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.
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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.
Car would squat RR less during cornering and hike lr more.
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.
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
Correct, Rogue1
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
Good to see ya back, Billet
Good thread.
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/...splacement.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?
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.
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?
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.
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.
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.
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.
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
Then i guess we should all be running rabuds purple bar
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
Acording to this theory, would moving shock mounts on frontend also change moment center?
One night I found the roll center to be on top of my roof! The car wasnt too pretty after that.
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.
Couldn`t agree more matt49.
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..
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.
Spring length is not what he means when he says upper coilover mount. The car doesn't load the top of the spring, it loads the top mount. Spring length doesn't affect handling in any way unless you are so short that you're in a coil-bind situation or so long that you are in a spring bow situation causing interference with the shock body.
But pretty common spring lengths are LF-10, RF-12, LR-14, RR-12. Longer on the LF if you're down pretty low (like under 350).
Bcollins82,
Good question because I have thought about that also. If you had a 1000 pound spring on the RF, it would certainly decrease roll rate and visually it seems that it would change roll center also but this is never taken into consideration in front roll center calculations. I think the reason that it doesn't is because front roll center has to do with how the two moment centers (or front view swing arms) interact with each other. And for lack of a better way to put, the left hand doesn't really know what the right hand is doing.
Im not sold 100% that spring length doesn't affect handling. When computing roll angles of front and rear suspensions, you have to measure from the top of each coil spring to the ground. I do know from experience that trying to match roll angles from front to rear, produces faster and more consistent handling, esp when the track slicks off.
Top mounting location on coilover should be used in roll angle analysis. Length of spring doesn't matter. The mounting location is where the force is transferred through. You can turn the coilover up side down or however. The location of the mount is all that matters unless like Matt49 says you are rubbing somewhere.
A heavy front spring as described would add resistance to roll. The easy way to look at this is by roll angle effects on the wheel rates. The roll center and center of gravity location relative to the center of the contact patches.
I don't have any special tools, just a tape measure and a plumb bob, but I did some testing a while back on this subject, with no shocks on and the car at ride height on jacks, I used the method of determining the rear roll center to be half way between the pan hard mounting points, then dropped a plumb bob from chassis down through that line, as I jacked the car into dynamic roll, left side up and right side down, the plumb bob moved to the right, which to me means roll center moved to left, using the between pan hard mounting point theory. my question is, how do you incorporate the spring mounting points into this?
Yeah, that is what I had always been taught. And I know most people subscribe to that theory. But I've also had people in the industry who are much smarter than me argue that theory. Lol I asked one to please clarify this when he mentioned it. He simply said if you have 500s across the front, then put a 300rf and 700lf in the car will it change the point the car actually rolls around? Yes, obviously it does... He then said OK, any argument that front spring rate doesn't change the dynamic roll center is invalid. It was hard for me to argue against that point... Definitely an interesting subject.
There is definitely something to the fact that we are dealing with independent suspensions up front. That being said, the engineering books that teach where front roll center is also usually assumed symmetrical suspension all the way down to spring rates. So my argument for why it matters in the rear could certainly be extended to the front.
That got me thinking...These days, most folks agree that a softer RF spring helps the car turn on entry. This is contrary to the old school thinking "heavy spring gets the weight" that was applicable when the cars ran much flatter. But it does seem to work today. My theory on WHY has been 3 pronged:
1) More RF travel allows for more RC migration to a lower position
2) More RF travel usually yields more camber gain
3) More RF travel usually means the LR stays in hike and gets more rear steer
All of the above would help turn the car and supersede any losses in tire loading due to the softer RF spring.
But now I'm wondering if we should also consider the theory that the softer RF is moving the "real" roll center to the left (toward the stiffer spring) which is giving the moment arm more leverage on the RF contact patch.
Thoughts???