Not what I was referring to.
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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???