Triumph TR2 Service Instructions Manual page 235

WHEELS AND TYRES
are taken into account when designing
the tyre and in preparing Load and
Pressure Schedules.
Pressures in warm tyres should not be
reduced to standard pressures for cold
tyres.
" Bleeding
"
the tyres increases
their deflections and causes their tem-
peratures to climb still higher. The
tyres will also be under-inflated when
they have cooled.
Speed
High speed is expensive and the rate of
tread wear may be twice as fast a t 50
m.p.h. as at 30 m.p.h.
High speed involves
:-
(i) Increased temperatures due to
more deflections per minute and a
faster rate of deflection and re-
covery. The resistance of the
tread to abrasion decreases with
increase in temperature.
(ii) Fierce acceleration and braking.
(iii) More tyre distortion and slip
when negotiating bends and cor-
-
-
ners.
(iv) More
"
thrash
"
and
"
scuffing
"
from road surface irregularities.
(d) Braking
"
Driving on the brakes
"
increases
rate of tyre wear, apart from being
generally undesirable. It is not neces-
sary for wheels to be locked for an
abnormal amount of tread rubber to
be worn away.
Other braking factors not directly
connected with the method of driving
can affect tyre wear. Correct balance
and lining clearances, and freedom
from binding, are very important.
Braking may vary between one wheel
position and another due to oil or
foreign matter on the shoes even when
the brake mechanism is free and
correctly balanced.
Brakes should be relined and dnuns
reconditioned in complete sets. Tyre
wear may be affected if shoes are
relined with non-standard material
having unsuitable characteristics or
dimensions, especially if the linings
differ between one wheel position and
another in such a way as to upset the
brake balance. Front tyres, and par-
ticularly near front tyres, are very
sensitive to any condition which adds
to the severity of front braking in
relation to the rear.
"
Picking up
"
of shoe lining leading
edges can cause grab and reduce tyre
life. Local
"
pulling up
"
or flats on
the tread pattern can often be traced to
brake drum eccentricity.
(Fig. 5.)
The braking varies during each wheel
revolution as the minor and major
Fig. 5
Local excessive
wear
due to Brake Drum
Eccentricity.
axes of the eccentric drum pass alter-
nately over the shoes. Drums should
be free from excessive scoring and be
true when mounted on their hubs with
the road wheels attached.
(e) Climatic Conditions
The rate of tread wear during a reason-
ably dry and warm summer can be
twice as great as during an average
winter.
Water is a rubber lubricant and tread
abrasion is much less on wet roads than
on dry roads. Also the resistance of the
tread to abrasion decreases with in-
crease in temperature. Increased abra-
sion on dry roads, plus increased tem-
peratures of tyres and roads cause
faster tyre wear during summer
periods. For the same reasons tyre
wear is faster during dry years with
comparatively little rainfall than during
wet years.
When a tyre is new its thickness and
pattern depth are at their greatest. It
follows that heat generation and pattern
distortion due to flexing, cornering,