


This is becoming VERY interesting! Doug referred me to Olley's roll steer discussion, beginning on page 169 of the Millikens' Chassis Design. He explains roll steer on the basis of roll axis slope AND on the basis of instant center relationship to the axle. If you have the book, I'm going to consider the 4link picture on page 179, but with the lower links parallel in plan view. This would provide roll oversteer, according to Olley. But, I'm going to consider the instant center well below the axle centerline, which...according to Olley's torque arm explanation...would cause roll UNDERsteer. Then, I'm going to attempt to add the effects, using, if necessary, CAD to validate my conclusions.
http://www.racetec.cc/shope

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There are two effects (causes). It would appear that the roll axis effect would be the larger, though the instant center effect is certainly significant. The linkage programs used by today's suspension design engineer would catch it, but, for those of us who must use cruder tools, it's nice to know. Here's an example:
The car has a rear roll axis that is inclined 0.349 radians up from the rear (roll oversteer). The roll angle is 0.0333 radians. This would mean that, in a left hand turn, the axle would rotate clockwise 0.349 times 0.0333 or 0.0115 radians The instant center is 60 inches from the axle centerline and is 10 inches below the axle centerline. The angle of the equivalent torque arm would be 0.1675 radians angled down from the rear (roll understeer). The spacing between left and right torque arms is 45 inches. The change in angle of the torque arm would be determined by the product of the roll angle and the ratio of half the arm spacing to the arm length. axle rotation = P{cos (B  S(phi)/(2P))  cos (B + S(phi)/(2P))}  S where "P" is the torque arm length, "B" is the radian measure of the orignal torque arm position, "S" is the lateral spacing of the torque arms, and "phi" is the roll angle. The "S" and the fraction line should be shifted to the right of the equals sign, of course. This comes out to a counterclockwise axle rotation of 0.00555 radians. http://www.racetec.cc/shope 


So would the results from both equations apply and be additive? I really have wanted to get Olley's but haven't yet. Maybe this will prompt me to get it.
The problem is the equations only work with small angles of roll. I am working on something right now that can get 25.5deg. of pure roll and a max axle to chassis angle of 27.9deg. (it has a little more droop travel than bump). At 25.5deg. of roll it has 0.14deg. of axle steer in the roll oversteer direction. At ride height it has a roll axis very slightly on the side of roll understeer but does have the IC higher than the tire radius with slightly triangulated lower links. Next I need to design an antiroll bar that will stand up to that much roll angle and not break not to mention handle the 24+ inches of wheel travel. Then sometime I need to figure out where your antisquat equation and mine don't mesh... I don't see it. 




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http://www.racetec.cc/shope 


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You definitely need a linkage program. Sounds like a fun project, but I've too many others in progress. Plus, I suffered a TIA a week ago, which adds urgency to my work. My ancient version of SolidWorks might be able to handle a 3link assembly. If I get a chance, I might give it a try. http://www.racetec.cc/shope 

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