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Old 08-21-2012, 07:12 AM   #36
Norm Peterson
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Originally Posted by Sleepy View Post
Is it true some of these anti effects causes undesirable behaviors? I have heard of WRX understeers on power during corner exit because of anti-dive. I don't really understand how they are related but Whiteline has a kit on the market to eliminate anti-dive for that car.
On any end of a car that provides both acceleration and braking there are two 'anti' effects, not just one. One deals with geometric resistance to suspension movement under braking, the other during acceleration. Unfortunately, in what I'll call "simple" suspension arrangements*** it is not possible to tweak one without affecting the other, and it is common for improvements sought by increasing one of the anti's to adversely affect the other one on that axle.

Up front, anti-dive is normally present for various reasons, and this forces some of the forward load transfer to go through the front suspension linkage directly. This fraction of the forward LT does not cause or contribute to nose dive, and happens almost instantaneously (does not have to wait for springs to compress or damper pistons to reach some velocity). The flip side is "anti-rise" - anti-dive's sometimes evil twin - and when the force at the contact patch is for acceleration rather than braking, some portion of the vertical tire loading is yanked away without the suspension being allowed to extend to fully compensate. This obviously lowers the total amount of front grip available, and then you're going ahead to use some of what's left for longitudinal acceleration so lateral grip suffers . . . you'd then expect that by removing some of the anti-dive you'd reduce the power-on understeer by the difference in the geometric effect.

FWIW, the same sort of thing goes on at the rear axle, with anti-squat having its evil twin in anti-lift. Most people who have autocrossed certain years of LS-powered 4th Gens know more about brake hop than they probably cared to, and anti-lift is part of that. You still have both of these anti's with your IRS, but it's unlikely that a 5th Gen is going to get into brake hop.

*** You'll find some "not-so-simple" suspensions, where the anti effects can be separated and more or less tuned separately. Mostly you'll see this in places like dirt track racing, although Unbalanced Engineering at least used to offer a "decoupled torque arm" for the 4th gen cars. There is some cost in both mechanical and setup complexity, and there may be other compromises as well.

Mark Ortiz sends out a free "Chassis Newsletter" on roughly a monthly basis that covers topics such as this.

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