1969 Torino

[FETorino]

Originally Posted by FETorino

Uh Yea? How about that?

Originally Posted by Ron Sutton

I'm not sure what you said, let alone how to respond.

.

Hmm I know of a car as you described above It sounds familiar.

Oh yea that would be mine

Pretty much non adjustable track width without a involved.
Track in front a little narrower than the rear. Although I have tightened that up a bit by and looking at .

there has been a fair amount of discussion on the spring, shock and bar package for my set up.

So it seems that leaves us @

Tune with roll center. Yep that's it

[Ron Sutton]

Rob,

I get it now.

Yes ... your car's wider rear track width ... by 2" ... helps us free up the rear of the big girl a bit. This is a step in the right direction to help overcome the bigger rear tires, but by itself wouldn't be enough.

In the suspension set-up I outlined for you, I bumped the rear spring rate & sway bar rate a "tad" ... but raised the rear panhard bar (roll center) more ... also to help free her up.

Once you hit the track, we'll need to "tune on her" to achieve a good balance & optimum set-up.

.

[Ron Sutton]

Alrighty then ... let's talk Roll Centers.

I’ll be very basic for any readers following along that are completely new to this & apologize in advance for boring the veterans with more knowledge of this. Cars have two roll centers … one as part of the front suspension & one as part of the rear suspension. I’ll first explain what role they play in the handling of a car … then how to calculate them … and finally how to tune with them.

Think of the front & rear roll centers as pivot points. 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. 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.

If you were to draw a line parallel down the middle of the car connecting the two roll centers … this is called the "roll axis" … that line would represent the pivot angle the car rolls on … again “typically” higher in the rear & lower in the front.


.

[Ron Sutton]

When a car is cornering … the forces that act on the car to make it roll … 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 9” above ground … the car has 47% of the 3500# weight with 11” of leverage to roll the front 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 9”.

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 9” above ground … the car has 47% of the 3500# weight with 9” of leverage to roll the front 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.

We’ll discuss moving the roll centers in the final section. Next, let’s cover how to figure out where your front & rear roll centers are at.

---------------------------------------------------------------------------------------------------------------------------------

Front: Measuring all the pivot points in the front suspension to calculate the roll center in the front suspension of a double A-arm suspension car can be tedious … but the concept is quite simple.

Quick Acronyms:
UCA = Upper Control Arm
LCA = Lower Control Arm
BJC = Ball Joint Center
IC = Instant Center
RC = Roll Center
CG = Center of Gravity
CL = Centerline

Your UCA & LCA have pivot points on the chassis … and they pivot on the spindle at the BJC’s. Forget the shape of the control arms … the pivots are all that matter.

.

[Ron Sutton]

If you draw a line through the CL of the UCA pivots & another line though the CL of the LCA pivots … they will intersect at some point (as long as they are not parallel). That point is called the instant Center (IC) … and the UCA/Spindle/LCA assembly travels in an arc from that IC point. However far out that IC is … measured in inches … is called the Swing Arm length. More on this later.

Next you draw a line from the CL of the tire contact patch at ground level … to the IC. Do this on both sides … and where the two “Tire CL-to-IC” lines intersect … is the front roll center. Look at the drawing below. The colored dots represent the IC for the same color LCA/UCA. The black dot represents the static RC at ride height.

Make sense?

.

[Ron Sutton]

Now I’ll throw you a curve ball. The static RC at ride height doesn’t mean much. It is the dynamic roll center “in dive” that really matters. “In dive” means when the front suspension is compressed & the car is in roll. So in the corner … when you have the front suspension compressed & the car is rolled over … all those angles change … and therefore the roll center moves. It typically goes down … and may migrate to the left or right of center. In the drawing below, the car is making a right hand corner … compressing the suspension 2” in the center and rolling over at a 3 degree angle.

Again, the colored dots represent the IC’s for the same color LCA/UCA. See how the IC’s move the swing arm lengths change? The black dot represents the dynamic RC in dive. Notice the RC is lower but also “migrated” to the left.

* We’ll talk about how to tune with Roll Centers in the final section.

.

