Rear roll center

[hpmaster]

I just spent/wasted 2 hours reading my notes, various books and a couple old RaceWise note books on rear roll centers. After reading that and these posts by guys I respect I officaly refuse to even think about where this make believe point is from here forward. I now think it doesn't do anything but add a piece of pretty much useless poo that can't be proven to me in a definitve way to do anything positive in tire loading adjustment.JMHO

[MasterSbilt_Racer]

Here`s my 2 cents. Roll center on rear is middle of both ends of panard bar. Shock angles and placement only locate cg in relation to that roll center. Moving your roll center 2" to the right will make your rocket handle like a rayburn, reeally good inthe mud and terrible in the slick.

You can move a left mounted panhard to the right side of the car and move the roll center to the left. Observation of roll angle and change in wheel travel confirms this.

[b1driver]

Then i guess we should all be running rabuds purple bar

[b1driver]

The only way any of this gets proven is with some data aquasition on one of these cars and whoever does it sure ain`t going to share it unless you got a fat wad of cash.till then its just guesswork in a firesuit

[b1driver]

Acording to this theory, would moving shock mounts on frontend also change moment center?

[MasterSbilt_Racer]

Then i guess we should all be running rabuds purple bar

Not what I was referring to.

[FlatTire]

One night I found the roll center to be on top of my roof! The car wasnt too pretty after that.

[hpmaster]

One night I found the roll center to be on top of my roof! The car wasnt too pretty after that.

That is the most definitive explanation of the existence of "roll center" so far in this thread! Now someone will say that was more related to center of gravity.........

[Matt49]

Acording to this theory, would moving shock mounts on frontend also change moment center?

No. Front end is independent wishbone suspension which is a completely different mechanical model than a beam axle suspension which is what we're dealing with in the rear end.

[Matt49]

Regardless of what theory we subscribe to on how to find the roll center, it is important to know that you are finding the STATIC roll center with the car at ride height. So if you're interested in knowing how the car will handle while you're following the pace truck, this is very useful information. But our cars are hiked up the majority of the time which drastically raises the CoG and shifts the RC to the left. It also changes shock angles, etc.
Oh yeah and by they way, when you apply the gas and the lift bar moves up, the j-bar pinion mount moves up also. Don't forget to factor that in :-)
In seriousness, the important thing to know if you wanted to understand it from an engineering standpoint, is the moment arm. The moment arm is the line between the CoG and the RC. Ultimately, its length and angle is what affects handling. This is what we are really changing when we move RC.

[JustAddDirt]

rc

Regardless of what theory we subscribe to on how to find the roll center, it is important to know that you are finding the STATIC roll center with the car at ride height. So if you're interested in knowing how the car will handle while you're following the pace truck, this is very useful information. But our cars are hiked up the majority of the time which drastically raises the CoG and shifts the RC to the left. It also changes shock angles, etc.
Oh yeah and by they way, when you apply the gas and the lift bar moves up, the j-bar pinion mount moves up also. Don't forget to factor that in :-)
In seriousness, the important thing to know if you wanted to understand it from an engineering standpoint, is the moment arm. The moment arm is the line between the CoG and the RC. Ultimately, its length and angle is what affects handling. This is what we are really changing when we move RC.

I think my head just exploded.

[b1driver]

Couldn`t agree more matt49.

[Bcollins82]

No. Front end is independent wishbone suspension which is a completely different mechanical model than a beam axle suspension which is what we're dealing with in the rear end.

If your solid rod theory applies to the solid rear axle in changing the effective roll center, why would it not change the effective roll center up front? If you move the upper shock mount it can change dynamic roll rate, making the car feel like it has a softer or stiffer spring, charging how fast and far that corner travels. Obviously it doesn't change the RC statically, but it doesn't change the rear RC statically by having a solid rod in place of a shock. I've heard many differing opinions on this, just curious on your take..

[MasterSbilt_Racer]

If your solid rod theory applies to the solid rear axle in changing the effective roll center, why would it not change the effective roll center up front? If you move the upper shock mount it can change dynamic roll rate, making the car feel like it has a softer or stiffer spring, charging how fast and far that corner travels. Obviously it doesn't change the RC statically, but it doesn't change the rear RC statically by having a solid rod in place of a shock. I've heard many differing opinions on this, just curious on your take..

You answered your own question. On the front, it only affects wheel rate.

On the rear, the point that the mass of the car contacts the rear axle assembly is the upper coilover mount.

[let-r-eat]

Matt's example of taking the spring out on the right side is a perfect example of BilletBirdcages illustration. The intersection of the top spring mount and the line between the panhard bars mounting locations will determine the lateral roll center. If you don't have a right spring then the roll center will automatically be at that left spring mount intersection with the line drawn through the panhard bar mounting points.

I agree that Matt's example of the rear ends *contact patch* lateral location underneath those mounting points as migrating would certainly be accurate.

Everything there is definitely dynamic for sure and getting this roll center coupled correctly with the front lateral location is most certainly an important concept to wrap your head around IMO.