The Delta independent., 1924-05-23
Automobile
Q's and As
(By Robt. B. Tilden.)
STOP SHORT

We usually smile when we see some
drivers swing up to the curb with
flourish and stop with the brakes lock-
ed, tires sliding on the ground The
usual thought is only that good tire
mileage is being used up to little
avail, yet the facts are, that the car
is not being stopped as quickly as if
the wheels had not been locked.
Brakes are more effective just be
fore the wheels are locked than after
they actually become locked although
few drivers realize this.
The force that enables us to brake
our car is friction. In every other
part of the car lubricants are used to
reduce this to the smallest degree pos
sible. In our braking system, however
especially where the wheels come in
contact with the ground, the greater
the friction, the quicker the stop.
Friction may be defined as the re
sistance of one surface, the change
in relative position of another surface
with which it is in contact. This
resistance is directly opposed to the
force tending to produce this relative
change in position.
There are two kinds of friction and
it is because of this fact that differ
ent degrees of braking effect are OBJ
tanned when the tires are sliding and
when they are not.
Takeing the definition of resistance
given above it is easily seen that if the
resistance is greater than the force
tending to produce motion no motion
occurs. This is Static Friction" or
the Friction OF Rest" If. on the
other hand, the force tending to pro-
duce motion is greater that the re
sistance to motion. motion occurs and
the static friction changes to "Kinetic
Friction" or the Friction of Motion" ="
We have had our definition of
friction but just exactly what is it?
tact interlock mechanically one with
the other. A surface may appear abso-
lutely smooth to our eye and touch,
1t may be ground to a mirrorlike fin
ish, yet actually it is not smooth, as
an examination OF it through a micro
scope would show Under the magni-
flying eye OF the miscoscope this ap-
parently smooth surface dissolves into
field of hills and hollovs, minute to
be sure but nevertheless a series of
high and low points. The friction be
tween two such surfaces is simply the
high spots OF one "meshing" with the
nigh spots OF the other. In exactly
ha ha sa- ~ frx-n
gears mesh with the other
In the case OF the gears, the smaller
the teeth the easier it is to strip the
gear and the less the gears must be
separated to unmesh them. In like
manner the smoother the surface, the
shallower are these minute hills and
hollows and the easier it will be to
move one surface on the other.
When one surface is stationary up
on the other. these myriads OF points
become most firmly meshed, the high
points OF one fitting into the low
points OF the other. When motion oc-
ours. however. these points do not
have the opportunity to mesh so COM
pletely. not sufficient time In reality
one surface is humping along on the
other. hitting the high spots, as it
were Naturally the greatest resist
ance to motion is when the points are
completely in mesh, when the surfaces
are stationary. 1t is for this reason
that Stau Friction is greater than
Kinetic Friction.
We ourselves demonstrate this fact
many times in our daily work. Prob
ably you are sitting in a chair while
reading this With both feet flat on
the floor very slowly try to slide the
chair with your feet. You will find
that the push required to start the
chair is considerably greater than that
required to keep 1t sliding after mo-
tion is once started
Applying this to our car we find
that the resistance to horizontal mo-
tion OF the tire on the road surface is
static resistance when the wheel is
rolling. for under this condition there
is no relative motion between that
portion OF the tire in contact with the
road and the road surface. For each
instant OF time the effect is the
same as II the car was at rest On the
other hand when the tire is sliding,
the resistance to the motion is one Of
Kinetic Friction, the lesser OF the two
forces, for then there is actually rela-
tive motion between that portion of
the tire in contact with the road and
the road surface.
II is for this reason that the hraking
effect is much more rapid just before
the wheels become locked than after
they are actually locked and sliding.
In the former case Static Friction with
its greater resisting power is bringing