[Ron Sutton]

Calculated the rear is easier, unless you have a double A-arm rear independent suspension … then it is exactly the same as the front. For all other common rear suspension types, here are the quick methods.

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.)

The RC is located horizontally exactly in the center of the two panhard bar pivots … which is why it makes sense to have the bar centered in the chassis on street, road race & AutoX cars … so the RC is centered in the chassis. Some oval track cars use a J-bar, which is not centered horizontally, and therefore neither is the RC.

Watt’s Link: The RC is located horizontally & vertically at the center of the bell crank pivot that is attached to the rear end housing.

Leaf Springs: The RC is located horizontally in the center … halfway between the two sets of leaf springs. The RC is located vertically at the height equal to the mating line where the leaf spring connects to the housing spring pads. If lowering blocks are utilized, the RC height is in the center of the lowering blocks.

Triangulated 4-links: Draw a line connecting the IC of the lower set of trailing arms … to the upper set of trailing arms … and where that line crosses the axle CL … is the rear RC.

Diagonal Link: The RC is located horizontally & vertically at the center of the two pivots. If the diagonal link is centered, so will the RC be. If the diagonal link is at 6” on one side and 7” on the other … the RC height is at 6-1/2”.

.

[Ron Sutton]

Tuning with Roll Centers:

First off, most people find tuning with the front roll center difficult, tedious, confusing & laborious … and therefore they don’t do it much. I love those people as competitors because they’re easy to beat. Getting fast … faster than everyone else … takes work, testing, work, smarts, more work & more testing. And the front suspension … which is the most complicated … is the most important key to cornering performance. Because I understand it so well, that has been to my advantage over the years.

You don’t have to become a tuner to have fun with your Pro Touring car. You can buy & install many good suspension packages available on the market that have a “much better than factory” set-up for your car … because the aftermarket manufacturer worked out a good basic geometry package. The car will handle well, outperform most factory cars and be a lot of fun. Just don’t disillusion yourself into thinking you’re going to show up at serious competitions & beat the “thinkers & tuners” with a bolt on package.

If running “good” isn’t good enough, and you want to compete at a higher level and win events … you need to learn about suspension geometry & tuning … and do lots of testing & tuning. I figure I have over 2500 test days under my belt in my 35 years of racing. I’m not the smartest guy at the track … but when he goes home … I’m still there testing, tuning, learning & getting faster. To win … you gotta be willing to do the work. If you’re not, be clear on that and set your goals accordingly. We’re all here for fun. Some of us find the fun of winning worth the effort & sacrifices it takes to do so.

---------------------------------------------------------------------------------------------------------------------------------

Tuning with Front Roll Centers:

Let’s start with the understanding that to move the front RC … you are changing the angles of the UCA and/or LCA … to achieve a different IC. Some changes affect RC only dynamically in dive … while most changes affect the RC both statically at ride height & in dive. I’ll put an * next to the item that only changes RC dynamically & doesn’t show up statically.

What are (or can be) your tuning tools to change angles:
1. Spindle heights and/or distances from spindle pin to ball joint surfaces
2. Ball joint pin heights
3. Control arm length*
4. Adjustable control arm mounts on the chassis.
5. Also, obviously, any changes in ride height.

Direction:
a. Raising the RC, places it closer to the CG, reducing the CG leverage, reducing roll angle … and working the front tires less.
b. Lowering the RC, places it farther from the CG, increasing the CG leverage, increasing roll angle … and working the front tires more.
c. For faster corners found at big road courses I’ve found the happy window to be 1” to 2.5” … and -0.5 to 1.0” for tight AutoX events.
d. If the RC migrates to the inside of the corner under dive … it will work the front tires more … but roll more if not controlled by the suspension.
e. If the RC migrates to the outside of the corner under dive … it will roll less work the front tires & roll less.

*KEY NOTE: For optimum cornering ability, you need to WORK the tires … and low RC’s combined with big sway bars work the tires while keeping the roll angle low. In other words, don’t use the front RC as your primary tool to control the car’s roll angle.

For the hot rod I’m designing & building for myself, I have two front RC locations. For AutoX it is 0” in dive & 1.2” in dive for road racing & high speed events. All I have to do is change the slugs in two control arms & reset the toe & bump steer shim packet. Yes, I have big sway bars.

