hydroplaning ad nausea

[Brett Dikeman]

At 8:37 AM -0400 5/29/04, Mike Arman wrote:

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

I certainly am, after this- I'm deleting further messages on the 
thread- this discussion's going nowhere.  Huw, myself, and others 
present established fact; you say "BUT that CAN'T be right!" Don't 
bother replying- I'll ignore it, I promise.

>  This tells me that going from 25 lbs to 35 lbs in a car tire isn't 
>going to make a HUGE difference.

It makes an enormous difference; you're assuming that the tire 
performs essentially the same no matter the contact patch area.  The 
tire is designed to have a -very- specific contact patch area and a 
very specific shape.  Anything less or more will result in much less 
than ideal performance, and if the tire is not the proper shape, it 
won't work as well as it could, it won't wear properly, and it will, 
at the extreme, get damaged.  The tire flexes more at the sidewall 
when underinflated, and more at the center when overinflated.  When 
it flexes it heats up.  The work done by the tire flexing causes low 
mileage in a street vehicle under normal driving- 3-4 psi can affect 
your mileage by several MPG.

When doing high performance driving, you want a)even contact patch 
and b)even tire temperatures.  Too high and not only will you have 
too small a contact patch, you'll overwork it and it will be flexing 
more; the result is that they'll heat up fast, get greasy, and you'll 
be all over the place.  Too low and the center of the tire actually 
buckles up and off the pavement, you destroy the sidewall 
construction(by both bending and overheating it) and you wear the 
edges off the tire, wearing them down eventually to the cords.

   Alignment and tire pressure, as well as the track or road, affect 
contact patch and temperature.  This is why you see pit crews 
measuring tire temperatures and pressures from a race car hot off the 
track.  Reading temperatures tells you if the tire is being 
overworked, and if it is properly inflated.

Yes, the contact patch changes size under acceleration, cornering, 
and braking.  This is part of the reason you're taught at a driver ed 
school to do ONE thing at a time- brake, then corner, then 
accelerate.  Tires do one thing better than they do two things, in 
part because doing two things at once puts an enormous amount of 
weight on one tire, and lifts it off the other three.

All this is covered in a club driver education event, and I highly 
suggest you go to one if you find this sort of stuff interesting.

Yes, real world conditions make things very complex, which is why in 
every case a driver with mild training will be able to go around a 
track faster than he/she can with traction control on- and why a 
driver with mild training will be safer than someone who knows 
nothing about car handling but has TC.

   TC and ABS are designed to keep the vehicle under control- not to 
get the most grip.  "Under control", contrary to the advertisements 
on TV, is NOT a 'Hand Of God' that picks you up entering that 180 
hairpin at 60mph and plants you at the exit safely.  No sir- traction 
control will keep the car under control right until you smack the 
guardrail, shoving your cell phone up your nose and spilling your 
latte grande all over the nice expensive wood dash in your Mercedes.

   Go buy a book on high performance driving, go to a quattro(pardon, 
Audi) club driver ed event, take a high-school level 
chemistry/physics class, and stop arguing with the rest of us. 
Please.  It was fun for the first ten messages, now it's not, you're 
being stubborn and arrogant, and we're annoying the rest of the list.

Brett
-- 
"They that give up essential liberty to obtain temporary
safety deserve neither liberty nor safety." - Ben Franklin
http://www.users.cloud9.net/~brett/