cosmick

What's the value of lowering it? What tangible result will be immediately obvious, how does the chassis response feel to the driver in daily spirited driving? Stock versus 3 inches lower?

TEDSgrad

iTrader: (4)

Roll Centers = Cars have two roll centers … one as part of the front suspension & one as part of the rear suspension, that act as pivot points. Because the front & rear roll centers are often at different heights, the car rolls on different pivot points front & rear … “typically” higher in the rear & lower in the front. When the car experiences body roll during cornering … everything above that pivot point rotates towards the outside of the corner … and everything below the pivot point rotates the opposite direction, towards the inside of the corner.
Roll Angle: is the amount the car “rolls” on its roll axis (side-to-side) in cornering, usually expressed in degrees.
Pitch Angle: is the amount the car “rotates” fore & aft under braking or acceleration, usually expressed by engineers in degrees & in inches of rise or dive by racers.
Optimum roll angle works both sides of the car’s tires “closer to even” ... within the optimum tire heat range … providing a consistent long run set-up & optimum cornering traction.
The car’s Center of Gravity acts as a lever on the Roll Center … separately front & rear. Higher CG’s and/or lower RC’s increases roll angle. Lower CG’s and/or higher RC’s decrease roll angle. Getting the front & rear of the car to roll similar is desired. Getting them to roll the same is not, because …to have optimum grip on all tires and disengage the inside rear tire (to a degree) to turn well … then re-engage the inside rear tire (to a higher degree) for maximum forward bite on exit. So, on entry & mid-corner, the car needs to roll slightly less in the front to keep both front tires engaged for optimum front end grip, while allowing the car to roll slightly more in the rear to disengage the inside rear tire, to a small degree, to turn better. For optimal exit, the car will have more roll in the front & less in the rear to re-engage the inside rear tire to a higher degree than it was on entry & exit, for maximum forward bite (traction) on exit.

The forces that act on the car to make it roll … when a car is cornering … … act upon the car’s Center of Gravity (CG). With typical production cars & “most” race cars, the CG is above the roll center … acting like a lever. The distance between the height of the CG & the height of each Roll Center is called the “Moment Arm.” Think of it a lever. The farther apart the CG & roll center are … the more leverage the CG has over the roll center to make the car roll. Excessive chassis roll angle is your enemy, because it is over working the outside tires & under utilizing the inside tires.

Some people like to look at the car as a unit. I look at it as two halves. Here are some examples … using a typical 3500# Pro Touring Car with 53% front weight… to provide more clarity:

If the CG is 20” high … and the front roll center is 2” below ground … the car has 53% of the 3500# weight with 22” of leverage to roll the front of the car.
If the CG is 20” high … and the rear roll center is 11” above ground … the car has 47% of the 3500# weight with 9” of leverage to roll the rear of the car.
* Rolling the car that much more in the front overloads the outside front tire & under utilizes the inside front tire when cornering.

If you lowered the car 2” … the CG drops 2”. The front roll center probably moved too … but it’s not linear … it is based on A-arm angles. Let’s say it dropped 1” in the front to 3” below ground and the rear stayed the same at 11”.

Now …
If the CG is 18” high … and the front roll center is 3” below ground … the car has 53% of the 3500# weight with 21” of leverage to roll the front of the car.
If the CG is 18” high … and the rear roll center is 11” above ground … the car has 47% of the 3500# weight with 7” of leverage to roll the rear of the car.
* The front now rolls over less & the rear too, making the car run “flatter” … not flat, just less roll angle … working the inside tires better.

Any weight you can remove from high up … or relocate to lower in the car … moves the CG down … reducing the leverage it has over the roll center … allowing the car to have less roll angle during cornering … working all four tires more evenly … and the grip of four tires is faster than two.

