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Old 10-19-2017, 09:03 PM   #18
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Join Date: May 2015
Location: Greater Seattle, WA
Posts: 534
Originally Posted by steved0x View Post
I was just thinking - they are rebound adjustable and not compression adjustable? Wouldn't compression control the rate of spring compression and contribute to the distance of compression?
From what the Koni double-adjustable tuning guide has to say about the topic...

"Bump [compression] damping controls the unsprung weight of the vehicle (wheels, axles, etc.). It controls the upward movement of the suspension such as hitting a bump in the track. It should not be used to control the downward movement of the vehicle when it encounters dips. Also, it should not be used to control roll or bottoming."


"The rebound damping controls the transitional roll (lean) as when entering a turn. It does not limit the total amount of roll; it does limit how fast this total roll angle is achieved. How much the vehicle actually leans is determined by other things such as spring rate, sway bars, roll center heights, etc."
I concur with what is said, but in a pinch when handling balance adjustment is needed, there is nothing faster than adjusting an accessible (pair of) knobs to change the rebound setting at one end of the vehicle. It's faster than changing springs, faster than changing swaybars. If you have adjustable swaybars where the adjustment mechanism is petty easily accessible, that would be the exception, but many cars don't provide this, or they're still a hassle to quickly adjust even when they are present, and you can still fine-tune dynamics further (e.g., refine specific handling issues only occurring at corner entry or corner exit) using adjustable shocks.

Many people use alignment settings and tire pressure changes alone to achieve the neutral handling balance adjustment, and tire pressure are certainly about as convenient and quick to change as knob-rebound adjustable shocks are. However, assuming you start out from a setting with optimal tire grip from optimal tire pressures and optimal alignment (especially camber), any tire pressure adjustment you do to deviate from that to "fix" neutral handling will give less overall grip than making an adjustment to the relative front/rear roll couple. Adjusting the roll couple will basically harness the grip of the better-gripping end of the vehicle to help out the more poorly performing end of the vehicle. It does this by reducing the lateral weight transfer at the more poorly performing end, and redirecting that lateral weight transfer to the better performing end.

This is all because reducing lateral weight transfer at one end of a vehicle increases grip, while increasing weight transfer at an end of the vehicle reduces grip.

And that's all due to an interesting property seemingly inherent to pretty much all pneumatic tires, which is the coefficient of friction of a pneumatic tire seems to be always better at lower loads, and always worse at higher loads.

So transferring weight to that outer tire when the vehicle is laterally accelerating, makes the coefficient of friction of the outer tire reduce. Although this weight transer increases the coefficient of friction of the inner tire get better, it's generally always an overall a net loss of grip across the pair of tires, since more weight is now pressing down on the more poorly performing outer tire, that poorer gripping performance of the outer tire becomes more significant than the inner tire. (There's a weighted average function at play here.)

The converse side of this coin is that reducing lateral weight transfer increases grip, so that's why being able to fine-tune the front/rear roll couple is essential to be able to fine-tune a neutral handling characteristic without sacrificing overall grip.
2001 Boxster

Last edited by jakeru; 10-20-2017 at 06:55 PM.
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