Sec. 10
Page 61
fo rm s of abuse and c o s ts truck ow ners la r g e sum s
of money annually. T his trem endous w aste can be
avoided if trucks are equipped with tir e s and rim s
of the proper s iz e and type, la rg e enough to carry
the load. The rated carryin g capacity of a tir e can
not be in crea sed by inflating it beyond the maximum
recom m en ded p r e s s u r e .
V alve Stem
A fter in stallation of t ir e s , make su re valve
ste m does not r e st on brake drum. If b r a ss stem s
a r e too c lo s e to the brake drum s, e x c e s s iv e heat
may dam age valve and cau se p o ssib le lo s s of air
p r e s s u r e .
WHEEL AND TIRE WEAR
CORRECTING IRREGULAR TIRE WEAR
H eel and Toe Wear - - T his is a saw -toothed
effect w here one end of each tread block is worn
m ore than the other. The end that w ea rs is the one
that fir s t g rip s the road when the brakes a re applied.
H eel and toe w ear is le s s n oticeab le on rear
tir e s than on front tir e s , b ecau se the propelling
action of the rear w h eels c r e a te s a fo r c e which
tends to w ear the opposite end of the tread blocks.
The two fo r c e s , propelling and braking, make for
m ore even w ear of the rea r t ir e s , w h ereas only
the braking fo r c e s act on the front w h e e ls, and the
saw -tooth ed effect is m ore n oticeable.
A certa in amount of h eel and toe w ear is norm
a l. E x c e s s iv e w ear is usu ally due to high speed
driving and e x c e s s iv e u se of b rak es. The best
rem ed y, in addition to cautioning the owner on his
driving habits, is to interchange tir e s regularly.
Side W ear — This may be caused by incorrect
w heel cam ber, under inflation, high cam bered roads
or by taking c o rn ers at too high a rate of speed.
The fir s t two c a u se s a re the m ost common.
Cam ber w ear can be read ily identified b ecause it
o cc u r s only on one sid e of the trea d s, w hereas
underinflation ca u se s w ear on both s id e s . Camber
w ear req u ires co rr ectio n of the cam ber fir s t and
then interchanging t ir e s . T here i s , of co u rse, no
co rrectio n for high cam bered roads. Cornering
w ear is d isc u sse d further on.
M isalignm ent Wear - - T his is w ear due to
e x c e s s iv e to e -in or to e-o u t. In eith er c a s e , tires
w ill revolve with a sid e motion and scra p e the
tread rubber off. If m isalignm ent is s e v e r e , the
rubber w ill be scrap ed off of both tir e s; if slight,
only one w ill be affected.
The scrap in g action against the fa ce of the tire
c a u se s a sm a ll feather edge of rubber to appear on
one sid e of the tread and th is feather edge is c e r
tain indication of m isalign m en t. The rem edy is
readjusting to e -in , or rechecking the en tire front
end alignm ent if n e c e ssa r y .
C ornering Wear
— When a truck m akes an
WHEELS AND TIRES
extrem ely fa st turn, the w eight is shifted from an
even loading on a ll w h eels to an abnormal load on
the tir e s on the outside of the curve and very light
load on the in sid e t ir e s , due to centrifugal fo r c e .
T his unequal loading may have two unfavorable
r e su lts.
F ir st, the r ea r tir e on the in sid e of the curve
may be r e liev ed of so much load that it is no longer
geared to the road and it s lip s , grinding off the
tread on the in sid e half of the tir e at the e x c e s s iv e
rate. T his type of tir e show s much the sam e ap
pearance of tread w ear a s tir e w ear caused by
negative cam ber.
Second, the tra n sfer of weight may a lso o v e r
load the outside tir e s so much that they a re la te r
ally d istorted resu ltin g in e x c e s s iv e w ear on the
outside half of the tir e , producing a type of w ear
lik e that caused by e x c e s s iv e p o sitiv e cam ber.
C ornering w ear can be m ost e a sily d istin gu ish
ed from abnorm al cam ber w ear by the rounding of
the outside shoulder or edge of the tir e and by the
roughening of the tread su rfa ce which denotes
abrasion.
C ornering w ear often produces a fin or ra ised
portion along the in sid e edge of each row in the
tread pattern. In som e c a s e s th is fin is a lm o st as
pronounced a s a to e -in fin, and in o th ers, it tap ers
into a row of tread blocks to such an extent that
the tir e has a definite "step wear" appearance.
The only rem edy for cornering w ear is proper
instruction of op eration s. D riving m ore slow ly on
cu rves and turns w ill avoid grinding rubber off
tir e s . To offset norm al cornering w ear a s much as
p o ssib le , tir e s should be interchanged at regular
in tervals.
Uneven Wear — Uneven or spotty w ear is due
to such ir r e g u la r itie s a s unequal c a ste r or cam ber,
bent front su sp en sion p a rts, o u t-o f-b a la n ce w h eels,
brake drums out-of-rou n d , brakes out of adjust
ment or other m echanical conditions. The rem edy
in each c a se c o n s is ts of locating the m echanical
defect and co rrectin g it.
Powe*r and Speed — E x c e ssiv e sp eed has a l
w ays been harmful to t ir e s . Speed c r e a te s heat -
heat soften s t ir e s .
Stops and Starts — Quick stops and s ta r ts grind
off tread in a hurry, may ca u se flat spots which con
tinue to grow for the life of the tir e .
T em perature - - C onsiderably le s s m ileage can
be e j e c t e d from a tir e u sed in a ll w arm w eather
driving a s com pared to a ll cool w eather driving, or
from a tir e fir s t put into s e r v ic e in w arm w eather.
MECHANICAL IRREGULARITIES (F ig. 10)
Follow ing a re som e w heel or v eh icle ir r e g u
la r itie s which may ca u se rapid or uneven tread
wear:
Toe-In — The w h eels on the sa m e axle are
c lo s e r together in the front than they a re in the
