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Alright, here it is, in a nutsack, er, nutshell. A big nutshell, but a nutshell nonetheless.
Simply put, understeer is when the front of the car hits the wall, and oversteer is when the back of the car hits the wall.
What creates this bias? Grip. If the front tyres lose grip before the rears, then the car will understeer. If the rear tyres lose grip first, you look like a new autocrosser who just bought a 911.
Great, but what determines how much grip a tyre has? Well, tyre compound is one factor. Contact patch is the other. The point of those arms is to move the suspension in a way that keeps the tyre perpendicular to the road. This lets one use the maximum grip of the tyre.
So, the suspension arms work, but what else is there? Bushing deflection. Wonder why some people put those pretty red bushings in their suspension? Because the floppy stock rubber bushings allow the arm to move in a way that it isn't supposed to. It creates undampened play. Also, when the tyre hits a bump, it's translated to the pivot point of the arm at both ends. With the lower lateral arm, there is the end by the wheel (a ball joint) which should have no flex in it (but there is a recall to fix it if you do!). Because there is no possibility for extraneous movement, this isn't a big deal. Then, there is a rubber dampened bolt connecting this arm to the crossmember. In order for the suspension to compress, this rubber bushing must also compress. Lastly, where the shock fork meets the arm, there is a rubber dampened bolt. Before the spring and shock can compress, the rubber bushings at the top and bottom of the shock must compress. Know what the spring rate of a stock rubber bushing is? Me neither. That's why they get shitcanned, it's another aspect of unpredictability.
Now, we have the arms working as they should... what happens if the car hits a bump? The suspension must compress, else the car will develop upward momentum. This wouldn't be a problem, but what happens when you finish going over the bump? The car must lose it's upward momentum, and it winds up being off of the ground for a moment. Tyres don't grip air very well. So, in addition to compressive dampening, it must be taken into consideration what the suspension does as it is being unloaded. The shocks and springs must keep the tyre in contact with the ground as much as possible in any condition. The front of a DSM, however, is much heavier, so it needs more dampening and a higher spring rate. If the shocks and springs are poorly matched to the car, things like bump-steer become a problem.
The weight dynamics of a car also play a role. A tyre that will provide 1G of grip on a 2000 lb car will provide signifiganly less then that on a 4000 lb car. Therefore, with everything else in the suspension being equal, the weight distribution of a DSM would cause understeer.
Any flex that occurs will change the angle that the tyre is at, and make it less perpendicular to the road. This can include chassis flex, bushing deflection, failing wheel bearings, failing ball joints, and anything that has the potential to move in a different direction then normal.
Now, back to your original question, shock tower bars (yes, shocks, not struts!) keep the tops of the shock towers at an equal distance from each other, preventing inward and outward flex, which creates havoc with dynamic camber. Lower tie bars, however, go on the bottom of the suspension, and tie the lower control arms together. This keeps the lower control arms equidistant from each other. With a solid STB and LTB, your suspension will remain parallel.
Sway bars are something else, altogether. They act like a spring, only when one side is under bump. A sway bar is meant to allow both sides to bump together, like going over a speed bump. If one side is under compression, and the other side isn't, the sway bar provides resistance. This comes into play on a corner, when the outside suspension is loaded, and the inside suspension is unloaded. The resistance of the sway bar tries to push some of the compressive force to the other side, and even the load. Stiff sway bars can make a car corner much better without the added harshness of stiffer springs.
Wow, that was quite a novel. My apologies for going off on a bunch of tangents. I think that they were necessary to make the whole picture come together, though.
Here are the cliff notes: Do you want to learn about what suspension parts do? Read my fucking post.
-=B-= Dude, Boeing called. They want their wing back.
Dark0ne95: There is a butthole on that girl that his going to feel the wrath of 23 yeras of worldwide hate.
Me: Can I put that in my sig?
Dark0ne95: GO right fucking ahead.
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