BMW and driving in snow

[John Smith <john@xxxxxxxxxxxxxxxx>]

I'd be interested in digesters informed thoughts/comments/opinions on
the
following observations...

My 325i ("standard" 205/55 R 15 87V tyres) has excellent
drivability (straight line, cornering, braking etc) in the dry.
If there is a limitation it seems to be that the back end
breaks away first on turning (seems to go quite "light"). However in the
wet its very easy to lose traction - more than you'd expect even given
the reasonable amount of torque available at the wheels. In the snow
it's probably one of the poorer performers I've driven and I don't think
it's entirely my driving :-) I've owned many other cars, most with less
power
I must admit.  (And I wouldn't swap my BMW). But before anyone goes
bananas at me about not having the correct tyres for snow please wait
- - I know that already, but I'm comparing with other (lesser) cars that
didn't
have snow tyres either. 

Now to my question (at last): *why* is it that larger (wider footprint)
tyres
seem to be (are) poorer performers in terms of grip, in the snow (and
wet)?

Maybe this is a silly question with an obvious answer - but I ask it
anyway! I'd
have thought that more area in contact with the ground means more grip?
Maybe that's
only true in the dry? I suppose for a given weight of vehicle, a tyre
with a
larger footprint has less downforce per-unit-area. If this is the case
then
downforce (into snow or onto wet tarmac) must be a factor too. So big
weights
in the boot/trunk will help. Also tyres with smaller footprint?

Having said the above, I thought that snow chains work because they
"dig-in" to
the snow and "pull" the car through it - but snow (I suppose) has quite
a low sheer-force, so maybe chains work because, overall, they have a 
very low footprint and so a high downforce per unit area - maybe?

The above may well be junk - if it is perhaps you can tell me why - I'd
really like to know.

- -John

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