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From: tron@fluke.UUCP (Peter F. Barbee)
Newsgroups: net.auto
Subject: Re: "The biggest farce ever"
Message-ID: <667@vax1.fluke.UUCP>
Date: Fri, 22-Jun-84 19:16:06 EDT
Article-I.D.: vax1.667
Posted: Fri Jun 22 19:16:06 1984
Date-Received: Wed, 27-Jun-84 08:48:38 EDT
References: <2104@ihnss.UUCP> <714@ut-ngp.UUCP>
Organization: John Fluke Mfg. Co., Everett, WA
Lines: 49


Two items of disagreement (I agree that 55 stinks but not for the same reasons)

1) The major drag force at speed (anything over 45 or so) is aerodynamic drag.
   This force is proportional to the velocity squared.  I know that the 
   effective coefficient of drag is, for many new cars, much lower than it
   used to be but this doesn't change the fact that the drag force is increasing   more than the velocity.  Let's look at some facts;

         F= cd * A * p * V**2 / 2        where  F = drag force
						cd = coefficient of drag
						A = projected area
						p = fluid density
						V = velocity

   now let's compare the following situations;

		car 1 : cd=.45 , V=24 meters/sec (55mph)
		car 2 : cd=.32 , V=34 m/s (75 mph) , and A= .85*A of 1

	so     .45(24**2)/2 * Ap = F (car 1)  =  129.6 Ap

	and    .32(34**2).85/2 * Ap = F (car 2) = 157.22 Ap

  I believe car 1 is representative of older cars, say a '75 Nova, and 
  car 2 is a similarly sized (interior room) modern aerodynamic car. I
  did not take engine friction, or wheel bearing friction, or tire friction
  into account as they increase linearly with velocity. Also note that the
  velocity here is relative free steam velocity, so the situation is worse
  witha headwind, and better with a tailwind.

2) It seems obvious that in the event of an accident more damage will be
   done if the cars are going faster.  They simply have more stored energy
   (call it inertia, potential energy, or momentum) to dissipate.  Since
   many accidents occur with both cars going the same direction you could
   argue that it's the relative speed that mattered but I disagree.  Usually
   the cars will come to a stop, they are usually out of control, and therefore
   are likely to hit something.  I can also mention the effects of reaction
   time, you travel farther (27%) in a given time at 75 than at 55.  I don't
   think many more accidents would occur if the limit were raised, I just
   think they would be worse.

				Peter Barbee

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