RE: Hydroplaning, was: Re: I have seen 225/50-15's on a 4kq

[Eaton Dave <dave.eaton@minedu.govt.nz>]

agree to a point. however, the issue to my mind about inflation pressure is
simply one of the ability of the *whole* contact patch to be in good contact
with the road surface.  as you know, an under-inflated tyre cannot maintain
contact pressure in the centre of the tread as it can on the outside.  hence
heavy tyre wear on the outsides of the contact patch.  i would also say that
an under-inflated tyre would not be nearly as successful at pushing the
water *outwards* and tend to leave a lot of water in the centre of the
contact patch.

this issue obviously is more of an issue with wider tyres than with narrower
ones.

it's a similar issue with gravel and rally cars as well.  again generally
speaking, the rally cars are keen to cut through the gravel to the surface
underneath.  excessively wide tyres don't help this of course...

dave 
'95 rs2
'90 ur-q

	-----Original Message-----
	------------------------------

	Date: Wed, 25 Nov 1998 07:21:57 -0800
	From: "Janet Scruggs" <scruggs@mbay.net>

	Now that the topic of hydroplaning has been broached, I'd like make
it more
	complicated.  Dave Eaton, and others, have made the case that what
we are
	dealing with is a 'weight per square unit' relationship and I agree
that it
	feels 'right' and there is probably no need to investigate
further... but.
	Years ago the trucking industry studied this issue and found that
there is a
	direct relationship between tire pressure and ability to resist
	hydroplaning.  Further they found that the loading of a tire was not
as
	significant a factor as the tire pressure.  I'm sure that on a graph
of
	these two influences the loading and pressure lines will cross
somewhere,
	but I've no idea where.  I do know that the front tires on semis are
	typically inflated to around 85psi and all the rest to around 65psi.

	I offer the explanation that with increased pressure the tire will
deflect
	less as it rolls into a non-solid surface, like water, effectively
'cutting
	through' the water layer to a solid surface more effectively than a
tire
	having less pressure.  Examining the extremes seem to support this:
Imagine
	two tires of the same width and loading one being inflated to 10psi
and the
	other made of aluminum (sorry Phil and Jim... aluminium).  Now roll
them at
	speed through your imaginary water trough.  Seems to me that the
stiffer of
	the two should 'cut through' the water and resist riding on top of
the water
	film.  Comments?