If they gave you a printout of the alignment, look at the caster readings and you'll see they are way too low. Ford is famous for building vehicles that can not be properly aligned but the Aerostar DOES have adjustable caster but it is almost impossible to set correctly. There are two places on each side to install shims. To adjust caster, a shim must be removed from one location and / or added to the other location. Since those two locations are so close together, changing one of the thinnest shims causes a huge change in caster. The actual value isn't critical. What's important is the readings are the same on each side to avoid a pull.
Most alignment mechanics become extremely frustrated running back and forth to both sides making changes, and the four nuts to loosen to change the shims are real hard to get to even with the special tools the alignment industry developed just for these vans, that they finally give up when they get it close enough. No doubt your alignment mechanic was ecstatic and relieved when the van went straight on a test drive that he overlooked the poor steering wheel return. He will have to realign it and increase caster by two to three degrees.
There are five ways to describe what caster does. The easiest one to understand is to know that positive caster pushes the wheel down when you turn that wheel out. Since it can't really go down into the ground, it raises that corner of the car. That means you are actually lifting that corner with the steering wheel. The wheel that is turning in goes up so that corner of the car goes down. The two corners want to balance each other out. That's what makes the steering system return to center by itself. When caster is very low, there isn't much difference from side-to-side when you turn one way or the other so there isn't musch force acting on the steering system after you turn. That makes for a very unstable vehicle that tends to wander on straight roads. Caster was increased on all vehicles to increase stability in the 1960s as we started driving faster. That was the main reason power steering became so common. It was needed to overcome the harder steering. Most older cars called for negative caster which allowed us to steer them easily without power steering, but the wander wasn't very noticeable when we drove 40 -45 mph on the highway.
Wednesday, May 18th, 2011 AT 5:02 PM