Date:         Fri, 20 Dec 1996 17:54:11 -0800
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         bayer@sybase.com (David Bayer)
Subject:      Re: What was that about lowering tire pressure on ice?

I am sure that Derek Drew can speak about this as one can view pictures of
his syncro westy climbing through deep snow with slightly squat tires, but I
will add my observations first...

>| Under-inflated tires actually buckle inward when in contact with the 
>| snowy ground due to low tire pressure, thereby leaving only the
>| edges of the tire to contact the ground as evidenced by the tire
>| track patern in the snow left by the spinning tire.  This means that only
>| the edges of the tire are really contacting and gripping the snow. 

	When you let pressure out of the tire, it may buckle in a little bit,
but more importantly, you are lengthing the amount of tire that comes in
contact with the snow and ice below. As the tire deflates, the top and side stay
round, the bottom of the circle flattens (when viewed from the side). When view
head on, the tires curls up a little bit - you're in snow not on roads when
using this method, right so that curl it still going to be filled; the tire
effectively becomes a short track... This will help you get through muddy
tracks, deep snow, not really deep snow, icy roads (where the greater contact
patch raises the amount of force needed to start you sliding), and when most
of all - when you are already kind of stuck.

>| In your experiment, you'll likely find that all things considered equal, 
>| the under-inflated tires will sit and spin more than the properly
>| inflated tires.

	This is also going to be very dependent on your driving style. Real
slow starts and stops on ice are crucial because the amount of friction to
start you sliding is a hell of a lot lesser than the amount of friction needed
to stop you from sliding. Also, how under-deflated are the tires?

	Also, I do not think that driving around with your tires deflated for
a long period of time was an option people where talking about in previous
threads. Deflating your tires will help you when things are real rough, but
it a solution to fix things temporarily. If conditions are horrible over a
long period of time or on a daily commute, I don't think lower tire pressure
to 15 psi would be the best solution, but then, I haven't lived in a snowy
climate for a few years. When I did, I just slowed way down and tried to
avoid traffic when the weather got too nasty... Still, the cold did lower my
tire pressure a bit in the little car (with little tires) I was driving then
(the tires didn't tend to curve inward on the little fiat project car) and
my concern was not really on the tires (usually on the electrical system) so
they were inevitably low. I slide only once in that car when I didn't mean
to, and it was when I was going too fast on an icy road in Maine.

dave