RE: BMW and driving in snow

["Lin, Gary" <Gary.Lin@xxxxxxxx>]

Alex replied:
John asked:
> > 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)?
> 
>Because they have a harder time pushing through water/mud/snow/slush to the
surface of the road.  The wider the tire, the larger the contact patch with
the road.  The weight of a car is constant. The larger the contact patch,
the lower the weight per square cm/inch of the contact patch. This is great
news for dry traction.

>However, the lower the weight per cm2 of contact patch, the lower a chance
that a tire will push through water/mud/snow/slush to the surface of the
road. The less grip a tire appears to have in rain/mud/snow. 

For a given air pressure, wider tires do not have a 'larger' contact patch,
they have a differently shaped one, short and wide vs, long and narrow.
This is good for dry but bad for rain/snow.  Snow calls for the long /
narrow patch, i.e. the wheel cuts through the snow instead of floating on
it.  Picture a pizza cutter vs. rolling pin.

John asked:
>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?

Yes, yes.  But remember, size doesn't change, shape does.  Going smaller (-1
concept, reverse of +1) makes the footprint shift to the long / narrow
shape.

>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?

Yes, it's like the high heel spikes putting a much deeper dent in mud vs.
large flat soles spreading out the weight.
- ---
Doc, using summer tires in snow is like using cough syrup to treat a
headache. ;-)  I could barely get my car on worn Yoke AVS Intermediates out
of a flat parking space, but putting on a set of Bridgestone Blizzak MZ-02's
today has it barreling through 8" drifts, hardpack, softpack, and ice.  It
can still spin wheels or kick in ABS, but grip is phenomenal compared to non
snow tires and the car remains very controllable once off the gas.  I can
probably slalom SUV's.  Can't wait for the Superbowl Ice Storm! :-P

Gary Lin   National Capital Ch.   '88 325is street   '85 325eis track

------------------------------