Polar moment of inertia and weight distribution

[Belatti]

Hi, recently I have read an F1 engineer (dont remember who or from what team, or even where I read it) saying that reducing the polar moment of inertia of the car it makes it easier to stay near the limit of adhesion. I gave it a thought but cant fully accept that explanation. I would like to hear some opinions here and gather experiences you all might have had about the subject

I engineer some 1250Kg 400HP front engine and rear wheel drive cars with enough DF to corner at 1.6G (1.8 peak). The tyres are same size (and thus rigidity) front and rear so I try to relocate the components I can and the ballast to get 50-50 % weight distribution. Now the problem arises: there are 4 "silhouettes" in the series and one in particular has a slightly heavier engine and a longer tail. Those cars are more propense to understeer.

[Tim.Wright]

For the moment of inertia i need to think about it a little bit... Perhaps it is because there is less yawrate overshoot to steering response if you have a lower polar inertia so maybe its easier to place the car on the limit on corner entry. It might be a single seater specific thing because with GT cars you don't drive them with such fast inputs on the steering wheel so overshoot generally isn't a problem.

But for the mass distribution I'd suggest that just because you have the same tyres front and rear, you don't necessarily want a 50% mass distribution. Generally you want it more biased towards the driving axle (rear in your case) to give you better traction.

How far you should go in doing this will depend on how much US you can recover using a more forward balanced roll stiffness and/or whether you start to see unacceptable wear rates on the rear axle. It will also depend on the track. Some tracks are more sensitive to good traction than others.

[Greg Locock]

PMI always seems to be an over exaggerated attribute for a vehicle. My argument in favor of low PMI is that if the car starts to yaw near the limit then smaller forces are needed to bring it back into line, so you are in less danger of losing control. The downside is that everything will be happening that much faster.

On the other hand on gravel roads or snow it seems to me that a car with a high PMI is easier to control when hanging the tail out on a corner.

[Jersey Tom]

Hi, recently I have read an F1 engineer saying...

Many people say many things...

[Belatti]

Hi, recently I have read an F1 engineer saying...

Many people say many things...

So what?

[olefud]

PMI always seems to be an over exaggerated attribute for a vehicle. My argument in favor of low PMI is that if the car starts to yaw near the limit then smaller forces are needed to bring it back into line, so you are in less danger of losing control. The downside is that everything will be happening that much faster.

On the other hand on gravel roads or snow it seems to me that a car with a high PMI is easier to control when hanging the tail out on a corner.

With more PMI, It's harder -for better or for worse- to get the car rotating. But once it starts to overrotate it's harder to stop it, usually for worse.

[riff_raff]

I imagine a lower PMI in roll/pitch/yaw would usually be a good thing for an F1 car. The car will respond quicker to steering inputs. A more consistent PMI over the course of a race, as fuel load changes, would also be beneficial.

[DaveW]

Hi, recently I have read an F1 engineer saying...

Many people say many things...

When rig testing a vehicle, I use a "Performance Index" (PI, a cost function) as guide for set-up changes. The PI is proprietary (it incorporates my prejudices about what is important) but it can viewed, and presented, as a multidimensional surface. The objective is to find the minimum value of the surface. The normal order of priority is dampers, springs, tyres, installation stiffness, and inertia parameters. A change in a lower priority parameter would normally require higher priority parameters to be re-visited.

It is fair to say that I do not often look at the last two, but they are calculated. Slightly tongue in cheek, I looked at what I would do to change inertia parameters for two vehicles with fairly representative set-ups.

An F1 vehicle would be "improved" by reducing the Pitch radius of gyration (PRoG). However, a "GT" vehicle would be improved by increasing the PRoG. In both cases getting the c.g in the "right place" is at least as important.

The difference between the two vehicles is interesting, and (perhaps) relevant - see Greg's comments. In my case, it has a lot to do with tyres, springs and damper selections. Whether the conclusions would still stand after re-visiting higher priority parameters is matter of speculation....

[hardingfv32]

….In both cases getting the c.g in the "right place" is at least as important.....

Are you able to expand on this statement? It is not the case that lower is always better?

Brian

[DaveW]

It is not the case that lower is always better?

Your right, of course. The position of the c.g. is defined by three coordinates, one of which (the vertical) cannot be determined accurately during a multi-post rig test. The other two can. Specifically, I was referring to the longitudinal position of the sprung mass.

[MadMatt]

Having worked in the WRC, I have to add that in rallying, PMI is probably as important (if not more) than in circuit racing. Due to the nature of the road (stage), the car tends to change direction much more often than on a race track. Therefore, having a bigger PMI will help you to carry the energy from corner to corner, therefore help you to turn in.

For sure in rallying they tend to target overshoot (they used to on tarmac and gravel, nowadays mostly gravel) but still, I think depending on the type of racing, you don't mind that much having a bigger PMI.