hydroplaning

[isham-research.freeserve.co.uk at pop.pol.net.uk]

>> that'd be a contact patch of 20 sq. in. at 40 psi.
>> For 8" wide tires, the rectangular length of the patch would
>> be 2.5".  Ok, sounds fine.  Now reduce the pressure to 10 psi.
>> The expected length would now be 10".  Not likely.  But, ok,
>> reduce the pressure to 5psi. I guarantee you won't see a 20"
>> long contact patch!

> the formula works until you are either riding on the rims or, slightly
> before that, the crumpled up sidewalls.

Sidewall-floor contact comes much earlier than that.  Your 8" wide tyre
becomes a 10" wide tyre long before it's deflated to 10 psi.  At 5 psi
it's on the rim.  Look at the pressures (around 60 psi) used in the
emergency spare.

>> The problem with the simple load x area assumption is that
>> it implies that the tire doesn't exist.  The air is *not*
>> supporting the car.  The *tire* is supporting the car, and
>> the air is giving the tire it's shape.

> the air is supporting the car.

I've noticed they go down quite a lot if you take the air out of the
tyres.  A trick used frequently where I used to live - to remove stuck
double-decker omnibuses from low railway bridges when the driver forgot
he wasn't driving a single-decker.

(For the locals - the Barton Bus Company's '15' route to Sawley.  The
 single-deckers could go straight down the A453 Tamworth Road, the
 double-deckers had to take a diversion.  Except, every now and then,
 a driver would forget and they'd have to let the tyres down to get
 the bus back out.)

--
 Phil Payne
 http://www.isham-research.freeserve.co.uk/quattro
 Phone +44 7785 302803   Fax: +44 7785 309674