Join Date: May 2008
Location: Bay Area, CA
Feedback Score: 5 reviews
Okay, I'll take a stab at this...
There are four wheels on the car. In order for the car to move, each wheel needs to somehow connect to the frame of the car. We use control arms to accomplish this. There are two CA's for each wheel - an upper and a lower. All eight CA's have an attachment point to the vehicle frame.
Why do you need two CA's per wheel instead of one? It adds stability and secures the wheel as you move over uneven surfaces, or turn. This may not be the best analogy but think about pushing a wheelbarrow. Suppose it only had one handle for you to grab it, lift it, and push it forward. That might be easy if you are lifting a light load and traveling in a straight line on a perfectly flat surface.. but if you had to move a heavy load over a rough surface with lots of turns... well that would suck. Hence, you have two handles aka, "arms" to gain better, "control" of the direction of the wheel. There is also a shock/damper and spring connecting the front and rear LCAs to the frame.
The two front wheels are held to the car through a knuckle. This is basically a large piece of funky shaped metal with a big hole in the middle, and three smaller holes. In the big hole, you have a wheel bearing with a hub that has the spokes that you secure your wheel against. In the very center of the hub, you have the axle which connects to the transmission. As I said, there are also three smaller holes. Two are for the UCA and LCA, and the third is for the tie rod - which is input from the steering column.
The rear trailing arm (RTA) is basically the equivalent of the knuckle, except its' in the rear of the car, and it works a little differently. It too has three holes, as well as a large hole in the middle for the hub, which you connect your wheel against. Two of the holes are for the UCA and LCA. The third is a toe compensator for adjusting wheel toe. Since the RTA is so long, there's also a bushing in the middle. That bushing is marked inside the yellow circle in the pic above.
If you look at the first pic with the purple rear LCA's, the innermost hole has a bushing which goes into the sub frame. The outermost hole also has a bushing but goes into the RTA. The third bushing on the rear LCA's is next to this hole, and is for the rear shock.
Hope that helps!