The "completeness" or thoroughness of the job. A front-wheel alignment only aligns the front wheels. A four-wheel alignment aligns all four wheels. All vehicles can benefit from a four-wheel alignment, but it is especially important on front-wheel drive cars and minivans as well as rear-wheel drive cars that have independent rear suspensions.
Until a decade ago, two-wheel alignments were the norm for all cars and trucks. But the arrival of front-wheel drive changed all of that. It's always been important to align all four wheels to one another because the rear wheels can have just as much influence on steering as the ones up front do. But on most rear-wheel drive vehicles, there are no adjustments on the rear suspension. So even though the need to check rear wheel alignment is there, there's not much a technician can do if the rear wheels aren't true with respect to the ones up front.
If the rear axle is cocked slightly to one side or the other, it creates a "thrust angle" that causes the steering to pull slightly to one side or the other. If the misalignment cannot be corrected by repositioning the rear axle, a "thrust angle alignment" (aligning the front wheels to the imaginary angle created by the rear axle) can at least make a vehicle steer straight.
The arrival of front-wheel drive in the 1980s increased the awareness of the importance of rear-wheel alignment. Though many of the early front-wheel drive vehicles lacked much in the way of "factory" adjustments (front or rear!), aftermarket kits allow most types of alignment problems to be corrected. Most newer front-wheel drive cars and trucks now have factory adjustments for the rear suspension.
According to a recent survey by Brake & Front End magazine, a trade publication for alignment shops, 44% of all alignments jobs being performed are now four-wheel alignments. Those who are still doing two-wheel alignments say price is a big issue with consumers. They don't want to pay more for a four-wheel alignment. Others say they don't have the proper equipment or training to do four-wheel alignments.
Two-wheel alignments still appeal to many technicians because aligning two wheels doesn't take as much time as aligning four wheels, nor does it require very sophisticated alignment equipment. A toe stick and a camber/caster bubble gauge are all that's needed to do a "quickie" alignment job. But aligning only the front wheels is like asking your dentist to only clean your upper teeth. All four wheels need to be checked because the rear wheels are just as important as the ones up front -- even on rear-wheel drive vehicles with nonadjustable rear suspensions.