Panhard Bar/Track Bar: The RC is located horizontally & vertically at the center of the two pivots. If the bar is level and both sides are 8” off the ground … the roll center is 8” above the ground. If the bar is at an angle with one side at 11” & the other at 12” … the RC is at 11-1/2”. (But this angle will make the car handle differently on LH & RH corners.)
For optimum cornering ability, you need to position the rear RC low enough to work the rear tires … allowing just enough rear roll angle to disengage the inside rear tire to a degree … but high enough to prevent the outside rear suspension from compressing so much that the car rolls diagonally … and unloads the inside front tire.
Total weight ... weight distribution front to rear ... and height of this weight (CG) act like a lever over the roll centers. As discussed earlier, lowering the CG shortens that lever, as does raising the RC ... but works the tires less. Raising the CG lengthens that lever, as does lowering the RC ... and works the tires more.

Your goal is to move them both... to the degree possible ... where you find the optimum balance of working the tires & roll angle.

This is Ron Sutton text not my own. Everyone should go over to Lat G and read his stickies in the chassis and suspension sub-forum. You have to realize that he is a double a-arm guy, so you have to be able to chew the meat and throw away the bone as it relates to our McStrut cars.

cosmick

I appreciate your effort, but I still don't understand what lowering the rear roll center, without lowering the ride height, will do for how my '86 Camaro would behave in my local canyons?

Thirdgen89GTA

iTrader: (5)

Per what was written.

Lowering the roll center, without lowering the Center of Gravity will increase roll angle. Aka, the car is going to have more body roll in corners, further loading the outside tires, and reducing weight on the inside tires. To counteract this you'd need to either stiffen the springs or swaybar to compensate.

Or, for simpler terminology, your rear end is going to jump around alot more, its going to lean more, its going to react more strongly to directional changes (ex when slaloming)

TEDSgrad

iTrader: (4)

Yup, it's in there.

If you only lower the roll center and keep ride height the same (CG), you increase the leverage and roll more.

Roll, roll me away,
I'm gonna roll me away tonight
Gotta keep rollin, gotta keep ridin',
keep searchin' till I find what's right
And as the sunset faded
I spoke to the faintest first starlight
And I said next time
Next time
We'll get it right

cosmick

So Founders makes a panhard relocation kit for no good reason? Good to know, nearly wasted some coin. Thanks!

TEDSgrad

iTrader: (4)

If used in conjunction with lowering, it is very effective. Like so many suspension components, it's the combo that produces results. Roll center adjustments are a very effective tool, but not the only tool.
Again, those stickies are very worthwhile to read.

Alice89

iTrader: (6)

cosmick, are you talking about this: http://www.foundersperformance.com/p...elocation-kit/

If so then that's more for exhaust clearance... It's not really meant to lower your rear RC like the jegs or UE piece.

I can tell you that the jegs piece in conjunction with heavier rear springs lessened brake dive coming into corners & kept the rear of my car planted more in the corners. It was a very noticeable improvement for just an $80 bracket.

Lonnie P

Typically when you lower a car, the panhard is no longer level. You can lower the axle side to keep it level, which is 1 way to use a panhard relocation bracket.


A second way would be to lower the roll center to alter the handling, by lowering both sides of the bar. Typically lowering the rear roll center will cause more rear body roll & resultant understeer in a stock application. Now if you increase the rear spring rate and/or rear swaybar to compensate for the added body roll, you can alter the handling in a positive way. There is not 1 correct answer as you have to tune for your driving style, tires, track etc. It is somewhat trial/error, but many proven setups are listed on this site, which provide a great starting point.

TEDSgrad

iTrader: (4)

Yeah, if that is the part, NOSHOWALLGO is correct that it isn't designed to alter RC.
If you want to keep your ride height, the other alternative is to remove high weight (that lowers CG): Lexan back hatch, Lexan T-tops, fiberglass hood, even adding some lower weight (sound deadening material to floor pan). All of these lower CG and lessen the leveraging going on. But lowering ride height is the big one.

Maybe understanding the front RC will help with understanding the rear RC.
In the front, the RC is with the worms - a negative number, -2" or so. Here, raising the RC closer to the CG really helps (DROP SPINDLES!). It is the relationship that counts. In the rear, the RC is,say, 14". As stated, you don't/can't get them equal, you just want to improve the ratio between them - RC and CG. AND, the rear RC change should not be that great; small changes yield big results. I'm not a fan of lowering bracket on both chassis and axle side - but some people use them. In the rear, you can't lower RC and lower CG a lot. If you really lower the RC a lot in the rear, you risk diagonal roll resulting in unloading the front inside tire (weight is shifting from fr inside tire to rear outside tire).

