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Originally Posted by Sirbelch
How does compression and rebound effect the way a car handles on a track and how does changing both compression and rebound effect handling?
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The idea is similar to spring rates. You want the compression/rebound at a rate where you don't lose traction over bumps/rough sections in the road. Excessively high compression/rebound makes the car skittish. In other words, you want a balance. Without rebound the car would endless jump up and down. Too much rebound damping and the rebound speed is reduced to a point where it doesn't react quick enough to a rough patch, say a bump on the road.
Too much compression damping and the wheel bounces off bumps. Too little compression and the car takes a dive when hitting bumps.
Aside from bumps, where damping settings come into play in corners is the weight transition. Excessively stiff rebound dampening, for example, means the weight doesn't transfer quick enough from inside to outside, possibly lifting the inner wheel. Insufficient dampening means it transfer too quickly. Hence body roll is also in part dictated by rebound dampening. This ties into a point about roll bars, and how ppl get locked into a mentality of "stiffer is better": just because race cars have less body roll doesn't mean they're using insanely gigantic anti-roll bars. It likely has to do with their compression/rebound dampening settings as well.
The two go hand in hand--changing compression has to be compensated with rebound damping. Basically, you want a happy medium where there's just enough stiffness that the wheel won't lift, and where there's always optimal load on each tire (outside and inside).
Going back to body roll, stiffer rear sways can be deceptive--when you're not at the limit, the setup will feel more aggressive, as though it's on rails... but at the limit it could have lost traction. So even though stiffer sways could "feel better," in reality you don't know until you've thoroughly tested on the track. You could always use a smaller sway up front, which could increase front traction... as opposed to lessening rear traction with a bigger rear sway bar.
I disagree with Highrev's semantics of wanting the "softest" spring setup (although his point is still valid, and he could kick my butt on the track). You want something not too soft, not too hard. Soft enough where it can do its job of compressing, hard enough where it doesn't make the suspension hit the bumpstops on every corner. Everything is about a happy medium--and to some extent, preference.
Edit: to cover my butt, a lot of what i've said are simplifications and general rules of thumbs. Stiffer rear sways doesn't necessarily "reduce" traction as much as softer fronts don't necessarily increase traction. My point is overdoing settings always have their downsides.