How to locate the mass centroid axis ?

[Warpspeed]

The commonly available suspension simulation computer programs require the entry of an effective centre of gravity point at each end of the car located above each axle centerline.

This may be at a different height at each end, depending on how the mass centroid slopes from front to rear. It will obviously pass through the main CG point at the vehicle centre, but how to determine the location of the actual mass centroid, and the respective front/rear heights?

With such a complex distributed structure trying to calculate it theoretically would seem to be an almost impossible task. Is there a practical way to actually accurately measure it?

 

[willeng]

I found this & it would seem if these guys are concerned about the mass centroid axis we all should be.

http://www.audiosignal.co.uk/McLarenF1.html

It would seem that Warpspeed is again onto something that gets overlooked.

MoreWing

I'm not so sure now if Caroll Smith was wrong with this if it is still a design consideration. Maybe he knew more than people think?

 

[GregLocock]

Um, that article talks about the height of the roll centre heights (front and rear) relative to the centroid of the body. Singular.



Cheers

Greg Locock

 

[willeng]

Greg, i guess what you me by singular you mean the Cg point & they have set the roll centres at each end the same height relative to this?

That would seem correct but!

Please knock me on the head if this is wrong, a (centroid) is the centre of rotation of the mass is it not?. If we veiw the car from the top the mass centroid would be the Cg point because this would be the centre of rotation of the mass.

But when stating roll centres (with) the mass centroid we are viewing the car from the front or rear & the mass centroid is again the centre of rotation but in the longtudinal direction. The Cg point is the point where the forces act upon as we know but it is not the mass centroid when viewed from the front or the rear of the car.

If a centroid means the centre of rotation of the mass it must be along the same axis as the roll centres they are talking about because the Cg is not the mass centroid unless viewed from the top if you get what i mean.

So in side veiw they have the mass centroid axis the same height at either end to the roll centres & have had to drop the engine lower to achieve this.

Hope this makes sense?

 

[GregLocock]

I agree with your last sentence, although I actually think the emphasis in the quoted statement is BS, you drop the engine to drop the CG, always. A low CG is the best single handling thing you can do. If your principal axis of inertia comes out horizontal as a result, well that's nice, but it is not why you do it.

I also agree there is a lot of logic in making the RCH much the same at each end. This reduces longitudinal load transfer when turning. (Note I am splitting load transfer into along the car and across the car here, rather than looking at each axle - it helps).

Intuitively I also suspect things works better if the roll centre axis along the car is parallel to the principal axis of inertia, but remember, the RCH argument above is concerned with steady state cornering, so the body is no longer accelerating in roll, whereas for the principal axes to be important we must be talking about a transient event.

Time to drink some more beer I think...




Cheers

Greg Locock

 

[willeng]

Greg,

HaHa, No beer i'm afraid, you'd have to be an Aussie!

Unfortunately i spend my life working with my hands not the keyboard & have trouble putting things into words.

I think it's a matter of horses for courses, if we want to go as fast as we can you have to look at every possibility & follow most things through to find out.
It is interesting though that the majority of (good) race car engineers find the (mass centroid axis) important. From Formula Ford to F1, Le Mans to Nascar to V8 Supercars.
Who is right i don't know, but if they look at it there must be something there.

Have a beer & think about it some more!

 

[GregLocock]

No F1 is solely concerned with CG location. I have never heard a F1 designer saying he was aiming at anything other than the lowest possible CG height, and the correct front/rear weight distribution.

I'll have a play when I get some free time, unfortunately (ha) I've got a 6 day sailing regatta so that's plenty of time for thinking and drinking, not much chance to do serious maths.


Cheers

Greg Locock

 

[sreid]

I believe Colin Chapman's quote was "It's how long, how wide and where you put the weight."

 

[NormPeterson]

I keep coming back to Mr. Smith's mass centroid axis being useful if you wanted to determine the chassis' torsionally deformed shape due to its own [mass]*[lateral acceleration] separate from any roll couple distribution effects. But it seems that in order to do that, you'd still need to determine the principal axis of the moment of inertia, along with some chassis' torsional stiffness vs longitudinal position data.

Norm

 

[willeng]

Greg,

Yes i agree with the Cg thing & what the F1 designers say, but it is what they neglect to mention that i am interested in.

 

[red300zx99x]

Sorry, alittle off topic, but the McLaren F1 read talks about subframes and bushing used for long. compliance, has anyone ever seen a pic of this, ive been searching for months with no luck