SlickTrackGod

That Founders piece DOES change rear RC height. They did not manufacturer it with any understanding of how it can cause danger to a car if someone does not know what they are doing. THis could cause an already "tight" (understeering) car not to turn even more so.

The JEGS unit allows for adjustment range as well as better level of the panhard rod in conjunction with parallel to the ground.

Now up to the Ron Sutton quotes. A lot of what is typed there is very generic. He really only delves into "static" form of RC's and overall roll axis. He talks about pitch and roll, but no YAW. YAW is a very important and overlooked part of a roll axis- it is mostly only in dynamic motion, but can be used in static form is specific applicationms like circletrack.(No need to get into that part here though.- just dynamic form which happens with any suspension vehicle)

You roll leverage as he lists in percentages does not work in linear numbers how his examples list. It is fine for a very basic generic understanding, but they can and often do change dramatically from static though into dynamic reality. How? I will give an example.

The rear RC alone is said to be center bar length and center vertically of the pivots which is the same thing (the quote above- Panhard Bar/Track Bar: The RC is located horizontally & vertically at the center of the two pivots. If the bar is level and both sides are 8” off the ground … the roll center is 8” above the ground. If the bar is at an angle with one side at 11” & the other at 12” … the RC is at 11-1/2”. (But this angle will make the car handle differently on LH & RH corners.) THIS ONLY IS TRUE IF SPRING LOAD RATES ARE THE SAME AT STATIC RIDE HEIGHT. They ARE NOT the same in dynamic loading into a croner, steady state, or corner exit. The chassis Roll Axis is different on all four of these senerio's all with the same "setting".

I knew this would require a novel, so here goes more typing-

What happens when a car rolls upon a panhard rod setup is the outside spring will load and increase overall spring rate more that the less loaded inside rear spring. This imbalance of spring loading due to body roll will MIGRATE the rear RC of a panhard rod towards the side of car (coming off centerline symmetry) and favor the side that is greater load.... So what happens is the overall roll axis from front RC to rear RC now YAWS. THose leverage numbers in change during braking, they change again during steady state, they change again during throttle application off the corner exit as the car rolls out of the corner. It is in no way a static numbers thing and is NOT WAY LINEAR.

Now weight of a vehicle and how the weight is controlled is every bit of influence on roll centers and mainly a roll axis. Just because it works for one car with certain weights, spring rates, swaybars, tire offsets, tire grip, stagger rim widths, and shock damper values both comp and rebound- does NOT mean it will work for someone else's car.

RC and Roll axis movement are purely dependant on many many factors all acting at the same time all in conjuction upon the RC's and thus Roll axis.

By lowering the rear RC only makes for the need of larger sway bars which is bad for a car by locking up the independant wheel travel of each wheel in relation to the body. this lockup causes loss of mechanical grip over imperfect roads. Best to limit RC travels (vertically that is- some lateral movement can be advantagous if you know how to use it but this is very advanced setup knowledge) I generally propose people with 3rd gens use the combination of extended front ball joints with the addition of the rear Jegs axle side PH relocator so as to not increase the need for larger sway bars and reduce positive roll weight "ESPECIALLY" on the heavier V8 cars.

TEDSgrad

iTrader: (4)

Thanks Dean,
Yeah, it's generic description from static - I always need a basic framework of understanding before I can bolt on more complex components.
Good points to keep in mind that dynamic is much more complex. Would your definition of YAW be similar to diagonal roll? - rr->lf; lr->rf?

Since pics/visuals are always food for understanding, here's the Jegster (axle side only bracket) just before welding.


After as I'm prepping the LCARB's before weld


And a pic showing the exaggeration of the panhard 'slope' - I wrestled the 12 bolt up myself and it's on stands as I bolt things up.


You can imagine what dynamic movement would do to the panhard.

