Rear roll center

[Pete]

Got a question. What would it do the handling if you moved the rear roll center to the right 2" roll center height stayed the same? On a rocket blue front.

[MasterSbilt_Racer]

Car would squat RR less during cornering and hike lr more.

[SuperEight]

It should increase drive and make the car calmer, but a lot depends on how you do it... If you do it by shortening the j-bar then forget what I just said, the car will be wilder and hike up more. If you somehow shift your current j-bar 2" to the right and everything else stays the same(length, angle and heights), then what I said at first would apply. Many variables play into this depending on how you do it.

[Rogue1]

I thought the upper pick up points of the springs determined where the roll center was from left to right, and the jbar determined the height

[Matt49]

Correct, Rogue1

[billetbirdcage]

Car would squat RR less during cornering and hike lr more.

I'd agree with this.


As to how the rear roll center is figured well: I have issues with the newer method that roque and matt are talking about. While I do think it helps people visualize what moving the frame side of the j-bar does as far as raising or lowering the roll center, I think it's vastly wrong on left to right.

That's my opinion and I'll leave it at that

[Dirt2727]

Good to see ya back, Billet

[let-r-eat]

Good thread.

[Matt49]

I didn't have much time when I posted before but here goes my best effort at trying to explain what I THINK is going on here.
Unfortunately, there isn't a lot of literature out there on this and most of it is in automotive engineering text books which when dealing with panhard bar beam axle suspensions, are dealing with very long bars and equal spring rates on both sides of the car. This greatly simplifies the roll center determination because almost invariably, it will be at the center of the car. For example:

http://www.timskelton.com/lightning/race_prep/suspension/images/panhard_displacement.jpg

In this crude but common example, the roll center is in fact on the point midway between the ends of the bar. I think this is where a lot of people get the notion that the roll-center on a late model is the point midway between the ends of the j-bar. But it simply isn't so. That roll center would be so far to the left and so high that the car would never transfer weight to the right side.
In modern dirt cars, we use a much shorter bar with a lot more angle and very often have a difference between the spring rates on each side. This complicates things.
With a beam axle configuration, spring rates and their lateral mounting location certainly play in part in determining roll center. In general the roll center will be closer to the heavier spring. Think of how the car would roll if you had no spring on the LR and a solid rod where your shock is on the RR. Left side would go up while right side stayed basically in place. Then think the opposite. Solid LR and no RR spring. Left side would stay and RR would go down. Obviously, the roll center isn't in the same place in these two scenarios.
Roll center height is still dictated by the points of the ends of the j-bar but you have to draw an imaginary line between the points and extend that line BEYOND the ends of the j-bar. In many cases, the roll center may not actually be between the ends; it may be past one end (e.g. further to the right of the pinion mount).

People talk about front roll center migration a lot but my goodness can you imagine how much the rear roll center migrates with all of the hike up the rear end gets?

[MasterSbilt_Racer]

Both shock mount and panhard/j bar location affect lateral roll center location. Don't ask me to show the exact location, but I have plenty of experimental data to see it.

[hpmaster]

Like others here I have heard several ideas/theory on what puts the roll center where in the rear of the car over the years of reading and classes I have taken. I do not have enough confidence after all this to say X makes Y happen for sure so I look at these roll center talks on the rear as good ones to sit out. Who here has what they think is a good cause and effect chart on changing the rear roll center by changing the different components that affect rear roll center location?

[billetbirdcage]

I couldn't find the picture of the system to find the rear roll center I was talking about, so I had to remake it. I think it used to be on afco's tech section as well as many others but seems to be wiped from the internet as it should be, lol. That is the one I was talking about, which I believe to be idiotic and not correct at all.

That one basically said the roll center left to right was at the top of the RR spring.

[billetbirdcage]

In my view, the most accurate is this one. However as you can see it leaves open a large area that it could be, which I think is really a mute point anyways. What I care about is tire loading more so then where the roll center actually is. You have a lot more forces going one with a current LM set up, then just roll loading of the tires. Where the J-bar is attached compared to the tires and the angle of the j-bar has a huge effect on tire bias loading.

I also think this is basically what matt was referring to also, I just wanted to makes sure people weren't using the method that I posted above with with intersection of the RR top of the spring as the defining place of the rollcenter.

[b1driver]

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.

[zeroracing]

The CG is 100% independent of any linkages, shocks, springs or roll centers on the car. It is only the center of the sum of all the masses.