Moment of Inertia in a car.

[Belatti]

Is a reduced moment of inertia in a car always better?

I was having a talk with a racecar engineer about ballast and other devices that can be moved to setup weight distribution in a sport car and he told me that you must always have to try the minimum moment of inertia.

I think that that may be the case in a track with many chicanes where you need a fast yaw response, but maybe in other tracks with open fast curves you may want a car that is less prone to a spin.

Thoughts?

[Callum]

I guess it could make the car less twitchy to have a larger polar MOI.

[dumrick]

I think that that may be the case in a track with many chicanes where you need a fast yaw response, but maybe in other tracks with open fast curves you may want a car that is less prone to a spin.

A larger polar MOI won't make the car LESS prone to a spin: when the car has some angular speed, in a turn, the inertia will fight against angular acceleration (neeeded for more angular speed, in case the radius becomes shorter, or less angular speed, like in the connection to a straight). Spinning is an effect of inertia and the higher the polar moment, more the system is induced to mantain angular speeds, even against the will of the driver.
Inertia also should not be mistaken for stability, that is a function of the positioning of the COG of the system, in relation with the addition of forces that are exerted on it.

[Belatti]

Thanks Callum, dumrick!

I guess the other "technical" people of the forum are busy with all the "gates" scandals and the drivers market speculation...

Its is true what dumrick says, that when you have got an angular speed it is more difficult to "stop" the "yawing". But it is also true that its more difficult to acelerate "angularly" the car in the X axis. It may help a novice driver without good and trained reflexes.

The MOI also has consequences in roll aceleration as well.

[dumrick]

That's right, a MOI is determined in a particular axis. My reply only concerns the MOI in the z (vertical) axis. Inertia in the x axis will affect roll behaviour and, in the y axis, pitch.

But it is also true that its more difficult to acelerate "angularly" the car in the X axis. It may help a novice driver without good and trained reflexes.

I'm sorry, but I'm not sure that is the case. In my view, less inertia is always better. It makes the car more responsive, not only in angular acceleration (like in turn-in), but also in deceleration (like stopping a drifting rear).

[F1_eng]

Hello,

It is not always best to have the lowest possible MOI.

A vehicle has a centre of precussion which is a point in which is rotates about the Z axis. The centre of precussion is dependant upon the inertia of each component and their relation relative to the centre of gravity.
The position of the centre of precussion is vital in determining how a vehicle behaves through a corner, particularly during the initial turning in. The reason it's so important is because the rotation point affects what happens to the slip angle of the tyres, in particular the rear tyres.
If the centre of precussion is ahead of the rear axle then you would see a considerable increase in rear slip angle during turn relative to the steer angle whilst if cop is far behind the rear axle, rear slip angles are considerably reduced.
Of course this determines how the car behaves, so its not just a driver skill thing. but drivers have preferences, for example a british touring car design company spent a fortune on light materials and reducing MOI by moving stuff to the middle, then after pre-season testing with the driver, he hated the car. They ended up putting ballast in the front and rear bumper, he then reduced his lap times considerably. I wont say which team or driver but it was a while back, some people on here might know.

Regards

[Belatti]

But it is also true that its more difficult to acelerate "angularly" the car in the X axis. It may help a novice driver without good and trained reflexes.

My bad, I meant Z axis.

... but drivers have preferences, for example a british touring car design company spent a fortune on light materials and reducing MOI by moving stuff to the middle, then after pre-season testing with the driver, he hated the car. They ended up putting ballast in the front and rear bumper, he then reduced his lap times considerably. I wont say which team or driver but it was a while back...

Thanks F1_eng! That was what I was trying to tell to this race engineer with suposedly a lot of experience...

Here in the TC series there are four types of cars:
Dodge, Chevy, Torino & Ford.

All brands basically weights the same, have different engine blocks (3 liter, 6 cil, 50s designs with OHC and 2 valves per-cyliner) with different bore/stroke ratio, but the same power (and not torque curve) and very very different aerodynamic shapes. Its not a silhouete series as the rulebook is pretty open in rollcage construction and suspension geometries are different between each brand.

The Dodge and the Ford being the two most extremes cases: Ford has hideous aero, with downforce variations through the speed range, its smaller (less MOI) and has an oversteering tendency. The Dodge has a smoother aero but its larger (more MOI) and has an understeering tendency.