SlickTrackGod

If you lowered the rear RC 3 inches on a stock height stock suspension car the steering wheel would get so heavy in effort and the car would not want to bite the inside front tire in order to try and turn the car. The outside rear tire would be leveraged so much the inside front would lift pressure and the right front would be overloaded in trying to turn left. The car would feel like it only wants to go straight.

SlickTrackGod

Yes, it is somewhat probably best described as diagonal in layman's terms.

Pitch, yaw, and roll. Pitch is nose up, nose down. Roll is roll left, roll right, yaw is nose left, nose right. This yaw ofd the roll center would pertain to the centerline of the vehicle and how much off center the front is left or right in proportion to how much off center the rear rc is left or right.

The front RC pretty much ALWAYS migrates off center. The bigger issue with the front RC is its ability to drop rapidly on nose dive under hard braking. This front pivot point will drop much more rapidly then the CG of the car will. Thus when braking and going into a corner the nose of the roll axis drops and canters to the inside-thus- throwing the weight of the chassis motion up over diagonally and onto the outside front tire overloading it.

I will show a few diagrams that will show the front RC of a McPherson Strut car looking from the front view as the car roll into a left corner and leans right.

Note the a-arm level 1) how they differ, 2) how the inverted relation drops the RC into the dirt real quick.

Note the strut 1) angle- more angle is more caster and thus raised the front RC, 2) the strut length - more strut length reduces camber and lowers the RC.

The RC is made up of the A-arm line intersecting the 90* strut mount pivot line to make up the instant centers ( IC) of each side of the front suspension. Then you draw lines front he individual IC's back to their center tire contact footprints. Where they intersect is the front RC. Note when the car rolls left and right suspension geometry is no longer symmetrical like it was at static. The front RC generally always goes towards the inside of the corner making the outside wheel under more leverage in a teeter tooter form.

NOW, when you migrate the rear RC in the opposite direction, the inside rear wheel chassis weight is increased due to leverage and holds down the weight front tossing over to the outside front (IF the car does not nose dive and cause the inside rear to toss itself way up high in canter) The front RC height needs to stay in check level wise because geometry will drop it at a much faster rate then proportion to the rear RC. This is where extended front ball joints come into play keeping the front RC up and more consistent to the static height and also to centerline.

I really should draw 2 more picture examples but I am not taking that time to

One shows a lowered car with inverted a-arms, the other shows more of a stock height car after braking release and in steady state. The 2nd would be much lower by far under dynamic braking before it settled to this point back up and over to the inside on a left corner.

SlickTrackGod

Now to take this further (since I saw Lonnie posted and it made me think of his watts link)

The watts link ALWAYS remains the rc at centerline (reverse watts is different story but I will not address that here for now) What this means is the car's rear RC stays put left to right and makes for a very predictable feel to the driver in slallom imputs of the chassis changing direction rapidly. THis does however call for the need of a larger rear swaybar to control the increased leverage since the RC no longer migrates towards the spring rate with higher loading values.

Now with all that said, the watts is a good addition to a car under normal cornering when entering a corner under braking and steadystate, but the position of the watts RC makes for a lively outside rear wheel under throttle. The inside rear does not load as well in reverse chassis loading of throttle application.

I know that is hard to grasp. What a watts is good for is basically it firms up the level of an OEM 3rd gen platform because ion stock form the cars suffer from notorious brake dive and a plow-itno-snap overseer situation to get them to rotate. The watts keeps the rear RC from migrating in after the Roll axis has inverted at such an angle under nose dive, If the rear RC migrates out under that condition the inside rear is tossed so high into the air it finally snaps around uncontrollable. The geometry of the watts keeps the inside rear from jacking as high under severe nose dive. It does nothing however to leverage the rear chassis weight when you use this as a tool to load tires for heat proprtion in long term racing temps. Hard to get heat into an inside rear wheel with a watts setup.

Why is a certain amount of inside rear wheel leverage good? Most corners on a road have at least a little degree of banking- unless you are strictly talking street light corners and autox type racing. Pretty much most others will have a bit of banking even in the slightest form in high speed cornering. A watts does nothing in banked compression wheel loading. THis is a leverage point that a panhard rod shines in due to the outside migrated leverage. It also helps in rear inside wheel traction under braking. Each have their advantages and disadvantages.