RE: BMW and driving in snow

[John Smith <john@xxxxxxxxxxxxxxxx>]

"Fadeev, Alex" wrote:
> 
> > 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").
> 
> John,
> It's not the car, it's you!
> If anything, your car will be understeering like a pig. If your car suddenly
> oversteers during a turn on a slippery surface, barring any suspension
> upgrades, that means your are WAY too fast with your right foot!

Alex,
Thanks for the reply - but you seem to have had some trouble
understanding what I wrote. I have had several other replies - none say
it' me (even
though I'm always willing to learn - and don't know it all - and it
could be I suppose). The breakaway
I mentioned happens at high speed too and actually is quite common in
E30's at the limit
apparently- (well several people have said the same thing anyway) - it's
not too much right foot (although I know it *could* be). It's simply
what I observe the car actually 
does. I've driven for about 20 years now many of them in Germany and
Switerland 
and drove daily at 130 m.p.h on Autobahn's - somewhat slower on mountain
passes :-) so
have just a little idea about things like that. Experience isn't
everthing and
I obviously need some more practice.

> 
> > 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.
> 
> Ha?
> How is increased torque supposed to increase traction???
> Respectable amounts of torque will _de_crease traction (throttle-on
> oversteer) if you are not careful.

I didn't say it did. Quite the opposite. The word "even" was meant to
imply exactly what you say. Ok it was not a very good sentence.
I am aware that the high torque of a BMW
can easily cause loss of traction (as can any car) - so I am especially
careful about
exactly that.  In fact I've managed to drive cars up *very* steep icy
inclines when some others
have failed to do so (with the same car), so I'm not just some
heavy-footed driver.

> 
> > In the snow it's probably one of the poorer performers I've
> > driven and I don't think it's entirely my driving :-)
> 
> wanna bet?

Yes!
(see above!)

I'm more than willing to accept that I've got the wrong tyres fitted
(well I know
that really) - I just wanted to learn a bit more about as to why certain
tyres
perform as they do - that's all.

> 
> > I've owned many other cars, most with lesspower 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.
> 
> yeah, but did all those cares have the same size/width/design/model tires?
> if not, you are comparing apples to bricks.

Of course not... otherwise what would be the point of comparing
*different cars". It is valid to compare one cars performance against
another surely! The
whole point of my posting was to get some light about tyre *widths and
footprints generally* - NOT to compare the particular tyres - and not to
knock BMW's
(which are great cars) - that's why I have one.If I was trying
to compare the same tyres on both car A and car B then what you say
would be true - but I'm not. Please try to read what I wrote first.

> 
> > 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.

Other's have replied saying that's not true! I don't know
if it is or not. Some say contact area is the same as the extra
width is compensated for by reduce contact length. See... a different
point of view *may* be right!

> 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.

Ah! That what I thought (and wrote, I believe) in my posting. However,
several people have emailed me to say that that is *not* the case and
put reasonable arguments as to why. I hope they post copies to the
digest
too. 


I keep saying "other have said" etc above - they have, but as they have
emailed me
privately I will not post their responses/addresses here - perhaps
however it would
be good if responses came to the digest so others can debate these
points?
No critiscm intended (and thanks to those that did reply to me) - I just
think it may be a good idea.
> 
> alex f


John S

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