Why do cars need ball joints? Why not just have a fixed pin going up into the steering knuckle? I never understood the need for the pin to pivot in the suspension.
Because the tie rods don't always make nice right-angles where they mount to the knuckle. This is for two reasons:
1. It's not always possible to mount the steering rack so that it is centered. In fact, it's usually good to have the steering rack ahead of the front wheels to improve handling behavior and feel.
2. All cars have some amount of camber and/or caster on the front wheels. Moreover, camber changes as you steer and/or when the suspension moves up and down.
The early VW Beetles had kingpin front suspensions. In the early '60s, they switched to control arms with ball joints.
The kingpin configuration was quite durable, and the Bug was marketed in many countries where the roads weren't that great. But you sacrifice handling. I'm not sure if VW switched to ball joints in all markets, or just in the US (and probably most of Europe). American consumers demand a more comfortable ride.
I haven't seen one for a while, but I bet heavier trucks still have king pins. They are more concerned with durability than ride and handling. Also, they tend to have high sprung to unsprung weight ratios. More road hugging weight.
so the ball joints rotate as the suspension moves up and down to keep the camber the same??
The point is this: As you drive, the knuckle moves up and down, and sometimes tilts in or out, while the steering rack always stays bolted to the frame. This means that the angle between the tie rod and the knuckle changes as the suspension moves (or as you steer). A kingpin can't accommodate those changes because it always sits at a right angle. With a balljoint, though, it can work at almost any angle.
Does that make more sense?
ok so the wheel stays the same, just the relation of the tie rod to the knuckle changes. My car has a strut suspension, is it the same for doublr A-Arm as well? Also, if the ball joint moves, how come the wheels themselves don't move as a result, like change cmaber when your turn, etc.
It's all in how it's designed. Whatever camber and camber changes your suspension is supposed to have, everything is designed to prevent ill effects. For example, the tie rods are supposed to be parallel to the lower control arms at all times. This ensures that the steering angle doesn't change when the suspension compresses. If your suspension has a lot of camber or caster in it, it'll be designed with that in mind.
For the steering ball joints, keep in mind that only the angle between the tie rod and the knuckle is what changes. The length of the tie rod is always the same.
This stuff generally applies to any independent suspension. It's implemented differently but the principle is the same.