268 Auto Suspension and Steering
Independent drive axles are usually called CV axles.
CV axles are solid steel shafts or hollow tubes that connect
the transaxle output shafts to the wheels. There are always
two CV axles on a front-wheel drive vehicle. Some front-
wheel drive vehicles have a transfer shaft on one side. A
few high-performance or sports cars use an independent
rear axle with CV joints. Some older independent rear
axles have U-joints.
On most cars and trucks with rear-wheel drive, the
drive axles are solid steel shafts that extend from the
differential assembly to the wheel hubs. Some four-wheel
drive vehicles have a solid front axle. All front-engine, rear-
wheel drive vehicles use a drive shaft between the
transmission and rear axle. Some drive shafts are two piece
types. Four-wheel drive vehicles also have a front
drive shaft.
Several types of flexible joints are used on modern
vehicles. The type of flexible joint used depends on the
type of axle being used. CV joints are used on all
front-wheel drive vehicles and a few rear-wheel drives.
They are able to transmit power through an angle without
causing variations in shaft speed. There are two kinds of
CV joints, the Rzeppa joint and the tripod joint.
The Rzeppa joint consists of a set of ball bearings
inside of two sets of races. The ball bearings can move
between the channels to compensate for changes in angle.
Tripod joints consist of a three point spider and trunnions
that rotate on roller bearings. The spider and trunnions are
placed in a housing. As the axle rotates, the trunnions and
spider are driven by the housing and tilt to compensate for
angle changes. CV joint boots keep lubricant in and dirt
and water out.
The U-joint, or Cardan joint, is used with drive shafts.
U-joints consist of a four-point cross and caps that turn on
needle bearings. The cross can twist as it rotates to
compensate for shaft angle changes. U-joints are used
when the angle change between rotating parts is
not too great.
Wheel bearings form a low-friction connection
between the wheels and the vehicle. All wheel bearings
are antifriction bearings consisting of three basic parts: the
inner race, the rolling element, and the outer race. There
are two kinds of bearing loads. Radial loads are caused by
the weight of the vehicle and by centrifugal force. Axial
loads are sideways loads that occur when the vehicle
is turned.
The three types of antifriction wheel bearings are the
ball bearing, the flat roller bearing, and the tapered roller
bearing. Ball bearings are found on the front axles of many
front-wheel drive vehicles. The balls and races are in a
single sealed unit. Straight roller bearings are often used on
the rear axles of rear-wheel drive vehicles. Tapered roller
bearings are used in front and rear axles and are always
installed in pairs to absorb sideways loads. Tapered roller
bearing preload is adjustable.
Most tapered wheel bearings are packed with wheel
bearing grease, which must be periodically renewed. Some
ball bearings and straight roller wheel bearings are greased
for life. Oil splash from the rotating gears is often used as a
lubrication method on solid rear axles.
The two main classes of wheel bearing lubricants are
wheel bearing greases and gear oils. Most wheel bearings
are lubricated by grease. Some solid rear axle wheel
bearings are lubricated with gear oil. The wheel bearings
on most modern vehicles call for EP lithium grease. Wheel
bearings used in solid rear axles are sometimes lubricated
with the gear oil used to lubricate other rear axle parts.
Bearing seals keep the oil or grease in, and water out.
The wheel hubs and axle flanges are the mounting
surface for the wheel rims and tires. The hub may contain
the bearing races. Front hubs used on front-wheel drive
vehicles are often pressed onto the axle.
Wheel rims are the connection between the hub and
the tire. The rim design used today is called the drop-
center wheel. Rims are made of stamped steel or various
alloys. Rim size determines what type of tire will be used,
and is composed of diameter, width and flange height.
Tires have the job of cushioning road shocks and pro-
viding traction. External parts of the tire are the tread and
the sidewalls. Internal parts are the plies and belts. Modern
tires are radial tires. Older tires were called bias tires. Tire
ratings and quality grades are stamped on or molded into
the side of the tire.
The wheel rim is mounted to the hub or axle flange
through wheel studs and lug nuts. A few vehicles use
tapered bolts instead of lug nuts and studs.
Review Questions—Chapter 12
1. Why are front-wheel drive independent axles called
CV axles?
2. While most CV axles are _____ steel shafts, a few are
_____ to reduce weight.
3. A four-wheel drive vehicle has two _____.
4. What is the purpose of a drive shaft slip yoke?
Matching
Match the drive axle with the type of flexible joint.
5. Independent front axle. a. CV joint.
6. Independent rear axle. b. U-joint.
7. Solid rear axle. c. No flexible joint.
8. Rear-wheel drive shaft.
9. Front-wheel drive shaft, four-wheel drive.
10. A Rzeppa joint uses _____ between the races to
allow angle changes.
11. A tripod CV joint uses three _____, which are sepa-
rated from the spider by needle bearings.
12. All wheel bearings are known as what kind of
bearings?