WHEELS
AND
TYRES
DYNAMIC
OR
COUPLE
UNBALANCE
EQUAL
EXCESS
MASSES
AT
'A'
AND
'B'
Fig.
17
Dynamic or Couple Unbalance.
The causes may lie in road conditions,
traffic conditions, driving methods and
certain features of design which are essen-
tial to the control, steering and driving of a
car. Close attention to inflation pressures
and the mechanical condition of the car will
not always prevent irregular wear.
It is therefore recommended that front tyres
be interchanged with rear tyres at least
every
2,000
miles. Diagonal interchanging
between left-hand front and right-hand rear
and between right-hand front and left-hand
rear provides the most satisfactory first
change because it reverses the directions of
rotation.
Subsequent interchanging of front and rear
tyres should be as indicated by the appear-
ance of the tyres, with the object of keeping
the wear of all tyres even and uniform.
7.
PRESSED STEEL WHEELS
S.M.M.
&
T.
standard tolerances are-
(a)
Wobble
The lateral variation measured on the
vertical iqside face of a flange shall not
exceed
A".
fb)
Lift
wheel the difference between the high
and low points, measured at any
location on either tyre bead seat, shall
not exceed
&".
Radial and lateral eccentricity outside these
limits contribute to static and dynamic un-
balance respectively. Severe radial
eccentricity also imposes intermittent load-
ing on the tyre. Static balancing does not
correct this condition which can be
an
aggravating factor in the development of
irregular wear.
A
wheel which is eccentric laterally will
cause the tyre to
"
snake
"
on the road but
this in itself has no effect on the rate of
tread wear.
At the same time undue lateral eccentricity
is undesirable and it affects dynamic
balance.
There is no effective method of truing
eccentric ~ressed steel wheels economicallv
and they should be replaced.
Wheel nuts should be free on their studs.
When fitting a wheel all the nuts should be
screwed up very lightly, making sure that
their seatings register with the seatings in
the wheel.
.
,
On a truly mounted and revolving
Fig.
18
Wire
Wheel
and
Hub
Cep.
12
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