WHEELS
AND
TYRES
Impact Fractures
In order to provide adequate strength,
resistance to wear, stability, road grip
and other necessary qualities, a tyre
has a certain thickness and stiffness.
Excessive and sudden local distortion
such as might result from striking a
kerb, a large stone or brick, an up-
standing manhole cover, or a deep
pothole may fracture the casing cords.
(Figs.
7
and
8.)
Impact fractures often puzzle the car
owner because the tyre and road spring
may have absorbed the impact without
his being aware of anything unusual
;
only one or two casing cords may be
Fig.
7
Severe
impact
has
fractured this Casing.
Fig.
8
A
double fracture.
fractured by the blow and the
weakened tyre fails some time later
;
there is usually no clear evidence on the
outside of the tyre unless the object
has been
suffciently sharp to cut it.
This damage is not associated solely
with speed and care should
be
exercised
at all times, particularly when drawing
up to a kerb or parking against one.
2.
SPECIAL TYPES
OF
IRREGULAR
TREAD
WEAR
(a)
''
Heel
and
toe
"
or
"
saw
tooth
"
wear
This is the condition where one end of
each pattern segment or stud is more
worn than the other (Fig.
9).
To
some extent
it
is latent
in
any non-skid
Fig.
9
"
Heel
and
Toe"
Wear.
pattern design and severe service con-
ditions may cause it to develop.
When each successive portion of a
running tyre comes under load the
tread is flattened and there is limited
pattern distortion and shuffle on the
road surface. Additional movement is
caused by braking, driving and the
tyre's own rolling resistance, which
acts
as
a constant retarding force.
On
rear
wheels
the effects of braking
and rolling resistance
are
offset by the
effects of
dri
.
Rear
tyres
usually
wear
evenly
"?
they
are
properly
maintained.
Front
tyres
are
at
a
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