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I understand your analogy now.
A baseball bat supported from the bottom (assume weight acting down) is unstable because upon any rotation from the vertical, the normal reaction force and the weight force are no longer in line, and an overturning moment is generated from this misalignment, tending to make the bat rotate away from its initial position.
If you instead support the bat from the top, then there's still a misalignment after any rotation from the vertical, but the moment generated is instead a restoring one, tending to make the bat rotate back toward its initial position.
The reason this works is because both the weight force and the normal reaction force act only in the vertical direction. This is where I find disagreement in the car analogy.
We can draw similar free body diagrams for front and rear drive cars. Look down on the car from the top, with the front of the car up. The drag force acts downward (this is why you need drive force from the wheels to maintain constant speed). The force from the drive wheels acts along the longitudinal axis of the car.
If the longitudinal axis of the car is straight up and down, then the drive force is also in the vertical axis, and the forces align (assuming left and right wheels give equal drive force), and we're balanced.
What you're saying J (I think) is that if we give the car some rotation from that up/down axis, as we did for the bat, then the force of the drive wheels will create a moment, overturning for RWD, and restoring for FWD, as happened with the bat. However, this isn't true, because when the longitudinal axis of the car is rotated from the vertical, the drive force no longer acts in a vertical line. The drive force is also rotated, such that it continues to act through the CG. There's no moment. This is unlike the bat, because in that situation, no matter how we rotate the bat, the normal force still acts in the same direction.
Stability of a car in an initially rotated position depends upon the ability of the tires to provide restoring moment due to lateral friction forces, perpendicular to the longitudinal axis of the car. Depending upon the location of your CG with respect to front/back of the car (i.e. the weight bias), the relative amount of force required to be provided from front vs. rear wheels in order to get a restoring moment varies.
If your CG is really far toward the back (old 911), then the rear tires must provide a large force, due to the small moment arm, to rotate the car back toward the vertical. The front tires could do it with a relatively smaller force (larger moment arm).
The situation is reversed for a front heavy car like a 9-3.
In either case, the weight bias is helping you a little, because more weight on the tires means you've got more capability to generate lateral force due to friction.
Assuming you're in a slippery situation, you've got minimal traction capability on all the tires. You want to have your best traction where it can most easily help you. Therefore, put your better pair of tires on the rear of your front-heavy 9-3.
Whaddya think, J?
'Roo
posted by 130.76.6...
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