Rear roll center

[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).

[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?

[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.

[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.

[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..

[b1driver]

Couldn`t agree more matt49.

[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.

[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.

[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.

[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.........

[FlatTire]

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

[MasterSbilt_Racer]

Then i guess we should all be running rabuds purple bar

Not what I was referring to.

[b1driver]

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

[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]

Then i guess we should all be running rabuds purple bar