hydroplaning

[Patrick Austin]

>
>How can that be if the weight of the car or the tire pressure hasn't
changed.  A wider tire only changes the shape of the contact patch, not its
total area.

Well, think of it this way:
If it takes more power to remove the water than it does to support the
weight of the car on that cushion of water, then the car will hydroplane.  

Say you've got a contact patch, moving through the water at a constant
velocity, V.   The total water moved per unit time is proportional to V
times the width of the contact patch times the depth of the water.  The
length of the contact patch doesn't matter here, because once the water is
gone, it doesn't take any power to keep it out of the "hole" left by the
tire.  Anyway, the contact patch doesn't change size, but it DOES become
more narrow when you go to a skinny tire.  So, it takes less power to move
all that water aside...so the car has to move faster to float on the water.
 It's like a boat planing.

Correct me if I'm wrong.  I just came up with that right now, so it may be
entirely bullshit..I'm a manufacturing engineer, not a mechanical
engineer...I've never taken a fluids course.  However, I _think_ it makes
sense. :)

  
						**************
						Patrick Austin
						paanta at bu.edu
						(617)782-9115
						**************

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