hydroplaning

[Mike Arman]

This DID turn out to be an interesting discussion . . . and I'm working 
this out myself to see if the FAA has any idea what they are talking about 
(uh huh).

For simplicty, let's take the bicycle example - bike plus rider weighs 200 
lbs, assume 50/50 weight distribution, each tire is supporting 100 lbs, at 
90 psi, contact patch is 1.11 square inches for each tire. The patch shape 
will be an ellipse, widest across the tire, narrowest fore and aft.

Let's change the tire pressures:

25 psi = 4 sq inches

50 psi = 2 sq inches

75 psi = 1.3 sq inches

100 psi = 1 sq inch

125 psi = .8 sq. inch

150 psi = .66 sq in

200 psi = .5 sq in.

Notice the tire pressures required to reduce of the size of the contact 
patch: To go from 4 to 2, we doubled the pressure. To go from 2 to 1, we 
doubled it again. to go from 1 to .5, we doubled it yet a third time. This 
tells me that going from 25 lbs to 35 lbs in a car tire isn't going to make 
a HUGE difference.

Now let's take the 9 times the square root deal:

sq rt 25 = 5, 9 times 5 = 45

sq rt 50 = 7, 9 times 7 = 63

sq rt 75 = 8.6, 9 times 8.6 = 78

sq rt 100 = 10, 9 times 10 = 90

sq rt 200 = 14, 9 times 14 = 126.

Note that the final number "45, 63, 78" etc. is NOT MPH - it is an "index", 
and I have no idea how to convert it to MPH - get out the skid pad!


We can see from this that it takes a GREAT increase in tire pressure to 
significantly raise the final number - to go from "45" to "90", we had to 
quadruple the tire pressure from 25 psi to 100 psi.  To get to "180", we'd 
have to have a tire pressure of 400 psi, quadruple again! (sq rt 400 = 20, 
times 9 = 180)


It gets significantly more complex when we have a vehicle with unequal 
weight distribution, with four wheels instead of two, plus braking and 
acceleration effects, plus variable road surfaces, plus undetermined 
amounts of water on the road, plus other material (sand, oil, etc.) on the 
surface as well, plus variances in tire compound, tread pattern, the 
hardness of the rubber itself, turning forces, and add them all together 
and you start to appreciate how much technology must be in MB's new 
traction control systems on the 600SL (assuming it works).

I have the distinct impression we are not done talking about this . . . ;-)


Best Regards,
Mike Arman