Alright somebody explain:

1) What is polar moment of inertia as it relates to the handling of a car

2) What is polar moment of inertia's relationship to a car's static weight distribution

3) What is polar moment of inertia's relationship to a car's wheelbase

1/22/2006 6:35:30 PM

I wish I could get paid money to answer questions like these...
Ahmet

1/22/2006 6:48:17 PM

polar moment of inertia is basically how difficult it is for something to to be moved in an arc (i.e., rotated around an axis). it's a function of mass and length of axis of rotation.

think of a baseball bat: if you choke up or get a lighter bat, it's easier to swing.

with cars, this has to do with how quickly the car can change direction (i.e., initial turn-in and transition). lower polar moment of inertia=better.

static weight distribution doesn't really directly affect it. having a shorter wheelbase would be beneficial. the principle way manufacturers reduce polar moment, though, is by centralizing mass (i.e., mounting a front-mounted engine as far back towards the firewall as possible. an RX-7 is a perfect example of this. For that matter, a Miata is a pretty good example. This, of course, is the primary handling advantage of a mid-engined car.

[Edited on January 22, 2006 at 7:08 PM. Reason : asdf]

1/22/2006 7:07:35 PM

Cornell experimented with increasing the polar moment on their 05 FSAE car. They hypothesized that polar moment was less important for handling than the roll moment, so they moved the weight in from the sides towards the center and front/rear. They did about as well as usual, but I'm not sure they found anything useful from that. The moment mostly involves the mass position in relation to the CG, not necessarily static weight distribution or wheelbase.

1/22/2006 9:26:08 PM

So having a low polar moment of inertia would explain why like CRX's are pretty good at slaloms etc. , even though static weight distribution isn't all that great?

does static weight distribution really matter much for handling then?

When you say a "low" polar moment of inertia... low as in what units? What is it measured in?


[Edited on January 22, 2006 at 9:47 PM. Reason : low?]

1/22/2006 9:45:20 PM

length^4

1/22/2006 9:49:26 PM

it seems intuitively correct that roll moment (related to suspension geometry and center of mass) has more of an effect on handling that the polar moment of inertia of the entire car about a vertical axis.

1/23/2006 2:19:07 AM

Maybe, maybe not. With a low roll moment (really only affected by cg and weight placement about the cg, not by suspension geometry) and high polar moment, the car would take a set faster. With a lower polar moment, the car might take longer to take a set on the suspension (weight transfer), but it will rotate faster. Having a lower polar moment has been the accepted approach for quite sometime in FSAE (as much mass centralization as possible), with Cornell just this past year going out on a limb experimenting with lowering the roll moment (not sure what happens with the pitch axis) and intentionally increasing the polar moment. Each approach has its merits, and I can't say if one is better than the other or not.

1/23/2006 7:34:44 AM

the roll moment arm is the distance from the cg and the roll center (which IS determined by suspension geomertry)

1/23/2006 10:02:01 AM

The weight transfer in roll depends on the moment arm between the CG and the roll centers, you are right. This is not the moment of inertia in roll of the car, though, which has nothing to do with suspension at all. The moment of inertia in yaw (polar moment of inertia) is more important to handling than the moment of inertia in roll.

Race Car Vehicle Dynamics or any Carrol Smith book (Tune to Win is a good one for suspension geometry) would be good references to answer these types of questions.

1/23/2006 10:57:00 AM

you said:

Quote :

"They hypothesized that polar moment was less important for handling than the roll moment"

i said:

Quote :

"it seems intuitively correct that roll moment has more of an effect on handling that the polar moment of inertia of the entire car about a vertical axis."

i was basically just agreeing with their hypothesis

1/23/2006 11:26:43 AM

Ok, if you want to. I wasn't really arguing if you agreed or not.

The moments of inertia have nothing to do with the suspension and everything to do with mass placement. I was just correcting what you said about the suspension affecting the moment of inertia in roll.

1/23/2006 2:13:09 PM

i didnt say the suspension affected the moment of inertia

i said that (without calculation), i would think that the suspension geometry (roll center location w/ respect to cg) affected handling more than the rotational moment of inertia about a vertical axis

1/23/2006 3:05:57 PM

^ I agree, but that's not what you said the first time

Quote :

"that roll moment (related to suspension geometry and center of mass) has more of an effect on handling that the polar moment of inertia"

Roll moment has nothing to do with suspension. I guess that's not what you meant...

1/23/2006 3:16:01 PM

the roll center is determined by the suspension geometry

the arm of the roll moment is the distance b/w the roll center and the cg

that is what i meant.

1/23/2006 3:26:57 PM

I'm not talking about the same roll moment as you. I'm referring to the moment of inertia of the car in the roll axis, which doesn't have anything to do with suspension design. I see what you mean, I just didn't want others to get confused.

1/23/2006 9:38:31 PM

1/24/2006 12:17:04 AM

So polar moment of inertia is a function of what factors? Mass placement while the car is rotating? I'm still trying to understand the relationship between static weight distribution and polar moment. It seems like they aren't completely unrelated.

1/24/2006 10:11:43 AM

Polar moment of inertia or the moment of inertia in yaw are the same thing, and refer to the moment of inertia about a vertical axis through the CG of the car. It affects how quickly the car is able to rotate through a corner. A higher moment of inertia will give more resistance to rotation (rotation will take longer), while a lower moment of inertia will have less resistance (it will rotate quicker).

Polar moment of inertia and mass placement/static weight distribution aren't totally related, but as you said aren't unrelated either. There's not really any way to calculate the moments of inertia based only on weight distribution. There are a few ways to measure the moments of inertia, but they involve swinging the car from the ceiling and calculating the frequencies of oscillation in the three axes (roll, pitch, and yaw).

1/24/2006 10:35:22 AM