From: mchaffee@ux4.cso.uiuc.edu (REAL LIFE?!?!  HA!!)
Newsgroups: rec.autos.driving
Subject: Re: Braking Distances
Date: 22 Nov 92 18:32:51 GMT
Organization: University of Illinois at Urbana

cal@codewks.nacjack.gen.nz (Cal Gulliver) writes:
> 
>Check out some AA road tests of new cars sometime. You'll see
>stopping distances that yield coefficients of friction of
>(usually) 0.8 to 0.95. But the maximum possible coefficient is 1.0,

Hate to say it but the max. coefficient is not 1.0 in any way at all.  It hap-
pens to be a convenient upper limit because it is most difficult to exceed
but it HAS been exceeded, and in cars too!  I think the tires on dragsters are
getting pretty close to 2.0, in fact.  The number 1.0 used to be like the sound
barrier; because it hadn't been broken, everybody assumed it couldn't be.  It
has been.

>And yes, the biggest truck can stop in the same distance as the most 
>British of sports cars, all else being equal (i.e. air resistance, 
>aerodynamics, road surface and weather, brake and tyre condition).

Again, I'm sorry, but this simply is not possible.  The only way a big truck
is going to be able to stop in the same distance as an MG is if it has the same
ratio of road friction to mass.  This would necessitate incredibly immense
tires, probably with more area than the truck has down on the road.  The prob-
lem is mass; the forward energy of the vehicle increases by the square of the
increase of mass.  Let's assume a British car that weighs 2000 lbs.  A little
heavy, I know, but it'll do.  Now let's assume a truck weighing in at 20,000
lbs.  OK?  Now, you assert that the only reason the truck takes much, much 
longer than the MG to stop is because the truck has more AIR RESISTANCE?!?!?
Truck brakes are always able to lock the wheels, so brake condition is not
a factor.  Now, if you assert that aerodynamic downforce is an issue, I have to
scoff.  The truck's raw weight creates enough more downforce than the aero-
dynamics of the British car to make this insignificant.  In fact, last time
I checked, at highway speeds, the bodies of most British cars LIFTED, they
didn't bear down!  OK, now tire condition.  Seems to me there's a lot more tire
under a truck than an MG.  So again, if I'm not mistaken, the truck should have
the advantage in this category.  You are failing to take something into account
in your calculations if you think a truck has a chance against my MG.  I would,
however, like to see a set of calculations that shows a cement truck and an MG
stopping in the same distance.  I'll tell you one thing:  You're only going to
see it in calculations; if I saw that happen in real life I think I'd start
believing in God again.
> 
>JW> ... locking all four wheels and sliding may be the best strategy
>JW> in some emergency stops if the idea is to minimize the stopping
>JW> distance.
>JW> ... one serious flaw, however: if the car is fully sliding, you
>JW> have virtually no control ...

Again, somebody's forgetting something here.  The Cf of sliding tires is sub-
stantially less than the Cf of tires at threshold.  Put another way, the
dynamic coefficient of friction (sliding) is ALWAYS less than the static
(rolling) coefficient, except in the case of Teflon, which doesn't apply here.
Whether or not the above came out of Scientific American, it is simply wrong.
The only case where sliding tires is a good idea is in the case of someone who
is not articulate enough to threshold-brake properly.
> 
> 
>AL>  ... (Of course, to determine the deceleration caused by that
>AL>friction, we need to know how hard we can brake without causing
>AL>the tires to lose their traction, and that *is* related to the
>AL>friction between the tires and the road surface.  But assuming
>AL>we're just teetering below that threshold, braking distance is
>AL>determined by the deceleration caused by friction experienced
>AL>by the brakes themselves, not the tires.)
> 
>With a strong leg and/or power brakes (and even little cars have 
>vacuum-assisted brakes these days) you can always lock the brakes,
>so the upper limit on braking performance is the tyre/road friction.
> 
>By far the best answer, of course, is ABS (Anti-lock Braking System).

On this I agree.
===
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