The Traffic Accident Reconstruction Origin -ARnews-

Re: speed from skid

Ed Livesay (elivesay@alltell.net)
Thu, 1 May 1997 21:01:04 -0400 (EDT)

Mike.... The derivation of the minimum-speed-from-skid formula does indeed consider
the weights of differing vehicles... there weights cancel out!... Consider the following:

The "work-energy principle" (the basis of the skid formula) can be stated as follows:

(with any luck the screen won't upset the manner in which I've written the following)

Wxfxd = .5xMxV^2

and since M (mass) is equal to Weight/Gravity, we can substitute and cancel;

Wxfxd = 1/2x(W/g)xV^2
_____ _____________
W W

proper canceling leaves:

fxd = 1/2G x V^2 ..... the "Ws" (weight) cancel out of the equation!

A few more algebraic steps yields the familiar sqr(30df) formula.

Now, you have to think of this carefully... nobody said that a larger (heavier)
vehicle wouldn't TRAVEL a little farther from an identical initial speed than
a lighter vehicle... we said it wouldn't SKID farther. The effects of heavier
weights do show up in factors like tire hardness and mechanical delay....
but should such factors be eliminated from the train of thought, all vehicles,
regardless of weight, will SKID the same distance.

There are several ways to account for the differences in tire hardness (read
traction) for larger vehicles like an eighteen-wheeler. Do some reading in
any of the well known recon texts for more data. In the meantime, when used
competently, you can rely upon the skid formula.

Ed Livesay
elivesay@alltell.net

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