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Re: Sorry. Just adding an interesting tidbit of info.
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Posted by 2Slow (more from 2Slow) on Wed, 14 Nov 2001 13:27:25 Share Post by Email
In Reply to: Sorry. Just adding an interesting tidbit of info., Schu, Wed, 14 Nov 2001 12:03:06
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If you stiffen up the rear in roll you will have more oversteer, or less understeer depending upon where you staretd.

You can stiffen the rear in roll by either increasing the stiffness of the rear stabilizer bar, or increasing the stiffness of the rear springs. Both effectively increase the rear roll stiffness.

The way that roll stiffness effects under/oversteer is a bit interesting. When rounding a corner (we'll keep the radius and velocity fixed to simplify things) there will be a constant force of MV^2/R acting at the vehicle Cg. This force times the distance from the Cg to the roll axis of the vehicle creates a roll moment. The roll moment is reacted by lateral weight transfer at the wheels. What roll bars do is play the game of where the moment is reacted. (If you do not chnage Cg height, or the roll axis position (which a stab bar stiffness change will not) the roll moment is the same regardless of roll stiffness. Since the roll moment is the same, the lateral weight transfer from the left to the right to will be the same. Anti-Roll bars play the game of moving the lateral weight transfer from front to rear. If the rear had zero roll stiffness, and the fron had infinitie roll stiffness (and the chassis was infinitely stiff) all the weight transfer would happen a the front. By varying the roll stiffness front to rear you play with where the weight transfer is going to happen.

Without getting too complicated let us define cornering stiffness. (we will call that the lateral force a tire can generate vs the vertical force on it.) As we increase vertical force, the available alteral force increases, but not as much as the increase in vertical force. (The slope of the curve is positive, but hte second derivitive is neg)

If we start with two whels loaded equally 1000 ls each for example they would be able to generate a certain combined lateral force. If we than change the distibution so that one is loaded with 1500 lbs, and the other is loaded with 500, the total lateral force available will be less than the first case. This will cause that end of the vehicle to generate less overall lat force

The roll bar game is to tune the lateral force transfer from front to rear so that the vehicle is never oversteer. (This is one reason why front anti-roll bars are bigger than rear ones) So if most of the roll stiffenss is up front, most of the weight transfer will be up front, so those tires will lose grip first.

By stiffening the rear, you cause less weight transfer up front and more in the back, so that you reduce understeer. The total weight transfer from left to right does not change though.

Stiffening bushings in the control arms or links in the rear could either increase or decrease understeer. Most rear suspensions have lateral force compliance steer in the understeer direction so stiffening them up would usually cause a bit more oversteer. It could go the other way though depending upon the geometry and bushing tuning.

-Joe



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