Re: Tire Survey for Audi Coupe GT

[Chris Idleman (FSAC) <cidleman@fsac3.pica.army.mil>]

You asked "why are wider tires worse in the rain"...  Here goes:      

Wider tires are worse in the rain for the same reason they're worse in the
snow: less ground pressure per unit area of contact with the road.  For maximum
traction on dry pavement, the ideal tire is a wide racing slick that distributes
the car's acceleration and braking forces over the largest possible area, thus
minimizing the force per unit area (the engineering definition of pressure) and
providing maximum grip when cornering.  Please note that I'm talking about
"shear forces" (aka: friction) which are parallel to the road surface, vs.
normal force (aka: downward weight) which is perpendicular to the surface of
the road.  This may seem a little backwards, but by *decreasing* your downforce
*PER UNIT AREA* with a wider tire, you also decrease your lateral friction on 
a *PER UNIT AREA* basis.  This means you are farther below the friction break-
away threshold *PER UNIT AREA* and therefore are farther below the friction 
breakaway threshold for the entire tire.  You now have greater grip in corners.

On to the next situation -- when you put a film of water or perhaps a layer
of snow on top of the pavement, the reduced downward force per unit area of 
contact afforded by the wider tire becomes a detriment, because now the surface
of the tire is not able to press down to the pavement through the water/snow
layer.  Each tire tread block tries to force the water or snow to the sides or
into the tire grooves -- any water or snow that doesn't get out of the way fast
enough becomes compressed underneath the block of tire tread and exerts an up-
ward pressure on the tire.  The result is hydroplaning where the tire lifts off
the pavement and "floats" on that film of water.  The ideal tire in this situa-
tion is one that can press through the water film or provide sufficiently large
water channels (Aquatred TM*) and retain contact (and traction) with the
pavement underneath the water/snow layer.  To get this effect, you have to use 
a narrow or knobby tire that distributes the weight of the car over a smaller
area and translates to a larger downward force on each block of tire tread.
Of course when the road dries up again, you're left with a tire that's less
than optimal for dry pavement because it will tend to break away when hard
cornering or braking/accelerating.  The more of the tire's surface dedicated
to channels and grooves, the less area is available for tread. 

Therefore when choosing tires you have to consider which one is more important
to you: good dry traction or good wet traction?  Most tires are designed to 
offer a good blend of both, but I have yet to see a "winter" tire outhandle
a "summer" tire on dry pavement, and vice versa.  That's why most people who
aggressively drive their cars in the winter recommend changing to snow tires
when the weather gets foul and their summer tires start behaving like banana
peels.  

Have I made it any clearer or have I hopelessly complicated everything?


					Chris Idleman
					cidleman@pica.army.mil