Polar moment of inertia

[Petrus]

The other side of the same coin.
Nów I understand why the effect of heavier wheels (17s) felt so wrong.

Yes, spot on; is exactly the same thing.

[AdamR28]

Less weight = more better. Easy

[Petrus]

Less weight = more better. Easy

and with the square of the distance to the CM more to the power of more better ;-)

[Ardent]

Short reply. Wow.

longer reply.
Caveat(s) first.
1, I am still getting used to my new tyres. They are not as stiff sided walled as the Yokos. (no surprise there)
2, Weather and prevailing conditions along with rural sludge covered roads. Not the best test parameters. Or are they?

1st things 1st. Everything you have said in this thread relating to weight, handling and the square rule is spot on!
So taking on board the suggested test suggestion, I removed the fold away chair and jump leads and replaced those with 50KG of weight plates.
You cannot view this attachment.
I then went, for what I call my short route joy ride, (26m) of rural roads; many changes of direction, bends, elevations and cambers. Sabine was not happy and I understood why. Not good.
I returned home, removed the added weight and the Spare and tool kit. (15kg)
You cannot view this attachment.
Now I have been joy riding with the spare, tool kit and a set jump leads for at least 7 years so the reference point is dialed in.
So for the first time have ran without the spare and kit.
Went back out and done the same 26 miles.
Flipping eck.
Might "only" be 15kg less than normal, but, Ye gods, what a difference.
Everything dialed in sooner and easier. Lines were held tighter. (easier)
left-right-left-right-right-left direction changes along with negative camber bends all taken (easier).
Came back. Parked and just sat for a moment.
Wow. What a difference, in less than ideal conditions.
All done at road legal speeds and more often below. But the sense of change was more than one would naturally expect.
The square of the difference is what makes it so much more.

Edit
The 50kg that was added and then removed, was put to good use elsewhere and was married to another 50kg +5kg and used here.
https://1drv.ms/v/s!AjKQRUlKMbdSgfQaNOFDnaJi6RdKTQ?e=F9LuoY

[Petrus]

The square of the difference is what makes it so much more.

Thanks for trying and the write up!

It indeed seems out of whack for relatively little weight but it ís real.

Now for us humans the sense of chánge of direction is most marked. For the car however, or more precisely the týres, mid corner which seems to ús constant is a change of direction; they ar pushing the car as much sideways as when pushing it from left to right p.e.
Reducing the pmi makes life easier on the tires under the same cornering conditions or in other words, shifts the limit to a higher corner speed / tighter radius.

How it affects ´yoú´; the individual person driving his/her car? Depends on what you like/ want etcetera.
Míne per example is probably most unpleasantly skittish to drive for many.
A good understanding however rarely go to waste.

[Beachbum957]

replaced those with 50KG of weight plates.

When you have a car that has approximately 950 lb (430kg) on the front wheels, 110 lb (50 kg) is over a 10% change and should have a significant impact.  I have driven our MR2 with about 100lb in the frunk (grass seed) and it was a pig.  We removed the full size spare on a 2003 and replaced with a bicycle spare and that made a slight change, but not very noticeable. 

We run with a spare because of the back roads we tend to travel, and the few times I have had a flat, a repair kit and compressor would not have done the job.  They won't fix a bent wheel.

[Petrus]

replaced those with 50KG of weight plates.

When you have a car that has approximately 950 lb (430kg) on the front wheels, 110 lb (50 kg) is over a 10% change and should have a significant impact.

Ofcoúrse it should: Point in case is that it has to the quare of the distance to center of rotation móre effect than that.

The PMI of the whole car is most likely under 1.000 kgm* (because the center of gravity is pretty close to the center of rotation) so adding lightness at the far ends has out of whack effect because of the square root of the distance the center of movement.

When coming back into the valley I was a bit too early too far in, in effect over the dotted line and ofcourse then and there a car comes up from the opposite direction. Só easy and quick; a doddle to put back on the right track. The (bucket-n-stuff out + p.a.s. out + crash bar out + lightweight washer bladder) x distance to CM squared = awesomely quick responding.

* I am rather surprised the value is not out there on the interweb.

[Joesson]

When coming back into the valley I was a bit too early too far in, in effect over the dotted line and ofcourse then and there a car comes up from the opposite direction. Só easy and quick; a doddle to put back on the right track. The (bucket-n-stuff out + p.a.s. out + crash bar out + lightweight washer bladder) x distance to CM squared = awesomely quick responding.

Response time hopefully less time than saying it!

[Petrus]

Response time hopefully less time than saying it!

Funny you should mention response time as that, the reflex mode of driving, is most often significantly quicker than you can thínk and react. A quick responding car is a huge bonus.
In this case I was back on the right track by the time I had consciously processed the events.

[Joesson]

Response time hopefully less time than saying it!

Funny you should mention response time as that, the reflex mode of driving, is most often significantly quicker than you can thínk and react. A quick responding car is a huge bonus.
In this case I was back on the right track by the time I had consciously processed the events.

Our Police driver's, I understand, do a running commentary as they make progress. Having tried this on occasion, as Petrus mentions, I have found that the "reflex" reactions seem quicker than the conscious thought/ speech.

[Petrus]

Our Police driver's, I understand, do a running commentary as they make progress. Having tried this on occasion, as Petrus mentions, I have found that the "reflex" reactions seem quicker than the conscious thought/ speech.

Racing would be impossible if it were not almost exclusively trained reflexes.
F1 drivers take a chicane quicker than a human can complete a single conscious eye, hand/foot sequence.
Same thing for most experienced drivers on the road. Most here will have plenty of experiences of it like already coming to a stop for a kid bolting accross the road while still processing having seen it.

[Petrus]

One of those weird memory lights and ...

                Wheelbase(m) CG loc(m)                   Inertia (kg*m^2)     Mass(kg)
                           from fr axle  above gnd             Pitch    Roll   Yaw

1985 Pontiac Fiero   2.375      1.389     0.507         1528    375    1619    1247
1986 Toyota MR2     2.319      1.3        0.500         1280    340    1460    1090

Source: Mitostile Prototipo

The yaw inertia is the PMI.
So my 1.000 kgm2 is probably too low but the point remains the same: That 10 or 15 kilo at about 2 m. from the rotation center has a lót more effect on the PMI than on the power/weight ratio.

On a side note, an overall weight reduction reduces PMI too ofcourse.

[Petrus]

For those mulling this over or having discarded it; skating to the rescue:

https://www.youtube.com/watch?v=k8dNEDpUXOs

Angular momentum and polar moment of inertia are for this purpose the same thing.
This skating pirouette illustrates that shortening the ´arm´ result in a increase in rotational speed.

In our car this translates to less force needed by a steering input for a change in direction; less force asked from the tyres.

For a practical trial go sit on a rotation office chair. Stretch out your legs and arms.
Ask someone to give you a rotational push/pull.
As you rotate, fold your arms and legs inwards.

Same for the car if you take off weight at the far ends or move it inwards: It will turn easier.
Traction wise it is also sáfer as you ask less of the rubber. It is after all the friction which pushes the car sideways.
Since a corner is a continous change of direction, the same corner speed will see you with a larger safety margin.