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

["Geoff Jenkins" <jenkinsg@pilot.msu.edu>]

Been thinking about this... I agree in re: the effect of pressure, but _how_
do you separate pressure from area? Assuming vehicle mass is identical, as
you lower the pressure, surely don't you soften and flatten the tire, so
that the force downwards remains constant?

I.e.

iff Vehicle weight (lbs) = constant

Decrease lbs/in^2 => Increase in^2

So that while you _could_ compare two different tire widths, each of which
is running at say 40psi (the narrower tire presumably having a "longer"
contact patch, and the wider one a shorter patch), you can't compare one
tire at 20psi with the same tire at 40psi, as you're changing pressures and
patches simultaneously...

The other question, presumably, would be in the way the tire's contact patch
deforms as the pressure changes. If the patch becomes wider, you'd be more
likely to hydroplane. If it becomes narrower but longer, front to back, it'd
be less likely to hydroplane?

Or am I (not too rare an event) missing some point badly?

Geoff

> -----Original Message-----
> [mailto:owner-quattro@coimbra.ans.net]On Behalf Of Janet Scruggs
> Subject: Hydroplaning, was: Re: I have seen 225/50-15's on a 4kq
>
>
> 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?
[Snip]
> Regards, Gross Scruggs