Polar Moment of Inertia

[RWebb]

The polar moment of inertia measures the resistance of a body to rotational acceleration about an axis. For a car, we are talking about pitch, yaw, and roll.







The basic idea is easy to grasp intuitively. A dumbbell is harder to spin than if the wt.s were placed in the middle of the rod. Another, and more aesthetically pleasing, example is to watch a figure skater as she spins with her arms out and then brings her arms in, or vice versa.


[insert pic of gorgeous ballerina babe here]

[Zeke]

I always thought of pitch, roll and yaw as orientation and not so much as energy. But there is no doubt that a car with everything packed into the middle, between the wheel base, is easier to bring back out of a lack of control. But, that assumes no aero ground effects.

That would really put the interest back in racing and driver talent to take a lot of the aero effects out of F1 and other series, AFAIC. I suppose that's why I still watch NASCAR because they drive the things, not pilot them. I don't approve of the antics, but that's off the subject.

The aerodynamics of stock car racing don't overcome the ability of the car to be driven with a little yaw.

Personally, I submit the characteristics of the 914 v. the 911, both of which I have tracked. Love the 914.

[RWebb]

You can measure it:



but not too many people have one of those in the back yard.



The other option is to calculate the polar moment:




There are also equations for a cloud of points. Those interested can plug that into their distributed parameter vehicle dynamics model.

[BReif61]

Ah yes, high school physics.

Also why the Cayman is a fundamentally better chassis than the 911.

Should I put my flame suit on now or later?

[wdfifteen]

Your description sounds like mass moment of inertia. I memory serves, polar moment of inertia is related to structural strength and mass moment of inertia is related to changes in rate of rotation.
But it's been a while since college physics. I may be wrong - again.

[M.D. Holloway]

Originally Posted by BReif61

Ah yes, high school physics.

Also why the Cayman is a fundamentally better chassis than the 911.

Should I put my flame suit on now or later?

Dang Brad! Just when I got the Cayman out of my head and rested into the idea of a 2003-2004 4S...now you have me thinking again!

[BReif61]

Originally Posted by LubeMaster77

Dang Brad! Just when I got the Cayman out of my head and rested into the idea of a 2003-2004 4S...now you have me thinking again!

I faced the same choice as you; I chose the 4S. I haven't regretted it at all.

It is really a win-win situation.

[RWebb]

Sometimes people get confused when they are planning a weight (mass) reduction program on their 911, and they only think about the weight distribution and how to change it. But, you can’t just think in terms of statics, you have to also consider dynamics.

One example on this forum involved how desirable it is to move the oil tank from behind the wheel to a position in front of the wheel. As one poster noted in that thread, it won’t change the wt. distribution much. But, it would have a greater change on the handling dynamics of the car. The oil tank is a difficult example because we really don’t know how much it weighs while the car is moving. If it is empty, it weighs 10.7 lbs. with all hardware, but no hoses, valves or the 1.2 OC-54 oil filter (at least, my 1973 tank does). If it is full of oil, add maybe 17 lbs. (10 quarts in tank and a specific gravity of ~0.84 for engine oil at 100 °C).

[rusnak]

The issue is how controllable is the vehicle at or over the limit and how much it lets you to react in a confident way. Otherwise you just drive slower, so as to avoid a nasty surprise.

[RWebb]

unless you hit some gravel or something in the road - happens all the timer here

----------------------------

The other thing is to think about what one should do to make their existing car more fun or more controllable.

Inspection of the equations in post #3 shows that in each the effect depends linearly on mass, but on the square of the spatial dimension.

Back to the oil tank...

- in terms of center of mass, you are moving ~~ 15 lbs. several inches and it is still behind the center of mass

- but think about the polar moment (comparatively speaking, to keep it simple) - the original distance from the center of gravity (Cg) is about 41" but with the tank in front of the rear wheel, that distance is ~~ 6"

so, 41^2 = 1,681

vs.

6^2 = 36

... a big difference.

And it gives some insight into why Porsche kept trying to move the oil tank in front of the rear wheel.

[john70t]

Having weight on the rear drive wheels increases traction, and the light front end increases toss-ability, while driver confidence is improved knowing he has to focus on what only one axle is doing.

All the mass in the center is not always best in all situations:
Remember the SWB early cars were lengthened for purpose, and the center-engine WW2 fighter P-39 had a nasty reputation for easily falling into permanent flat spins.

[rusnak]

My 914-6 is a good example of low pmi. It will feel balanced because it is, and you can pull almost 1G in a 4 wheel drift. But push it beyond it's limit and it will spin so fast you might not catch it. I had to react blindly to a spin induced under braking to avoid a deer. I sprained two fingers and my thumb in the process of catching the spin. The 911 never spins like a 914 does.

[RWebb]

yup - part of that is the longer wheelbase; Porsche went from a SWB 911 to the LWB partly for that reason

[M.D. Holloway]

[wdfifteen]

Originally Posted by john70t

All the mass in the center is not always best in all situations:
Remember the SWB early cars were lengthened for purpose, and the center-engine WW2 fighter P-39 had a nasty reputation for easily falling into permanent flat spins.

For directional stability the center of mass needs to be ahead of the center of aerodynamic pressure (that's why arrows have feathers at the BACK end). The P-39's problem was the center of mass was too near the center of pressure.

[kach22i]

I deal with this topic on my hovercraft and with that the center of lift should be ahead of the center of gravity, else a nose dive and end over end flipping could result upon power loss.

With the center of gravity aft of the center of lift, the tail comes down first just as with aircraft making a landing.

I read this recently - Aside:

Our own car designs - Page 2 - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

Originally Posted by HydroJim

One could argue that reverse trikes are in fact more stable when corning since the rear wheel stays planted while on a 4 wheel car, the trailing wheel has a tendency to come off the ground. On a trike, the car is trying to flip itself over the front tire in a turn. This means, that the turn would have to generate enough force to lift the vehicle up and over the axis of the front wheel. On a 4 wheeled vehicle, the force generated by the turn only has to be enough to roll the car over onto it's side. As long as the trike is properly designed, a trike is just as stable as a 4 wheeled vehicle in a turn. The only negative of trikes is for the back end to break loose, but with less weight in the back, this effect in somewhat nullified. If you're taking turns in normal driving fast enough to cause the back end to break loose, you need to reevaluate your driving and not your vehicle.

Again, with proper design and weight proportions, a trike can out handle a 4 wheel drive vehicle any day.