How does one determine a car's theoretical roll center. The point came up in another thread that a C3 chassis has a roll center 2" below ground, how can that be determined in a home garage, if at all?

 

bump

 

take sups. measurements and do some math, both front and rear, connect front susp and rear susp. roll centers and you have the roll axis.

 

Can anyone expand on this, if it's not too involved, perhaps using an example? This theoretical point would seem to me to be influenced by other factors like the center of gravity of a given vehicle, not just suspension mounting points.

 

I was hoping someone else would answer this.
You take a piece of paper and draw a ground line on it. You are looking at the front of the car.
You draw a line representing the upper and lower A arms on each side.
Just short lines for each.
This is the hard part. You have to accurately measure the distances for the inner and out rotating points from the ground and mark them on your drawing.
It is hard to measure the center of a ball joint to the ground and the center of the A arm bushing to the ground.
But say you did accurately measure all these distances.
You then do trig to determine where the instantanous center is. The instantanous center for the left side of your drawing which is the passengers wheel is actually on the right side of your drawing and way outside the car. You project the upper and lower A arms towards the center of the car and where they meet is way outside the car. I don't know it this is making sense but your instantanous center is where the 2 lines meet and this is below the ground and could be 10-20 feet outside the car.
You then draw a line from this instantanous center to the ground surface of the wheel from which you projected these lines.
Do it for both wheel and where the lines drawn from the instantanous center to the ground level of the wheel meet is the ROLL CENTER.
To raise the roll center the slope of the UPPER A arm needs to be steeper, or the pivoting shafts need to be lowered or the outer ball joint of the upper A arm raised to make this angle steeper so the instanous center is moved closer to the opposite wheel.
This causes that inst. center to be higher up so when projected back to the underside of the wheel it raises the roll center.

If you took the center of the crank for example as the center of gravity. The distance from this center of gravity to the roll center is the arm that is rolling the car over in a turn. If the arm is short/ a high roll center the roll over of the car is little. A longer arm more roll over.

I don't know if I am making sense to you. I know what I mean but might not be explaining it well.
I thought Twin Turbo could do a better job. He includes drawings alot of the time.
Anyway this is a rough attempt at explaining where the roll center is.

 

I think the GM performance manual has some C3 specific information on this subject, I will look when I get home.

This is a fairly good explanation (although they could use some illustrations for clarity).

http://www.circletrack.com/howto/181...ion/index.html

 

You only need to do it for 1 suspension corner, you draw a line through the Ic and the middle of the wheel contact patch and where it crosses the car's center plane, that's where the roll center location is. You don't have to do the other side because it's assumed to be identical/a mirror image so the 2 will always meet in the mid plane.

 

Your right Marck. Only one side is necessary. The circle track guys actually have different slopes to the upper A arm to move the roll center towards the outside for banked corners.
We have ours equal so it falls in the center like you said.

 

Here's a pic showing where the engineers calculated it to be.

 

That explains it beautifully, thank you all!

 

what does it all mean.

squat.

 

No it means if you don't like how your car behaves in a corner there are other alternative to hard hard springs. The guy who only runs the conventional setups never is the hot shot on the track. The guy that has the edge, his car setup just a little better then anyone elses is the guy setting all the track records , winning all the races.
I am fortunate in having a friend who while old is a hot shot Cascar racer. He sets all kinds of track records, wins nearly everything he shows up at and Nascar did a big writeup on him this past fall.
He discovered this stuff long before it ever appeared in print and that was his edge.
He obviously still has it since he became the Cascar champion last year but it cost him $200,000 to do it.

 

I run 1 1/2 inch longer then stock spindles and honestly I felt the difference as soon as I installed them. The car feels so stable on high speed corners, no lean but again I run a very rigid sway bar but light 400 pound springs.
I feel the spindles made a big difference because they raised the roll center.
Look at this welded spindle beside a stocker. Notice the difference in height from the large bolt to the top. If you remove this bolt and look how deep that hole is, how much material is missing in this cross section and then look at my welded joint and see how heavy the crosssection, the smoothness of the surface and the top of the spindle takes very little load.
Done properly a weld is as strong as the parent material and in this case with the increased cross section area I believe it is stronger.

 

I run 1 1/2 inch longer then stock spindles and honestly I felt the difference as soon as I installed them. The car feels so stable on high speed corners, no lean but again I run a very rigid sway bar but light 400 pound springs.
I feel the spindles made a big difference because they raised the roll center.
Look at this welded spindle beside a stocker. Notice the difference in height from the large bolt to the top. If you remove this bolt and look how deep that hole is, how much material is missing in this cross section and then look at my welded joint and see how heavy the crosssection, the smoothness of the surface and the top of the spindle takes very little load.
Done properly a weld is as strong as the parent material and in this case with the increased cross section area I believe it is stronger.


How much did this raise the roll center? BTW you did a great job on that spindle.

 

Guldstrand bushings and raised ball joints should almost accomplish the same thing. Along with my 660 front springs and 1 1/4 sway bar I should have minimal roll