Before we go any farther, let's more
precisely understand what is meant when you say that wider tires improve braking distance.
I assume that we're talking about braking distances vis-a-vis threshold braking (as
opposed to just stomping on the brakes and locking it up). This means that we're applying
the brakes JUST to the point of braking free, then very quickly releasing them, and then
very quickly reapplying them. You repeat this sequence many times until the car actually
stops. (In effect, you're acting like a human ABS system.) The distance between first
touching the brakes and the point where you actually stop the car is what we'll call the
braking distance. Further, let me assume that we're comparing two tires that are identical
in all respects (diameter, material, etc), except for their width.
Now let's look at what happens at the tire-to-road interface. In a perfect physic's
laboratory world, one usually asserts that the friction force (i.e., the braking force)
between two objects is a function of two things: the coefficient of friction at the
interface, and the normal force. (Muddying the waters more than a little bit is the fact
that this is only true for two "smooth" surfaces sliding against each other. It
doesn't take into account that rubber is relatively malleable, and tends to fill the small
nooks and crannies in a roadway, thereby changing the nature of the "braking"
problem into one containing both frictional and shearing-type forces. This turns out to be
a VERY difficult complication. Worse, the vulcanized forms of rubber used in most
automotive tires has rather weird coefficient properties that don't behave nice and
linearly. But I digress...)
Regardless, let's assume for a moment that we're testing the brakes in a laboratory, where
we can just consider coefficients of friction and normal forces. For two identical cars,
the normal force (or vehicle weight) is the same, so this is a non-factor in affecting the
braking distances. That leaves only the coefficient of friction. If we assume that the two
tires are constructed of the same material, then they should have the same friction
coefficient, right? Well, yes, except that the wider tire has more surface area than the
narrower one. I'm talking about the "circumferential" area around the outside of
the tire. This is given as Pi x Diameter x Width. Pi and Diameter are constant, but the
Width's are not, by definition, the same. Therefore the wider tire has more surface area.
Why is this important? Well, it turns out that the coefficient of friction of rubber on
asphalt is dependent upon temperature (actually, it's usually measured as a function of
sliding velocity, but it ultimately is a function of the heat generated at the sliding
interface). In our case, during threshold braking, we get instantaneous, "micro"
sliding action between the rubber and the road. This causes heat to build up, which drops
the coefficient of friction. A wider tire, having more surface area, can more easily
dissipate this heat energy than can a narrower tire. One reference I have claims that this
effect can be as much as 5-20%.
But wait a minute! Why do F1 drivers want to warm their tires up? Isn't this to improve
their tire's "stick?" In a word, yes. But as I alluded to earlier, rubber ain't
linear. It ain't even progressively constant. Racing tires need to get up into a bounded
temperature range to exhibit their best performance. Too little heat, and the coefficient
of friction is lower. Higher temperatures, oddly, have the same effect-- namely lower
*While Mark is no tire company engineer, he is
a degreed and licensed mechanical engineer, so
is probably as qualified as anyone to answer the posed question.