what exactly is instant center and how would i be able to visualize it? (sketch) what does it do?

john

Check it out--this is a decent explanation of IC.
http://www.dickmillerracing.com

penguinjohn,

Let me try to explain it. The reason I want to try is so hopefully I can figure it out tooo. And if all else fails, I'm sure David won't leave us out in the cold.

You look at the front of your car from standing in front of it.

Then you draw a line through the upper ball joint and A-arm frame mount pivot point, then the lower A-arm frame mount and ball joint for each side.
Continue the lines out until they cross each other on both sides.
Then where they cross, draw lines from that point to the center of the tires closest that cross point where they sit on the ground.
Continue the lines past the tire/ground point until they cross each other. At the point where they cross is the roll center point we are talking about. And this point should be on the centerline of the car.

Now for a stock first gen front end this point will have the lines drawn to it pointing to the outside of the car and then come back and cross under the ground. This is because the Upper A-arms ball joint ends are below the frame mount ends.

To raise the roll center, you gotta raise the ball joint ends higher then the frame mount ends which will cause the lines drawn outward to NOW point into the car and out the other side from where each was pointing.

Then continue to redraw the lines until they again cross each other.

At this cross point, then redraw the lines from that point to the opposite tire/ground contact point.

And where these last lines cross the center of the car vertical line is where the new RAISED roll center is.

The roll center height is the distance from the roll center point either below ground stock or above ground modified to the CENTER of GRAVITY point on the vertical centerline of the car.

How's that, David???

Confusing isn't it.

I haven't figured out why you gotta shorten the upper A-arm to keep the alignment right yet, so David, if you will tell us please. pdq67

Pdq67,
I think you got it.
If you are looking at front suspension roll centers.
The IC is where the lines extended from the upper and lower A frames intersect.
You draw a line from the IC to the tire contact patch centerline, use the tire opposite the IC. Where the line crosses the center of the vehicle is the roll center height.
If you are drawing the car in roll the roll center is where lines drawn from each side cross each other. So as you can see, the roll center can move laterally.

If you are doing rear suspension or front anti-dive, there are Instant Centers associated with that too. It's usually just the linkage extended to a point where they intersect. Lines get drawn from the IC to the tire center of contact.
When drawing antidive where the line from IC to tire crosses the center of gravity location front to rear, the height from ground to gravity ht is the percent antidive, front or anti squat rear.

As far as front center of gravity height to roll center is concerned, It's only important in that it needs to stay in similar relationship of movement from front to rear.

The problem with trying to design your own front suspension geometry is, what is the ideal?
There are so manny trade offs that only vast knowlege and experience can tell you what is the right number.
If tire traction under cornering is the only criteria, you could come up with the correct geometry fairly easy.
But under braking, the tire would camber in a lot more than is good and braking would suffer.
You might get too much side scrub of the tread which would tend to break loose the tire when the car is braked into the corner.
Also tires make a big difference.
As you go to wider and wider tires the geometry becomes more important to keep the tire flat on the ground and working.
Tire construction is a big factor too. The radial tires need more negative camber than the bias ply tires.
Racing "radial" tires are not true radials, they are a cross between a bias ply and a radial.

There are a lot of books out there but none give you the real numbers you need to re design your front suspension yourself and get it perfect the first try.
Personally, I feel it gets more risky the more neg camber change you achieve. You run the risk of screwing up something else.

A recient mag article I read showed where they got the perfect front geometry for keeping the tire flat when the car rolled into the corner and the rear suspension raised up due to braking.
This is ONE point in time where the car spends a split second as it enters the corner.
Once the car brakes a little it turns in to the corner. That is the point they optimised. There may be other attitudes the car stays in longer when cornering, that may NOT be optimum.
Also driver tecnique can vary what happens to the cars suspension and when.
Are your brains scrambled yet?
David

------------------
Check my web page for First Gen Camaro suspension info:
David's Motorsports page (http://ourworld.compuserve.com/homepages/David_Pozzi/)
First Gen Suspension Page (http://ourworld.compuserve.com/homepages/David_Pozzi/first_gen_suspension.htm)
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer

David,

So the instant centers are the line cross points. Great, I learned something. My drafting instructor always talked about imaginary points and instant centers. Boy, that was a long time ago. Thx. pdq67

In general, the front camber related IC should not be too close to the car or it will cause the roll center to move up and down too rapidly.
But if you look at a Winston Cup front upper A frame, some have very steeply inclined A frames that are breaking the "rules"
I think they are trying to get a weight jacking effect from using a high roll center. This would allow a softer front spring to be used.

Books refer to keeping the IC one to one and one half car widths or more away from the car.
The IC is the point the wheel is pivoting around. But every inch the IC moves.
If you get the front RC about 1 to 3" above ground and the rear an inch to two inches higher you will be close.
I haven't looked this stuff up lately so I'm just throwing out what I think I remember of it.

