One of my first experiences investigating accidents occurred early in my career when I was working as a police officer. When a call came in involving a traffic accident and all units arrived on scene, the first assignment was to ensure that everyone involved was unhurt. Once the scene was secured, several basic investigating areas needed to be covered. One of these was to obtain an accurate roadway frictional coefficient. Back then, most of us relied on the use of a drag sled for this purpose.
A drag sled is an apparatus that uses a weighted object with a tire tread on the bottom that is pulled by a scale to determine the coefficient of friction of a surface. After trying several different versions, I created the following:
- A local steel and pipe supply company donated a piece of 8-inch H beam steel approximately 20 inches long. Using this as my weighted object, I then used a large piece of tire tread that I found along the interstate.
- After cleaning up and painting the piece of H beam, I then cut the tire tread to size and adhered it to the bottom of the beam.
- Locating the center mass for a straight parallel pull with the surface was as simple as measuring halfway up the beam, where I drilled a quarter-inch hole.
- Attaching a small pintle hook through the hole completed my drag sled.
My drag sled turned out to weigh 32 lbs. After carrying it to several accidents, I found it to be easy to use, clean, and indestructible. Since that time, I have purchased a commercially available measuring device which is quick and easy to set up and can be used from the convenience your vehicle, keeping you dry on rainy days and warm on cold winter days. After using both models in different locations under a variety of weather conditions, I have found that my drag sled’s readings have been very consistent with those of the commercial device. I have also found that by keeping my drag sled handy, I could turn it upside down and use the “top” smooth steel side to resemble a vehicle sliding on its side or roof. Another advantage of my drag sled is the ability to use it over a short distance, such as across a small median or grassy area leading to a tree or wall that cannot be tested by driving over it with your test vehicle. Although my commercial measuring device is extremely accurate, I’ll always bring my drag sled along for those unpredictable situations that may arise.
WILLIAM BREM is an ACTAR-certified Accident Reconstructionist specializing in traffic violation assessments, police report and procedure analysis, speed analysis, visibility studies, perception/reaction time, vehicle dynamics and “who is at fault” incidents. He also has extensive knowledge of CDR (airbag) downloads and is experienced in assessing staged accidents, enhanced damage cases, and other law enforcement issues. Prior to joining ARCCA, Mr. Brem served as an active duty police officer and police accident reconstructionist.