SET OHIOPD AS YOUR HOME PAGE Go to Skidmark Analyzer

YawMarks Analyzer

Below is a calculator for analyzing the mimimum speed of a vehicle from the skidmarks it leaves as it goes around a curve, called Critical Scuff marks. Simply enter the blanks and the computer will show the mimimum speed of the vehicle. Directions are provided below.

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Enter the Longest Skid of the Vehicle being used to determine the drag factor.
Skip to Enter the Drag Factor if you already know it.
Place the longest skid measurement, in feet, of the test skids that you took with your police car or similar vehicle as to the one that crashed. Be sure inches are converted to feet: 1"=.08, 2"=.16, 3"=.25, 4"=.33, 5"=.41, 6"=.5, 7"=.58, 8".66, 9"=.75, 10"=.83, 11"=.91
Enter the Speed of the Vehicle being used to determine the Drag Factor.
Enter the speed of the above Test Vehicle, in miles per hour, at the time you slammed on your brakes to take the Test Skid.
Compute Drag Factor.
Click on the 'DragFactor' button and the Drag Factor will be shown in the entry below it.
Enter the Drag Factor.
If you already know the Drag Factor, Enter it here and skip the above fields. If you need to make some kind of slope adjustment, you will have to change the number yourself.
Enter the Chord.
Take the measurments in the first one/third of Yawmarks. Try to obtain a chord length over 30 feet to get better results.
Enter the Middle Ordinate.
Measure to the outside of the Yawmark.
As always, be sure inches are converted to feet: 1"=.08, 2"=.16, 3"=.25, 4"=.33, 5"=.41, 6"=.5, 7"=.58, 8".66, 9"=.75, 10"=.83, 11"=.91
Enter the Track Width or Guage.
Measure the Distance between the Center of the Tires in feet.
Click on Compute.
The Minimum Speed of the vehicle at the start of the skid will be computed and presented.
Enter another crash.
To clear the screen and refresh the program, you may use the refresh button provided or hit your 'F5' button.

The information on crash investigations used on this page was acquired after attending the Ohio State Highway Patrol Academy - Basic Crash Investigation Course. However the State Highway Patrol is in no way responsible for the information presented here.

The formula used to compute the drag factor is as follows: the drag factor equals the known speed of the test vehicle squared, divided by the result of the constant of thirty times the distance of the longest skid in feet. The vehicle used as the test vehicle should be a car of equal size and weight as to the car that crashed or if possible the crash vehicle itself, more often it is likely to be your police cruiser.

The formula used to compute the critical speed of the crash vehicle around a curve is as follows: first the radius of curve is computed by taking the square of the chord divided by the result of eight times the middle ordinate, then the division of the middle ordinate divided by two is added, and then one half of the track width is subtracted. With the radius of the curve now determined the critical speed is found by multiplying the constant of 3.86 times the square root of the product of the radius times the drag factor. The result is the mimimum speed in miles per hour as the vehicle entered the skid.


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