Assuming the lower A arm is level and the upper usually winds up between 1.5 and 2 degerees inclined down as it goes toward the centerline of the car.
The best way to measure the car and draw it is to measure between left and right sides of a particular component, then measure the height of it from the floor.
Cut the measurements in half and draw one side of the car from the centerline for an easy quick shot at the camber curves and roll center ht.

I like to draw the chassis with suspension and move the tire and ground level instead of moving the chassis, but you can do it the other way too.

I have some blank drawings with spots for all the relevant measurements. I just fill it in, take it home and draw it.
I have a suspension geometry program but havent used it yet. I'll probably do my 67 and 69 camaros with it. The 67 has the Guldstrand mod, the 69 has a taller spindle by Stock Car Products. The 69 spindle is around 1.5" taller than the stock spindle.
David

------------------
Check my web page for First Gen Camaro suspension info:
David's Motorsports page (http://ourworld.compuserve.com/homepages/David_Pozzi/)
First Gen Suspension Page (http://ourworld.compuserve.com/homepages/David_Pozzi/first_gen_suspension.htm)
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer

[This message has been edited by davidpozzi (edited 11-15-2000).]

Thanks, David, and wow!!

How do you raise the rear roll center??

I raised my front leaf spring mounts 3/4" but haven't even considered balancing RC's front to rear ratio wise like I'm doing with the four springs. What do you think?? Is it really worth the trouble in the rear. pdq67

david & pdq,

thanks for the replys. from what you've described, i realize that i knew more about this than i thought, i was just having a hard time visualizing it. ive done the guldstrand operation on my '68, i just didnt take any measurements because i didnt know how to. i remember reading a recent article in one of my magazines that detailed a new computer program for IC, RC, etc but i dont remember which it was. anyone? my a arms are positioned like you described, david, so i think im pretty much in the ballpark, however i am interested in the theory behind suspension stuff like this. do you have any material that you could email me (like that form you were talking about) that i could draw some of it out with? id like to learn more, so whatever you guys can dish out i will gladly accept. thanks.

john
penguinjohn@hotmail.com

I'll see what I can put together on the blank form thing.
I use a mac computer so I'll have to put it in a format you can use, probably .PDF.

When I get around to modeling my 67, I'll put the measurements on my web page for you to copy so you won't have to measure yours.

pdq67,
The rear roll center is determined by the shape of the rear spring, not much you can do about it.
Raising the front end of the spring moves the anit-squat IC from a side view and helps increase the anti-squat.
How much did you manage to raise the front spring eye?
David

------------------
Check my web page for First Gen Camaro suspension info:
David's Motorsports page (http://ourworld.compuserve.com/homepages/David_Pozzi/)
First Gen Suspension Page (http://ourworld.compuserve.com/homepages/David_Pozzi/first_gen_suspension.htm)
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer

Here's a form I use to make shure I get all the proper info to draw a suspension.
It's made for forumla cars but should work for a Camaro.

When measuring you will find the lower A frames are not square to the cars centerline.
Try to measure across the center of the lower ball joints.
David
Just copy the .gif image below to your hard drive. you should be able to print it from almost any graphics program.
http://ourworld.compuserve.com/homepages/David_Pozzi/susp_dimensions_.gif

[This message has been edited by davidpozzi (edited 11-15-2000).]

I raised the rear leaf front spring eyes 3/4", David. I'm going to study this stuff real close tonight when I get home. pdq67

David,

This might sound STUPID but what would happen if you used a solid axle up front and jointed Kingpins to it so that the spindles could pivot up and down on each end. As well as turn.

Then add pivoted drag links to something like a steering gear that would push/pull both, just like the steering linkage does, but the second setup would be tied to body roll and as the car rolls the mechanism would reverse the tilt of the tires.

Kinda like an active counter-roll system. I know that there would have to be one big bunch of bugs worked out, but what the hay, if it would work why not!!!
Just DREAMING.

And have I got an idea for an IRS!!! It would be located only be the springs and the frame!!! I need a FEA program to design the "springs".
DREAMING AGAIN. pdq67

I did some advising on a friends sprint car. It had the old style cross springs just around the time coil springs were beginning to be used.
I plotted roll centers and calculated wheel weights.
The suprising thing was these cars had very nice looking components with crome plating, etc.
But very poor geometry, and a bunch of other problems. The this was, one of the "worst" cars was winning!
Ever seen a rack and pinion steering rack attached to a straight front axle?
David

------------------
Check my web page for First Gen Camaro suspension info:
David's Motorsports page (http://ourworld.compuserve.com/homepages/David_Pozzi/)
First Gen Suspension Page (http://ourworld.compuserve.com/homepages/David_Pozzi/first_gen_suspension.htm)
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer