Rear roll center

[drtrkr244]

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.

MBRacer, do you think that is the main reason chassis builders designate rear spring lengths?

[Matt49]

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).

[Matt49]

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.

[drtrkr244]

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).

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.

[MasterSbilt_Racer]

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.

I believe it was carerra that did testing with data acquisition at Eldora and proved to themselves that spring length didn't matter.

[let-r-eat]

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.

[fastford]

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 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?

[Bcollins82]

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.

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.

[Matt49]

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???

[MasterSbilt_Racer]

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???

What happens when the lf leaves the racing surface?

[Matt49]

My hope is that it doesn't. But if it does, I hope I'm done turning. My feeling is the roll centers further to the left are assisting in keeping the LF down all the way around the track as opposed to the older setups where it was sometimes never on the track. To lift the left front, the roll center would have to be pretty far to the right assuming you aren't getting a lot of help by squatting the RR...something else we're seeing less and less of.

[save the racers]

I tried these theorys on a car.The problem is when you change r/c it will change other things.Camber curve ,bump steer etc in the front,motion ratio it the rear.The car responded in the rear like the spring mounting points set the r/c.If you are going to offset the rear r/c be ready to put springs in it that are outside the box to normal.You want to now where the r/c is mostly because you don't want it to be in an odd location causing the car not to respond in a predicable manor.

[let-r-eat]

I agree with your comments Matt49 but why do you think the old saying of "the stiff spring gets the weight" actually happens?

Wouldn't it happen because of the reasons you just described?

[7uptruckracer]

A LF oriented Roll Center puts more weight jacking into the LF the extra RF travel we are seeing would raise the dynamic RF upper control arm angle which will tend to migrate the RC right as well which would take off some LF loading BUT at the same time the RC is dynamically lower which will help load the front end.. So by moving it farther left you are in a way accounting for that migration. I do think MATT hit on the tendency for it to travel lower which a lower roll center will definitely make the front end steer and load in a more positive manner good point. Another thing that was hit on was the Camber gain. With the extra travel we are seeing you tend to see more dynamic gain and you don't see the RF bleed back as much height as we use to on the straights the RF stays pretty pinned. We use this on asphalt front ends a lot. You don't have a tire transitioning through a camber curve as you are trying to turn it you are already at optimal camber for corner entry at the moment you turn in. Also higher camber settings like matt has discussed in other threads with Camber Thrust theories. I think we are seeing the higher camber to 1) Load the tire more and 2) Like we use on asphalt to stretch our contact patch which i think helps fight a dirt tires tendency to deflect which will roll up under the tire and distort your contact patch You are seeing more LF rebound and softer compression so you aren't rolling the front ends as much as before either that helps with Roll Center Migration. Just my 2 cents. I don't pay attention to all the rear design i just tune how it was built for the power and package I have lol You can really make that act so many different ways it ends up being a mute point just know how to tune it for where you need to be

[FlatTire]

Track width hasn't been mentioned, but it also plays a role in how the lateral roll center migration effects chassis roll.