Weight Distribution vs. Polar Moment (Yaw Intertia)

[TensorWorld]

I don't post often but I have a question about the PMOI vs F/R weight bias. I am interested in the curious case of the Miata. Clearly with a low PMOI it would seem to fit the bill for autoX and indeed this likely explains why everyone and his brother seems to drive them at autoX's (at least from my experience).

 

However, they are also often toted as the closest thing to a "race car" and I always presumed it was because of the low PMOI and the connotation has always been that being like a "race car" is a good thing. In fact when inquiring to a friend who used to race for Porsche about cars to consider for track days he recommended the early miata's attributing their low PMOI as one of the reasons. He did seem to think this made it easier to wreck them too.

 

This got me thinking about F1. They should definitely have a lower PMOI than just about anything else on this planet you can sit in and I would assume that while the drivers have little trouble driving them around the track I would likely kill myself in one in several seconds. If I didn't, the low PMOI would make it impossible for me to drive anywhere near the limit, which seems to be the argument against too low a PMOI. (ya the power and downforce etc. would make it impossible too but just go with the thought experiment).

 

So why would there be too low a PMOI for a street (touring) car if the F1 cars can be driven just fine with the lower PMOI. It even seems intuitively right from my experience that a car with too low a PMOI would hurt my times because I would be scared to death of it. But sense I am not anywhere near an F1 driver I was wondering if I missed something. Thanks.

[Challenger]

Does Kim Blough have an account on HBZ or have you guys talked to him personally?

[JMortensen]

I believe he was banned.

[pparaska]

Kim chose to leave us behind as far as having a presence here that he is public about. Who knows, maybe he lurks.

[pparaska]

I'm guessing that car control with very low PMOI can be troublesome if the tiniest little thing upsets it and starts it yawing. I can see how such a car would be very busy to drive, and very tiresome on the street.

[johnc]

So why would there be too low a PMOI for a street (touring) car if the F1 cars can be driven just fine with the lower PMOI. It even seems intuitively right from my experience that a car with too low a PMOI would hurt my times because I would be scared to death of it. But sense I am not anywhere near an F1 driver I was wondering if I missed something. Thanks.

 

There are many things that contribute to vehicle stability. PMOI is just one aspect of that and can be tuned for or against with springs, bars, tires, etc. A low PMOI car is not always a spin waiting to happen and a number of high PMOI cars can span spin pretty easily (think short wheelbase 911).

 

When PMOI makes its presence known is when the suspension is mostly taken out of the equation - as the rear tires lose all grip and the yaw rate increases. Quick hands are needed at that point in a low PMOI car.

 

Also, comparing a F1 car (or any other open wheel, purpose built race car) to a production sedan is an exercise in futility. And being scared of a car spinning during an autocross or a race is a sign of driver inexperience or poor chassis setup. More seat time and a good chassis setup will allay those fears.