Ford is way better for chicanes, although a bit nervous. Dodge is better for high speed curves, but a bit slow reaction for chicanes. There are some drivers that can handle anything, but some others that just cant go from one car to another that easily and driving styles counts a lot here...

Hence my MOI question.



About the centre of precussion I will search and study a bit more about that. Thanks again!

[dumrick]

It's great to hear about new dynamical variables I've never heard before, in my studies or in specialized literature. I also will look into the centre of percussion to learn more from it.

[F1_eng]

It is also sometimes refered to as an internal conjugate

[riff_raff]

Belatti,

I don't mean to insult your English (because it is actually very good!), but the term "moment of inertia" can mean different things in engineering terms, depending upon how it is used.

For example, the polar mass moments of inertia, such as the roll, pitch, and yaw axis polar moments produced about the instantaneous C of G in a car chassis, are generally best when kept to a minimum. Since this results in a chassis that is more responsive to steering inputs and suspension spring/dampener tuning.

However, the term "moment of inertia" also can apply to structural member sectional properties. In regards to structural members, such as an F1 composite chassis tub in torsion or bending, having a higher structural moment of inertia would be very desirable. Since that would mean a more rigid chassis (remember basic Euler stress=Mc/I).

So high "moments of inertia" can be good or bad, depending upon how the term is used.

Best regards,
Terry

[dumrick]

Thanks for clarifying, Terry and don't blame solely Belatti, because I was also feeding this confusion. Clearly, throughout the whole discussion, the theme was the polar mass moment of inertia.

Apparently, the centre of percussion is a design variable well-known in sword- and baseball bat-making. Wikipedia has a nice article on it, with the distance to the CoG of the centre of percussion of the simplified model (b) being a factor of I(polar mass moment of inertia)/(A(distance of point to the CoG)*M(mass)), which, in a car model, I guess it can translate to the sum of each individual component b=sum I/(sum A*sum M).

Which I would like for somebody using the concept to explain me, is if there is any relation between the placement of the centre of percussion with steering geometry, if it's placed by feel or if there is a constant ideal relation between it and the rear axis. This last hipotesis would mean that wheelbase changes by moving the rear axle location would imply a recentering of masses (if shortening) or locating aditional masses behind the rear wheel axis(if lenghtening), effectively increasing the polar mass MOI of the whole car.

PS: I missed a good subject like this one in F1Technical.

[F1_eng]

A term used to define the cofprec in a usable manner is a vehicle's dynamic index. It is simply a number which defines where the centre of precussion is relative to the rear axle, a dynamic index of 1 being on the rear axle.
Most good "drivers" cars fall in to roughly the same range, having a dynamic index of around 0.8.

This method is used differently by different people so I wouldn't like to imagine what Google or Wikipedia would throw up on the subject.

[xpensive]

I guess the other "technical" people of the forum are busy with all the "gates" scandals and the drivers market speculation...

Sorry gaucho, I have been busy trying to empirically estimate the "mass polar moment of inertia" of my Porsche 968, with my Argentinian mistress in various positions.

[dumrick]

A term used to define the cofprec in a usable manner is a vehicle's dynamic index. It is simply a number which defines where the centre of precussion is relative to the rear axle, a dynamic index of 1 being on the rear axle.
Most good "drivers" cars fall in to roughly the same range, having a dynamic index of around 0.8.

Thanks for the information. If I understand correctly, the index is the ratio between distance to the COP and distance to the rear axle. Where is your point of origin of those measures (your xz=0)? Front end of the car, front axle or COG? And don't worry, I'm not making any racecar, just curious

[Belatti]

Terry,

I was obviously talking about the polar mass moments of inertia. Arround the yaw (Z) axis to be precise. I name that "I" and to keep it simple -> M=I*ɤ (units Kg*m^2 in order to -> Nm = Kg*m^2 * 1/s^2)

A 1st course teacher in the Uni taught us that, in order to avoid confusions, when we refer to structural member sectional properties we should call "second order moment" or "centrifugal moment" to "J" (not "I"). "J" can be referred to an axis or to a point (polar), it can assume positive or negative magnitudes according to its axis position and it is the one used in the formulas to calculate the tension Tau or Sigma of a structural element.

@ F1_eng, thanks a lot!