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BillyShope
Automotive
- Sep 5, 2003
- 263
(In the following, I'll be referencing only a RWD live axle car, but the conclusion also applies to a RWD IRS car. I realize the no squat/no rise line, for the IRS car, is displaced upward...negative Z direction...from that for the live axle car by a distance equal to the loaded rear tire radius and that the two lines are parallel.)
When I was in school, I was taught that the no squat/no rise line, for a RWD live axle car, passed, in the rear, through the rear tire patch and, at the front, through an intersection of two other lines, one a vertical line through the front tire patch and the other a horizontal line through the center of gravity.
Later, I realized that the center of gravity referred to that for the entire car LESS the mass of the rear axle assembly. With that correction, I went happily on my way.
But, a few days ago, I encountered a small error in a CAD layout I had constructed to check out a derivation. It was a very small error and, in the days of pencil and paper, I might have simply dismissed it as a layout error. But, CAD precision forces one to admit that there might be an error elsewhere. I finally realized that the rear point defining the no squat/no rise line was in error. It does NOT pass through the rear tire patch. It passes through a point which is a distance mR/M below (positive Z direction) the tire patch, where "m" is the mass of the rear axle assembly, "M" is the remaining mass, and "R" is the loaded radius. This is because, when there is no squat or rise, the only inertial forces are horizontal, meaning the vertical loads in the suspension links are unaffected by the ratio of "m" to "M."
Yeah, it's a very small error and, for the most part, it can be ignored, but, for those who might also encounter that frustrating little error in a CAD layout, I pass on this information.
When I was in school, I was taught that the no squat/no rise line, for a RWD live axle car, passed, in the rear, through the rear tire patch and, at the front, through an intersection of two other lines, one a vertical line through the front tire patch and the other a horizontal line through the center of gravity.
Later, I realized that the center of gravity referred to that for the entire car LESS the mass of the rear axle assembly. With that correction, I went happily on my way.
But, a few days ago, I encountered a small error in a CAD layout I had constructed to check out a derivation. It was a very small error and, in the days of pencil and paper, I might have simply dismissed it as a layout error. But, CAD precision forces one to admit that there might be an error elsewhere. I finally realized that the rear point defining the no squat/no rise line was in error. It does NOT pass through the rear tire patch. It passes through a point which is a distance mR/M below (positive Z direction) the tire patch, where "m" is the mass of the rear axle assembly, "M" is the remaining mass, and "R" is the loaded radius. This is because, when there is no squat or rise, the only inertial forces are horizontal, meaning the vertical loads in the suspension links are unaffected by the ratio of "m" to "M."
Yeah, it's a very small error and, for the most part, it can be ignored, but, for those who might also encounter that frustrating little error in a CAD layout, I pass on this information.
