was 150 feet?
Do the vehicles hit? At what speed?
Vehicle 1______________________
Vehicle 2______________________
Vehicle 1______________________ Vehicle 2______________________
TIME
& DISTANCE - MULTIPLE EVENTS
PROBLEM 12: A vehicle accelerates from a stop (fa = .2) for 25 feet. It then coasts (f = .01) for 15 feet before decelerating (f = .6) to a final speed of 5 miles per hour. What is the time and distance ?
SOLUTION:
Should you wish to plot this maneuver, all 3 "events" will be diagrammed. Since acceleration was chosen as the initial event, the diagram shows the maneuver from right instead of the usual left to right format.
Total Time = _________________ SECONDS
Total Distance = ________________ FEET
PROBLEM 13: A vehicle goes through the following maneuvers:
1. Acceleration factor = .2 Initial speed = 30 M/H Final speed = 60 M/H
2. Constant speed Distance = 264 ft
3. Acceleration
factor = .2 Initial speed =
60 M/H Final Speed = 75 M/H
4. Constant speed Distance = 300 ft
5. Deceleration factor
= .3 Initial speed = 75 M/H Final Speed = 60 M/H
6. Deceleration
factor = .6 Initial speed =
60 M/H Final Speed = 45 M/H
7. Constant speed Time = 5 seconds
8. Deceleration factor = .6 Initial speed = 45 M/H Final Speed = 0 M/H
9. Stopped for 30 seconds
10. Acceleration
factor = .2 Initial speed =
0 M/H Final Speed = 45 M/H
11. Constant speed Time = 5 seconds
12. Acceleration factor = .3 Initial speed = 45 M/H Final Speed = 60 M/H