266 Auto Suspension and Steering
itself. The three grades are A, B, and C. A provides the
greatest resistance to heat; C provides the least.
The traction rating is also designated by the letters A,
B, and C. A tire with an A rating has the best traction on
wet surfaces, while a tire with a C rating has the least.
The tread wear rating is designated by a set of
numbers. These numbers range from 100 to 500. A tire
with a rating of 200 should last about twice as long as a tire
rated at 100.
Wheel Fasteners
An important factor in wheel and tire design is the
way the rim is mounted to the hub or axle flange. On
almost all cars and trucks, the hub or axle flange contains
the wheel studs, Figure 12-40. Most wheel studs are
threaded bolts or studs pressed into the hub or flange. A
knurled area on the rear section of the stud cuts into the
hub or axle metal to keep the stud from loosening. The
head of the stud resembles a bolt head and is wider than
the hole in the hub or flange. The head keeps the stud from
coming completely through the hole. To precisely center
the wheel, a central part of the hub or flange is slightly
raised and holds the center of the rim in position.
To install the wheel, the holes in the center section of
the rim are placed over the studs and lug nuts are threaded
onto the studs. The lug nuts can then be tightened in a
cross or star pattern. The tapered end of each lug nut
matches a tapered area in the wheel mounting hole. The
matching tapers help center the wheel. On most steel
wheels, the lug nuts can be tightened by hand or with an
impact wrench. Custom wheels have different metal
expansion rates than the steel and iron hubs, and the lug
nuts must be tightened to a specific torque.
A few imported vehicles use a somewhat different
design. Instead of studs, the hub or flange has threaded
holes. Tapered lug bolts, Figure 12-41, are installed
through the wheel and threaded into the holes. As with lug
nuts, tightening is very important.
Tire Pressure Monitoring
Every set of tires has an ideal pressure for best han-
dling, braking and tire life. Even new tires lose some air
over time. For this reason, tire air pressure must be moni-
tored and air added when necessary. The simplest way to
check tire pressure is with a mechanical pressure gauge.
Most drivers, however, rarely remember to check tire pres-
sure. For this reason, many modern vehicles are equipped
with a tire pressure monitoring system. The tire pressure
monitoring system routinely checks pressure in each vehi-
cle tire. If low pressure is detected, an instrument panel
Lug nut
Center
cap
Aluminum wheel
Flat screw
Rear
hub
Brake disc
Wheel studs
Figure 12-40. This aluminum wheel is fastened to the rear hub with wheel studs and lug nuts. (Honda)