Excellent Article Explaining "Polar Moment of Inertia"
11-14-2000, 02:15 PM
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My point is that I think his conclusion is incorrect, not just that his explanation was too simplified. I think a rear engine car is more agile (assuming agile is the ability to quickly change direction) than a mid engine car (given everything else is equal).
I've been pondering this point since considering buying the S2000. One article stated the the S2000 had a very low polar moment of inertia, and I wasn't sure if that is exactly true.
I've been pondering this point since considering buying the S2000. One article stated the the S2000 had a very low polar moment of inertia, and I wasn't sure if that is exactly true.
11-14-2000, 03:37 PM
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Sorry, pre-mature reply. I'm trying to be very specific. I'm not trying to compare a 911 to an S2000 (I own one and find it very nimble compared to any other car I've been able to drive hard). I mentioned the 911 because I now that there are more than a few 911 owners on this board. My point is: on paper, a rear engine car is will be more agile because the mass is closer to the point of rotation. I noticed that the article states that the mid engine car's engine is closer to the "center of rotation", which is WRONG. The engine of a rear engine car is closer to the center of rotation. He is getting "center of rotation" confused with either geometric center, or center of gravity.
And I agree that this is a very small aspect of the overall handling of a car.
And I agree that this is a very small aspect of the overall handling of a car.
11-14-2000, 05:47 PM
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Back to the ice skater I also think thats a bad analogy just because a skater can bring their mass closer to the axis, whereas the car cannot, the car is easier to spin but also easier to stop spinning.
A fun thing to do is grab two gallon jugs of water and sit in a rotating chair and spin with the jugs held at arms legnth, when you're done getting yourself going bring the jugs in towards the center point, you will accelerate like the skaters. What happens is that since it takes less energy to spin the same amount of mass that is closer to the axis, if you put x amount of energy into it with the mass towards the outer point then when you bring them in you will accelerate to the rate that you would get if you applied x amount of energy to the inner most position.
whats the point? I dunno.
Just means this car is cool, Im glad I was awake during physics.
I like roast beef. . .
A fun thing to do is grab two gallon jugs of water and sit in a rotating chair and spin with the jugs held at arms legnth, when you're done getting yourself going bring the jugs in towards the center point, you will accelerate like the skaters. What happens is that since it takes less energy to spin the same amount of mass that is closer to the axis, if you put x amount of energy into it with the mass towards the outer point then when you bring them in you will accelerate to the rate that you would get if you applied x amount of energy to the inner most position.
whats the point? I dunno.
Just means this car is cool, Im glad I was awake during physics.
I like roast beef. . .
11-14-2000, 09:03 PM
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I'm just trying to reply to the authors implication that a mid engine car would handle better because of it's low moment of inertial about its center of rotation (I think he really means the center of gravity, the only way to get the center of rotation of a car near the geometric center of the car is to have 4wheel steering, or take the car into space and spin it). Number one: that is wrong, a rear engine car has it's engine closer to the (real, not his idea) center of rotation. Number two: having the lowest moment about the axis of rotation is more important than the center of gravity or geometric center.
I do beleive that for the overall real world handling characteristics a 50/50 weight distribution is most important, and a mid engine car can acheive that. I guess I AM trying to simplify it as much as possible, because optimum is often to simple to realistically achieve. Let me first give my definition of agility: the ability to quickly change direction, not the ability to hold a turn. Now back to the car that can turn its fron wheels 90 deg. Put 100% if its weight on a point on the center of its rear axle, turn the front wheels 90 deg and spin the car about it's rear axle. Now move the point of weight to the geometric center of the car (and keeping the wheels in the same positions) and spin it about it's rear axle. It will clearly take a higher torque to spin the car. Now to my point on the ability to hold a turn: I'm still grappling with your comment on during a constant turn (no acceleration, constant turn radius) that there is a slight angular acceleration. IF the angular velocity is constant (constant turn radius), than the angular acceleration is 0 and the moment of inertia doesn't matter. Now I must ponder if the angular acceleration truely is 0.
I'm not sure why you're using a moving coordinate system. Sure, if the coordinate system moved with the car, the car will rotate about it's CG, but wouldn't that coordinate system have to be transformed to a fixed coordinate system, because the the tires of the car is acting upon a fixed system?
Your missiles may get to outer space, but my missiles can fly up canyons, over ridges, and split a field goal (American Football) upright (OK, maybe a slight exageration) more than a few tank fulls (S2K tanks) away, and I investigate the designed flight software when it doesn't.
I do beleive that for the overall real world handling characteristics a 50/50 weight distribution is most important, and a mid engine car can acheive that. I guess I AM trying to simplify it as much as possible, because optimum is often to simple to realistically achieve. Let me first give my definition of agility: the ability to quickly change direction, not the ability to hold a turn. Now back to the car that can turn its fron wheels 90 deg. Put 100% if its weight on a point on the center of its rear axle, turn the front wheels 90 deg and spin the car about it's rear axle. Now move the point of weight to the geometric center of the car (and keeping the wheels in the same positions) and spin it about it's rear axle. It will clearly take a higher torque to spin the car. Now to my point on the ability to hold a turn: I'm still grappling with your comment on during a constant turn (no acceleration, constant turn radius) that there is a slight angular acceleration. IF the angular velocity is constant (constant turn radius), than the angular acceleration is 0 and the moment of inertia doesn't matter. Now I must ponder if the angular acceleration truely is 0.
I'm not sure why you're using a moving coordinate system. Sure, if the coordinate system moved with the car, the car will rotate about it's CG, but wouldn't that coordinate system have to be transformed to a fixed coordinate system, because the the tires of the car is acting upon a fixed system?
Your missiles may get to outer space, but my missiles can fly up canyons, over ridges, and split a field goal (American Football) upright (OK, maybe a slight exageration) more than a few tank fulls (S2K tanks) away, and I investigate the designed flight software when it doesn't.