362 Auto Suspension and Steering Copyright by Goodheart-Willcox Co., Inc. Introduction Correct wheel alignment is vital to vehicle operation. Incor- rect alignment can cause handling problems, steering dif- fi culties, pulling, poor tracking, and rapid tire wear. Th is chapter builds on the information presented in Chapter 16 to explain the purposes and methods of two- and four-wheel alignment. Types of alignment machines and tools are also covered. Th is chapter will explain how alignment methods diff er between rear-wheel and front-wheel drive vehicles. Th is chapter will show you how to perform all steps of an alignment on any type of car or light truck. Aft er study- ing this chapter, you will understand the basic principles and methods of both two- and four-wheel alignments. Wheel Alignment Fundamentals As discussed in Chapter 16, wheel alignment involves mov- ing various parts of the suspension and steering systems to obtain the proper relationship between the wheels, the road, and the vehicle’s body. To perform a wheel alignment, the technician must make a series of pre-alignment checks, attach the alignment measuring equipment, determine the vehicle’s alignment, and then, if necessary, move the adjust- ing devices to obtain the correct alignment. Th e technician must keep in mind that: • The vehicle must be realigned anytime a suspension or steering part is changed. • Normal wear makes alignment necessary on a regular basis. Very few vehicles operate for more than a few thousand miles before requiring realignment of some angles. • Vehicle defects and outside factors can affect alignment. • All alignment angles are related and a change in one can alter the others. • Factory-installed adjustment devices are used to adjust some angles. • Aftermarket devices are available to adjust some angles that are not equipped with factory adjustments. • Some angles can only be adjusted by changing parts or bending the vehicle’s frame. • Toe should be adjusted last on almost all cars and trucks. Two-Wheel and Four-Wheel Alignment Until about 20 years ago, almost the only type of wheel alignment was the two-wheel, or front-wheel, alignment. Most cars and trucks were rear-wheel drive, with a heavy solid rear axle holding the rear wheels in alignment. Th ere- fore, only the front wheels needed adjustment, and align- ment racks were designed to check only the front alignment. Today, there are as many front-wheel drive vehicles as there are rear-wheel drive vehicles. Rear-wheel drive is mostly used on trucks, vans, and sport-utility vehicles. Most cars are front-wheel drive. Most front-wheel drive vehicles have provisions for adjusting the alignment angles of the rear wheels. Some newer rear-wheel drive vehicles have independent rear suspensions, which must also be adjusted. Because of these design changes, four-wheel alignment is the most common type of alignment performed today. Checking alignment at all four wheels also makes it possible to set the thrust angle to ensure perfect wheel tracking, even on vehicles with solid rear axles. Most manufacturers rec- ommend the four-wheel alignment, even on their solid axle, rear-wheel drive vehicles. Figure 17-1 shows the diff erences between two-wheel and four-wheel alignment. Wheel Alignment Equipment In the past, satisfactory wheel alignment could be done with simple equipment. Some of this equipment is still around and can be useful for making rough adjustments to get a repaired vehicle to the alignment rack. Bubble gauges, which are used to check camber and caster, are held to the wheel hub by magnets, special clamps, or the spindle threads. See Figure 17-2. Toe can be roughly checked using trammel bars, Figure 17-3, a drive-over plate, Figure 17-4, or a tape measure. On older cars and light trucks, using this equip- ment may result in a reasonably close alignment adjustment. However, to do an accurate four-wheel alignment on a mod- ern vehicle, more elaborate equipment is needed. Note that most of the equipment discussed in the following section was also discussed in Chapter 3. Alignment Racks For any type of alignment, you must have an alignment rack with turning plates to allow the front wheels to be turned when measuring caster. Th e turning plates also allow the wheels to move outward when the vehicle is lowered aft er checking for worn parts. Th is allows the vehicle’s suspension to settle to its normal ride height. Th e alignment rack should be high enough to permit easy access to the underside of the vehicle. Some alignment racks are installed over a pit, which allows access to the suspension components of the vehicle. Th e most convenient design is to have an alignment rack installed on a hydraulic lift , so it may be lowered to drive the vehicle on, and raised to make adjustments under the vehicle. Figure 17-5 shows a modern alignment rack. Alignment Machines Th ere are many brands of alignment machines. Th ese machines diff er in some details, and operating instructions may vary. However, all modern machines contain inter- nal computers, are able to check the alignment at all four wheels, and show alignment readings on a display screen.