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Re: Truck stopping distances?



----- Original Message -----
From: "Brian Whatcott" <inet@INTELLISYS.NET>


At 06:59 AM 3/1/02, you wrote:
Why does it take a truck further to stop than a car? Data on many web
sites
gives about a 50% increase in stopping distances.

BUT

HS physics friction problems typically ask students to calculate the
stopping distance of a vehicle with speed x and COF y. This shows that
the
mass of the object is cancelled when finding the acceleration and
therefore
should be independent of the stopping distance.

Same tires, same pavement.
What is different about the braking systems?

Web searches only reveal that the distances are greater. No mention of
why -
besides the fact that trucks are heavier - useless.

Scott



*****************************
S.Goelzer


And come to think of it, loaded cargo trains can take a mile to stop.
Looks like a kinetic energy/available braking force ratio issue to me


With the train you also have to deal with steel to steel frictional forces
rather than 'rubber' to asphalt. One tends to see (in the movies) a steam
locomotive that tries to suddenly stop, reverses the driver wheels, only to
have them spin.

Note that a Lincoln Navigator will also take farther to stop then a Geo
Metro. I think everyone basically agrees that it is a function of the
brakes, not the tire/road frictional force that is crucial. Things get
complicated if we talk tandem trailers and keeping them from jack-knifing,
but a cement truck has the same concerns. Avoiding brake overheating has
been offered, but in practice this seems to be a question of design--how big
an area given to the brakes, how the brake/wheels are designed for cooling.
Material limitations was offered, but again it would seem that brakes
systems _could_ be designed to handle the increased forces. When this
argument was applied to the higher pressures were cited, but not the fact
that large trucks have _more_ tires so that any argument that still takes
tire/road contact area into account must account for that. I would still
suggest that a cost/benefit ratios come into play. Large, expensive,
sophisticated braking systems on trucks could probably stop them in
approximately the same distance as a car, but everyone expects that trucks
will take farther to stop, so save some bucks by staying with the current
moderately efficient braking systems.

[Anyone ever watch Nascar racing where they sometimes have a camera mounted
under the car to look at the brakes. They get hot enough to glow! ]

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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