The Traffic Accident Reconstruction Origin -ARnews-

Re: Drag Sleds

Ed Livesay (EdLivesay@worldnet.att.net)
Sun, 9 Jun 1996 12:55:20 -0400 (EDT)

Richard: It has been my experience that installing the pull eyebolt
at a point which is lower than the sled's center of mass will eliminate
a majority of the wobble. I am aware of several author's suggestions of
using water/pails etc. to find the center of mass point but the height of
the sled is not usually considered. Even though the eyebolt may be at the
precise center of mass the greater the height above the road surface the
more of a lever effect is available to produce wobble of the sled and
in the scale reading.... placing the eyebolt lower than the center of mass
seems to help tremendously... another idea (thanx to the Drag Sled (C)
commercial manufacturer) is to employ a hydraulic scale meter versus the
ususal spring scale... the fluid movement also smooths out wobble.

The lowered eyebolt is still at the center of mass in the lateral sense..
The scale is still pulled straight away from the eyebolt so that no sine
error is introduced in the scale reading. It is also my experience that
sled results are typically about 5% higher than accelerometer (VC 2000)
readings... I (and the commercial manufacturers) suggest that a reduction
be made in the final reading (I suggest about .05). This accounts for
what some authors refer to as the difference in the loading of a real
skidding tire (the accompanying weights above it) versus the loading
of a typical skid sled device.
Ed Livesay
EdLivesay@worldnet.att.net

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