The Traffic Accident Reconstruction Origin -ARnews-
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The Traffic Accident Reconstruction Origin -ARnews-
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I must agree and disagree with colleague Brendan... An improperly designed and used
sled can give erroneous values for a skidding vehicle's deceleration rate. Done
correctly though and a useable tool can result.
Several persons/entities exist that make and sell sleds. I would suggest that
the simple fact that a particular sled is commercially purchased doesn't inherently
cause it to be more accurate. After all is said and done, a sled ends up being
nothing more than a piece of tire tread glued to the bottom of a weight. The sled
is dragged with the use of a spring scale, strain-guage, or dynamometer. (period)
Considerable argument currently exists, but I (and a growing number of reconists)
recommend employing an empirically determined "proper pull angle" versus pulling
the sled horizontally as high school physics texts might imply. Pulling flat
yields simple "mu" or coefficient of friction for the road surface/rubber interface.
What you want is a value for the average deceleration of a skidding vehicle... which
is NOT a low-velocity value for coefficient of friction but an average value
for quite a number of frictional values encountered as a vehicle skids from one
speed to another.
Some formulae are coming to exist for first approximations of "proper pull angle,"
but the easiest and surest way to determine this is to select some "reference
standard" that you give credence to (i.e. a VC-2000, a velocimeter, bumper gun,
G-analyst, simple test skids,... whatever peels your banana). Carry out a few
tests in a flat, safe area. Then pull your sled while you experiment with the
pull angle (pulling up decreases values, pulling a little downward can even
increase the values) until you find the angle which yields the values determined
by your reference standard. Devise some kind of "pull truck" or other method
which causes your pulling angle to be consistent.
The only other advise is to thoroughly understand what your sled is and is not
revealing.... and there are a few twists... i.e., for slopes, ADD the value for slope
when sliding downhill and SUBTRACT the value when sliding uphill (I know this
sounds backwards and shouldn't be necessary but try it first... you'll be
pleasantly surprised!)
(Now for the fellas who's hair just fell off.... please don't whoop out your old
college physics texts and start quoting... we both have learned those RUDIMENTARY
basics and have built and employed sleds with them in mind.. and we found that
the resulting values were too high... There's more going on than is explained in
those texts... Please give me a break and try these concepts before flaming me.
I've been doing research on this topic now for two years and have argued with the
best and have experimented until I've got something going right... my resulting
book is being published by IPTM this year and the details of my research and the
resulting designs will be available for all to scrutinize... Thanx!)
Ed Livesay
elivesay@alltel.net
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