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[Phys-L] Re: Stopping Distance



There is another problem with truck braking; namely, weight
distribution. Partially loaded trucks typically have a large fraction
of their weight on the forward 10 wheels, and much less on their rear
eight wheels. Thus there is a high propensity for the box to break
loose and fishtail if the brakes are applied too hard.

Truckers learn to compensate for this effect, but that leads (along with
all the other factors mentioned) to longer stopping distances.

Dr. Mark H. Shapiro
Professor of Physics, Emeritus
California State University, Fullerton
Phone: 714 278-3884
FAX: 714 278-5810
email: mshapiro@fullerton.edu
web: http://chaos.fullerton.edu/Shapiro.html
travel and family pictures:
http://community.webshots.com/user/mhshapiro


-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu] On
Behalf Of John Denker
Sent: Monday, October 31, 2005 7:46 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: Stopping Distance

This is a complicated problem.

I did some googling, and was appalled at how inane the resulting
"information" was. It reminds me of the blind men and the elephant,
except that describing the elephant was not a life-and-death issue,
whereas braking sometimes is.

I was unable to find a scatter plot of braking distances. In the
absence of actual data like that, I am not going to believe any
one-sentence summary of the data. The summaries I've found do not
pass the giggle test.

1) I assume everybody on the list knows the obvious "coefficient of
friction" argument. But that is only a tiny part of the whole
argument.

2) For instance, on a wet road, vehicles are sometimes vulnerable to
hydroplaning. A truck will be *less* vulnerable than a sports car,
because the truck has higher tire pressure. According to conventional
wisdom, the hydroplaning threshold scales like the square root of
tire pressure. In contrast, the IIHS boldly and unreservedly claims
that big rigs do "much worse" on wet roads.
http://www.iihs.org/research/qanda/antilock.html#7

Big rigs commonly operate with tire pressure in excess of 100 psi,
in contrast to sports cars which are commonly 30 psi or less.

3) In the other pan of the scale, there are "strength of materials"
arguments. Suppose the tire surface sticks perfectly to the road
surface. At some point, the braking force will shear the rubber,
in the same way that a cheap rubber eraser leaves behind bits of
rubber when you erase something. The shearing tendency should
scale roughly like the tire pressure, so in the (questionable)
scenario where the sports car is right on the threshold of erasing
its tires, and the truck tire is made of the same material, the
truck should have worse performance by a factor in excess of 333%,
not the 147% tossed off by IIHS. In reality, the truck tire will
be made of harder material, but that will reduce traction in other
ways, observable in a reduced coefficient of friction.

Strength of materials arguments also apply to the road surface. A
truck in a panic stop can literally rip up the pavement.

I tend to buy cars with a usable coefficient of friction just shy of
unity (i.e. cornering at .95 Gees). I've never seen a truck that
could corner at .95 Gees or anything close to that.

4) There are stability issues. A vehicle with poor design and/or
poorly maintained tires might spin out during a panic stop. In a
big rig, this can result in jack-knifing. Antilock brakes help
with this, but are not a panacea.

5) If the road surface is bumpy, it may be nontrivial to maintain
contact between the tire and the road. This depends on many parameters,
including how much unsprung weight there is in the tires and suspension,
and other details of the suspension. It also depends dramatically on
speed and on the degree of bumpiness. A sports car is likely to be very
much superior to a truck in this department.

*) etc. etc. etc.

It's just physics ... but it is not trivial one-parameter plug-and-chug
physics.

==========

There are AFAICT no enforceable regulations on truck stopping distances.
That means it is left up to the discretion of the individual trucker.
In turn that means it might be very, very bad.

This leads to some very important action items for all drivers,
including
us and our students. For starters, never allow a truck to tailgate.
One particularly bad scenario would be a one-lane road with a sports car
close ahead of you, and a big rig close behind you. If the guy ahead of
you slams on his brakes, you are going to hit him, or get rear-ended by
the big rig, or both. You need to get out of that situation as soon
as it develops. If you wait until the guy slams on his brakes, it's too
late: there's nothing you can do at that point.

I routinely call 911 on my cell phone and fink on any truckers I see
tailgating. I doubt the police take action on my reports, but at least
I can tell myself that *I* did the right thing.