Ralphy wrote:

Jim,
I think I finally see how to calculate roll center!

You've Got IT!  Now try a set sloping down to the wheel - The Instant Center is Outside the wheel - then draw back through the contact patch - AND Voila! RC on centerline -- but MUCH lower.      Nice graphic BTW.

Going by this, on my MII front end. If I kept the factory A arms and lowered my chassis, I would lower my roll center?  So simple!

Only if you keep the wishbones attached at the same location on the chassis  - change that too,  and the whole package needs to be re-viewed.

And as you said, front roll center should be below the rear?

Right - front engine car for sure. Another reason the front is lower (raked) than the rear - keeps the nose (lift) down at speed!

You know when I was a kid I could not grasp base two numbers. When I realized the answer, I felt so stupid. I was looking for something way more complicated, I just couldn't see the answer.

So you get the joke, "There are 10 types of people - those that get binary and those that don't!"

Cheers - Jim

I know these are all static numbers, suspensions are a dynamic system, however.

So the roll center should be higher in the rear vs. front. The lower the roll center, the more need for a heavier sway bar. If the RC were equal to the center of gravity CG. The suspension would not roll in a corner. RC controls (effects) body roll, NASCAR acknowledges this in use of the Track Bar. They use TB adjustments during a race. A higher RC puts less weight on the outside tires. So I guess the RC controls weight transfer and changing the direction of force in cornering?

TRACK BAR

(Also referred to as a "Panhard bar.") This bar locates the vehicle's rear end housing from left-to-right under it. In calibrating the vehicle's "suspension geometry," raising or lowering the track bar changes the rear roll center and determines how well it will travel through the corners. During races, this adjustment is done through the rear window using an extended ratchet. Typically, lowering the track bar will "tighten" the vehicle and raising the track bar will "loosen" it.
http://www.nascar.com/2002/kyn/nascar_101/02/02/glossary/


NASCAR adjustments, pit member adjusting wedge not the track bar. Track bar is the rear adjuster.

Then you have roll couple, the difference of CG and RC. I'm guessing the lower the roll couple would be a more complete indicator.

Jim,
The more I understand the RC the more I see the need to not overlook. I was going to throw in the hat and call it voodoo.

Last edited by Ralphy (1/17/2012 9:46 am)

LOL  love that pic Ralphy, that's about how I feel after spending time reading here !!

I'm rough graphing  the C3 and Jag. I am completely wrong about the Jag RC, as you narrow it, the RC rises. The Instant Center IC shortens when narrowed. I halved the length only to see visible changes.

OK, this exercise for me was to tell where I was going when my install was done. The two rough sketches C3 and Jag. Second lower view is at half length on the lower link in each sketch. What I see is that as I narrow an IRS, the camber increases and the roll center goes up. Changes are in red, total angle is not inserted. As I said this is a rough sketch. The points are fairly close. Notice how the Instant Center decreases by a fairly large amount. Raising the RC diminishes the need for as heavy a sway bar. Since a higher RC creates less roll. The upward motion in red is equal in both views.

So you narrowed the IRS and you did it for a reason. More than likely it came out of a wider bodied car. So does the camber really increase? Being that the body you have is narrower, 1" of body roll will give a greater angle. So now the added camber per inch is needed in a narrower car. So all the geometry should come back to a pretty good formula for the car you have. You will however, still have a higher roll center. Narrowing, mathematically works out good. Moving the inner LCA pivot lower, will even bring the RC back on a C3 or I think a Jag.

The only situation you would end up with to much camber would be when you start going with excessively wide tires and your fitting them inside the wheel wells. Like this one. You would need to do some serious narrowing for the IRS to clear the wheels.
http://www.bryanf.com/510/images/rsusp15s.jpg
http://www.bryanf.com/510/images/rsusp8.jpg

Last edited by Ralphy (1/17/2012 10:09 pm)

Jim,
You got mail.

My posts are pretty long I know. But in it maybe you missed where I wrote this.
"Narrowing, mathematically works out good. Moving the inner LCA pivot lower, will even bring the RC back on a C3 or I think a Jag."

In real world you have to look for the vulnerable area. The halfshaft is never going to change position. The Outer camber rod pivot point to move, would require changing out the outer hub. Yes the inner camber rod is the point that can be altered the most easiest.

Here is such a device on a C4. It lowers and shortens the LCA or in Vette terms the camber rod. But also remember this will reduce the camber curve also.

http://www.racecars360.com/Corvette_Tech/C4_Rear_Suspension_-_2.html

Last edited by Ralphy (1/18/2012 10:59 am)