As most of you are aware, the RX-8 has been touted for having a low yaw moment. What exactly is a yaw moment? Is Mazda talking about the moment about the axis the car creates when it spins? Mathematically speaking, does yaw moment follow the form M[eff]=Ia, where I is the moment of inertia, and a, which is actually the Greek symbol Alpha, is the angular acceleration? Using tools I learned in taking mechanics, I assume the low yaw moment is caused by the car's center of gravity being much lower than most cars. That is to say, M[eff]=Fd, where F is the weight (I think), and d is the distance between the center of gravity and the level surface. So I suppose the M[eff] goes to Fd=Ia. Now, is the yaw moment measured when the car is spinning about its vertical axis? I think I asked this question twice, but only to try to make sense.

 

When ice skaters spin with their arms out, they spin slowly, then as they bring their arms in and they spin faster until they bring their arms over their heads in line with the axis of rotation along their body and spinf really fast.

Pop the hood and look at the engine and notice where it's located in relation to the front axle. Now crawl underneath the car and notice where the fuel tank is located in relations to the rear axle.

Relate that back to the spinning ice skater example.

 

Yaw is rotation along the axis that is perpendicular to the ground shooting into space.

 

I remember the relatio from my physics book. So i suppose the yaw moment is in only one direction (perpendicular to the surface). Actually, I probably just answered my own question. Thank you guys for your help.

 

yep, just like an ice skater...

toyota celicas have their mufflers before the rear 'axle' to reduce polar momentum (yaw). Our engines are pratically in the transmission tunnel, and the fuel tank is inside the rear axle as are all 4 seats. think of the center of the car as a fulcrum, and every atom of mass acting on it's own lever. longer the lever (farther from center), greater the torque applied to the 'fulcrum' thereby creating yaw. that's why low overhang is so touted amongst sports cars, and why most FWD (and most AWD cars too) cars are handicapped by design, as most always the entire motor sits in front of the front axle.

the same holds true on the roll axis (front to rear of the car), which is why a lower center of gravity is better.

 

I have noticed the position of the engine. It is really in there, which would account for its lower yaw moment.

 

The next time you go shopping, pile all the heavy stuff (cubes of Mountain Dew) in the front of the shopping cart... and try taking the corners really quick. That's a typical car.

Then pile all the heavy stuff in the center/rear of the cart and try again. That's the RX-8

 

Next time you go to the gym, look at a bare barbell. It's 45 lbs.

Then look at a dumbell which is also 45 lbs.

You can hold either one in your hand and lift it off the ground. But the dumbell is going to be vastly easier to rotate back and forth. So when you're talking about low yaw moment in a car, you're talking about the ability to change directions quickly.

 

The beauty of this car is the placement of the engine AND the placement of heavy item on the opposite end, the gas tank. Man talk about good engineering.

 

After reading those replies, I don't think anyone answered my question. There were some decent analogies (the ice skater and the weights), but no one really derived my set of equations. Oh well, that's okay. I just wanted to clarify what yaw moment was. Thanks guys .

 

Actually they did. Your questions were:

1. What exactly is a yaw moment?
2. Is Mazda talking about the moment about the axis the car creates when it spins?
3. Mathematically speaking, does yaw moment follow the form M[eff]=Ia, where I is the moment of inertia, and a, which is actually the Greek symbol Alpha, is the angular acceleration?
4. Now, is the yaw moment measured when the car is spinning about its vertical axis?

I'd say answering three out of four isn't bad.
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I will tell you with a straight face, after fives out in the "real world" as an engineer, equations are meaningless. Worry more about the overall concept such as, kinetic energy quadruples as velocity doubles. Same thing with drag. Ever since I left school, I have never worked on a problem that can be solved easily with a closed solution.

 

Besides most of us have been out of school so long, these things are kinda fussy:D

 

Now the truth comes out.

 

Math=life :D.

 

But thanks anyways for helping me out. I think it's better to learn about concepts rather than chat about whether Brand X wheel looks good or if coilovers are useful. It's pretty obvious I'm not done with school, since all I talk about is applying math to real world situations.

 

So let's take this to the real world.
The RX8 handles more closely to a formula racing car, than any street car I've ever sat in. I've raced Formula Fords and Formula Atlantics, off and on, for nearly thirty years.
The big difference in formula style cars and most street cars is the Polar Moment of Inertia. It's very low on formula and sports-racing cars because all of the mass is concentrated as close to the center of the car, as possible, with as little weight as possible at the ends.
The Porsche 944 is the exact opposite of the RX8, with the engine at the front of the car and the transaxle, gas tank and heavy back window at the far rear of the car. This means the momentum to not rotate for a turn is high. It also means that the momentum to CONTINUE rotating is high. This is known as a high polar moment design.
With the RX8 the polar moment is low which means it will rotate quite easily and quickly upon steering input, but also will STOP rotating easily. This explains the rabbit quick handling of the RX8 and the ability to trail off the throttle entering or in a turn and then catch the slide so easily. I can literally pitch the car to any angle I want, then nail the throttle at the desired angle and the bite of the rear tires will "catch" the car, wherever I want it.
Seldom do I need to counter steer to catch it, and when I do, I've screwed it up.
Keep the same horsepower and cut the weight to one third and you know what it's like to drive a Formula Atlantic car. Now put some of those HUGE gummy slicks on your RX8!!!!!
Both cars also have the rotating axis of the car right about the top of the drivers head so the driver is not being slung around by the car's rotation.
If you've ever driven a 'vette or similar front engined car where you sit almost over the rear axle, you'll know what I'm talking about.