The Traffic Accident Reconstruction Origin -ARnews-

Re: Seven Drag Factor Questions

Ed Phillips (EPHILLPW@CO.SAN-DIEGO.CA.US)
Thu, 6 Mar 1997 18:16:20 -0500 (EST)

Exactly the point Larry. The speed at the initiation of braking would not be the same as the speed at the initiation of wheel lock-up and tire mark deposit. In the spin down of the tires the vehicle also slows a little. So a radar lock at the intiation may read 40 mph and a radar lock at the intiation of skid mark deposit may be 38 mph. My point is to get the most accurate friction value you need to either measure the overall length of slowing which includes spin-down (as with a bumper gun), and get the intitial radar speed; or get the tire mark length and the radar speed when the tires lock up. That way the speeds and distances that are chucked into the derivative of the KE equation are from the same (nearly) point in the time sequence of the event. Your logic is good, the original vehicle speed must have been closer to 40 mph than 30 mph, all things being equal. I've heard that was the way to test before things got all scientific-ey. If you had 50 foot of skid mark, you got
in the car (if driveable) and tried to close in on the speed by trying to skid for the same length. Empirical testing.
Ed Phillips
EPHILLPW@CO.SAN-DIEGO.CA.US

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