[s-cars] Re: S-CAR-List digest, Vol 1 #1863 - Contact Patch

[Larry C. Leung]

Sean posted to list:

>Rich:
>
>I agree with you, maybe I wasn't explaining myself very well. Yes, the
>area of the contact patch is fixed, it's the shape that changes. Now,
>I'm not sure if the shape has any relationship to directional motion,
>i.e. if the shape is short and wide, does it provide more lateral grip
>versus a shape that is long and skinny that provides more longitudal
>grip (forward motion). The former would favor cornering power and the
>latter would favor acceleration / braking.

This is generally correct.

A long skinny patch (such as truck, trailer tires) favor braking and
accelerating.
Part of the reason for the tall sidewalls and high aspect ratio of drag
racing tires
is due to the contact patch shape. The low pressures also allow windup
and also
alter the contact patch to a launch traction favoring shape on take-off.
The other
advantage would be the larger patch with lower pressures. With braking
this would
also apply, but control and modulation would suffer.

Wide short patches (due to wide, low profile tires) add to cornering
grip, pressure
adds modulation and control, but does, to a degree, reduce availible
contact patch.
Fortunately hard cornering adds enough weight transfer to the widen the
patch again.

Conceptually, all of these things should change proportionally with
pressure, BUT,
tires are NOT consistant in construction (unlike a balloon), so the tread
area is
not the same in general stiffness and response to pressure as the
sidewalls. This means
some of the above conclusions are not anything more than generalizations,
but they
are generally true. The fine tuning are what tire engineers are for.

As always, YRMV.

Sean also writes:

>Maximum stopping distance is obtained the moment before the tires lose
>grip with the road surface (just before lockup). Any braking system is
>easily powerful enough the "lock the brakes" with any given tire and I'm
>not sure that the friction developed by tires varies significantly
>enough to make a difference.

Within reason, true. The grippier tires/compounds fight lock up noticably

better, so even with ABS, result in shorter stopping distances. In short,

they have the ability to provide more stopping force by having a greater
coefficient of friction.

>I realize that on a track situation bigger
>brakes dissapate heat better and provide more pedal feel which would
>contribute to shorter stopping distances, but I find it difficult to
>believe that the advantages of bigger brakes do not apply in a one time
>situation as in normal street use.

Better modulation DOES effectively reduce stopping distances. More
important, however, is the greater force (torque really) provided by
larger brakes allows a faster RATE of build of stopping force, for those
who follow, the area under the braking torque curve is wider and flatter
below the tire slip force, so larger brakes also contribute to shorter
stopping distances by basically having greater power. It's similar to the
reverse situation for horsepower and torque. For example, an I-5 turbo
engine (high
torque, broad flat torque curve, just somewhat weak at the very bottom
end)
that produces say 240 HP and around 240 ft-lb torque, vs say a Honda
S-2000 engine that produces 240 HP and around 170 ft-lb of torque, if
asked
to move the same car (assume that they weigh the same) will easily beat
the
Honda powered car to say, the 1/4 mile, even though they both will have
VERY similar trap speeds at the end of the quarter. It's the broad torque
curve (i.e. torque) that makes this happen. It's the same for brakes. The
larger
brakes by, default, have more availible torque (T = r x F, r being rotor
diam).
So, they WILL shorten stopping distances, in spite of the fact that the
PEAK braking force is limited in both brakes cases by the tires
coefficient
of friction (grip).

Now, on the street, you're probably safe with stock brakes, and you drive
according to the capabilities of the car, with appropriate safety
margins. Bigger,
better brakes will allow you to have a GREATER safety margin (or if you
are
a heavy traffic jockey, run closer tolerances within your own reaction
times!).

BTW, not ALL cars can easily lock brakes, my brother's '95 Mazda, with
totally
functioning (per factory) brakes, is scarily inadequate in it's ability
to lock, and
the beater '90 Subie I'm driving is NOWHERE as effective at braking as
any of
my (ex and current) Audi's or VW's. I drive accordingly.

HTH,

LL - NY