Center of gravity

[dirtdiggerracing]

Can someone tell me where the roll center is on the front of a GenX ?? Static and dynamic ?? Thanks

[Bubstr]

I really don't follow on this two COG thing. When talking applied forces, there is only one COG. That COG can apply force in only your tire contact patches.

When applying force to the side, there are two roll centers that determine how much side or down force is applied to each side tire, but that force always comes from THE COG. This also happens in front to rear forces. Instead of roll centers, you have anti squat values(4 bar settings) and anti dive (angles of control arms) for front. If you asymmetrical settings, Different side to side and front to rear, you can juggle how much weight and in what direction it is applied.

Think of it as a man pushing a refrigerator. Push too high the fridge will topple over away from the push. Too low and it will topple over on the man pushing. Think of where the man pushes, as your roll centers or anti squat values. These values or roll centers relationship to the COG of the fridge, determine how much down force is at his two feet (tires).

It is the relationship of the COG and your roll centers and bar settings that determine how much force is applied downward ( side bite / forward bite) or sideways at any tire (lack of bite).

[Matt49]

Wow...
There is absolutely no such thing as TWO centers of gravity in a car...unless the car is physically articulated (which it is not).
The reason there are two roll centers (front and rear) is because the front and rear suspensions are completely independent of one another. But there is only ONE chassis and ONE mass that is being sprung so to say that there are somehow two centers of gravity is completely incorrect. The ENTIRE mass of the car acts on the front suspension and the ENTIRE mass of the car acts on the rear suspension. There are not separate masses therefore there are not separate centers of gravity.
I'm always open to new and fun ideas with vehicle dynamics but we do need to work within the laws of physics.

[hpmaster]

I sent a link on the first or second day of this thread to the guy asking on how to figure COG by PM. Last time I saw the COG at work was when two wreckers picked up a wrecked mod at both ends and it flipped over, guess they didn't find it. Past that I will sit in my corner and say nothing except to say what Matt said, WOW.

[Bubstr]

Every oz of weight from the bolt fastening the radiator in, to the decal on the spoiler all adds to the center of gravity.

When you talk about overhang weight, I believe you are talking about polar moment. Weight closer or farther away from the COG. This weight still is part of the Mass of the car but can change the rate of transfer when applying forces.

Think of the merry go round at the park. If your way out on it, it takes effort to stay on. If your in the center, you almost need no hands to stay there. These are the forces at work with polar moments. High polar moments can turn your car into a bicycle or low polar moment can rob you of side bite. Too much rear can spin you out and not enough can make it hard to turn. What you want is a compromise that allows your roll centers, anti squat and anti dive to become tools to fine tune the force.

I know putting bar A in hole C is the way we fine tune chassis now days, but I think it adds something, if you know why. It's all based on physical law and knowing a few of those laws helps your confidence on choices of bar A in hole C. or D.

[Matt49]

I think the term you're looking for is moment of inertia which is directly related to the distribution of an objects mass independent of it's center of mass. An object with most of its mass toward the center is easier to rotate than an object with its mass distributed further to the outside. This is why a spinning figure skater speeds up when she brings her arms and leg in: decreased moment of inertia.

Careful with the merry go round analogy. You're describing centrifugal force which is something completely different and is completely independent of the moment of inertia. BUT as you move to the outside, the merry go round would be harder to spin which IS because of the increased moment of inertia you created by moving your mass way from the center.

http://en.wikipedia.org/wiki/Moment_of_inertia

http://en.wikipedia.org/wiki/Centrifugal_force

[Bubstr]

Matt your right. Why I said polar moment is because it is a secondary, even third modification of force from the center of mass due to location of height and lateral weight components that make up the center mass.

I'm sure it's in wikipedia too. http://en.wikipedia.org/wiki/Second_..._area_products

[SuperEight]

Wow...There is absolutely no such thing as TWO centers of gravity in a car...unless the car is physically articulated (which it is not).The reason there are two roll centers (front and rear) is because the front and rear suspensions are completely independent of one another. But there is only ONE chassis and ONE mass that is being sprung so to say that there are somehow two centers of gravity is completely incorrect. The ENTIRE mass of the car acts on the front suspension and the ENTIRE mass of the car acts on the rear suspension. There are not separate masses therefore there are not separate centers of gravity. I'm always open to new and fun ideas with vehicle dynamics but we do need to work within the laws of physics.

Love it!!! I was thinking thats how it was but didn't wanna be difficult. (2)-RC's, (1)-COG I played around with an earlier version of the race wise software and learned a lot(1999). Sadly some of the recommendations it made didn't work as theorized, but learned some valuable info along the way. Seems like the one we had even wanted us to input sway bar diameters, which haven't been seen on DLM's since the wedge days...