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For the Techies-Polar moment of Inertia

648 views 9 replies 8 participants last post by  jtfrankl  
#1 ·
Awhile back, when the MR2 spyder was coming out, I read an article that talked about this. It seemed like a great piece of information when comparing the handling of two cars. They were able to say the PMI was X% better in one car vs. the other. Thus one car would be much more fun to drive because the car would be able to turn faster and handle better. Since this subject was mentioned in an article from a European car mag, and I have never seen US mags talk about this measure, I am just wondering why? and can you find this info anywhere for production cars?
 
#2 ·
I'm not familiar with that term but I would assume it means where the center of gravity is. The mr2 sypder is a midengine sportscar, that means all of the weight is in the middle, and therefore it pivots better. Midengine cars are always a blast to drive.
 
#3 ·
Yeah, I've heard the same thing about mid engine cars but, the 350Z is supposedly going to have a 48/52 weight distribution, would that give it the kind of feel that we're talking about here (aka the MR2's mid-engine pivot?)
Mike DiaZ
 
#4 ·
Definition I found

Rather than some scientific explanation, let’s look at an example that illustrates the concept. Picture your car as a set of barbells with 50 pounds of weights at each end. Picture trying to quickly turn and stop the bar bell while holding it. Because of momentum, it’s hard to turn, and hard to stop turning once it starts. Now, move the weights in close to the center of the bar and try it again. The bar turns, and stops turning, much more easily because the weights have less distance to travel. Reduced polar moment of inertia makes a car feel like it "wants" to change direction. The car feels better and the suspension is easier to tune.
 
#5 ·
Re: Definition I found

mike diaZ, what cobalt is saying is, the 350z will have nowhere near the feel of the MR2 spyder. sure the weight distribution (48/52) is pretty good, but the weight isnt all in the center of the car like it is in the Mr2.
 
#6 ·
The 350 Z will be a front mounted mid engine. Close. But I would still prefer the Mid rear. The reason is basically from your perspective you turn a lot faster with the rear engine, since all the weight moves around that point, including you.
 
#8 ·
Re: Definition I found

the weight distro of hte 350z IS in fact 52/48... so that when the car is accelerating out of a corner, it has perfect 50/50 bias.

and btw, mid-rear engine cars have about 40/60 weight biases, not 50/50
 
#9 ·
Re: Definition I found

The most widely used example for this is the spinning ice skater. As you see sometimes when the skater is spinning, she will spin faster as she brings her arms closer to her body and spin slower when she extends her arm. So if you have the more of the weight closer to the pivot, the object will be easier to turn.
 
#10 ·
Re: Definition I found

M D Geist wrote:
>
> So if you
> have the more of the weight closer to the pivot, the object
> will be easier to turn.

The corollary to this is that the object is also less stable.
Mid engine cars have good handling but things can get out of hand very quickly.
 
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