---------------------------------------------------------------------------------------------------------------------------------

Also know, when you are changing these control arm angles, you are changing the camber gain. Use this to your advantage.

A lot of stock production cars have the swing arms so far out … there is little to no camber gain … often camber loss. Plus, in many stock production cars the A-arm angles put the roll center so low it is below ground ... and the CG is high … giving it a ton of leverage to roll the car … which is part of why many stock production cars roll so much.

Typically, when you dial in your front geometry … you’re goal is to place your RC for optimum handling for the type of driving you do (or find the best compromise) … and end up with the desired camber gain.

Some quick tips:
Anytime you’re shortening the “swing arm” … you’re increasing camber gain … regardless of how you did it.
Anytime you’re shortening the swing arm length … & keep the IC at the same height … you’re raising the RC.
Conversely, lengthening the swing arm length … & keeping the IC at the same height … lowers the RC.
Anytime you’re raising the IC of the swing arms … and keeping the same swing arm length … you’re raising the RC.
Conversely, lowering the IC of the swing arms… and keeping the same swing arm length … lowers the RC.

There are several software programs out there to calculate roll centers. I own & use several. I suggest Performance Trends to car guys & gals often because it is the easiest to use.

*KEY NOTE: When you change the swing arm IC … length or height … you are changing the bump steer … because you are changing the arc the UCA/Spindle/LCA assembly travels in. Another reason why so many people don’t like tuning on it.

For the hot rod I’m designing & building for myself, I have two front RC locations & have worked out the bump steer for both setups … which includes a tie rod slug & shims. I simply keep a “set” for each of the 2 RC locations, making the change over quicker & easy … after the initial work is done.

Last Key Note about Front Roll Centers
once I have tested & worked out an optimum front end set-up for a type of track … we “lock it in” and don’t change it at the track. So this is NOT something you’re constantly tuning on … just initially. At the track, the optimum front end geometry is the optimum front end geometry … so as the track changes throughout the day … we’re tuning on other stuff to keep the car “balanced”.


.

[Ron Sutton]

Tuning with Rear Roll Centers:

If you have a double A-arm independent rear suspension … then it is exactly the same as the front. Change the term from “spindle” to “upright” and party on. Everything else is the same.

For all other common rear suspension types, here are the quick methods to tune.

Panhard Bar/Track Bar: Each end has a rod end as a pivot point. In race cars, we use a variety of different styles of mounts on the chassis & housing that allow us to relocate these pivot points up or down on both sides. I see a lot of aftermarket suspension kits & frame clips that have no adjustability.

My first thought is “what the … @$&% !”

Then I realized they’re doing this so tuning rookies don’t get themselves in trouble. Frankly, I can not fathom not having the rear roll center adjustable. It is one of the most predictable tuning tools & with the right mounts, simple, quick & easy. It is so easy, that in racing circles, if a racer is “lazy” that’s the tuning tool they use first & most, which isn’t correct, just reality of human nature.

Watt’s Link: Very simple to move the bell crank pivot up or down, as long as there are holes available.

Leaf Springs: Do not have an adjustable RC unless you change the mounting points of the springs or use lowering blocks.

Triangulated 4-links: Do not have an adjustable RC unless you change the mounting points of the links. Great for street performance cars with owners not desiring to tune.

Diagonal Links: Are for drag racing … as they are not independently adjustable for RC in the typical mounting. They mount to the ends of the rod end bolts, so change only when the lower bars are being moved to different holes.

The two best methods of centering the rear end … and have tunability … are the Watt’s link & panhard bar. Many people favor a Watt’s link because it keeps the rear end perfectly centered during travel & roll. I like them, but they’re not my preferred method, because the method of mounting leads to limited adjustability … sometimes only a few holes … farther apart than I like to make changes … and often mounted with the RC too high for modern low roll suspensions.

This limited tunability of the Watt’s link requires the tuner to rely more on other tuning items such as springs, shocks & sway bars. It reminds me a little of torque arm suspensions. They work well, just not much tuning adjustment. I think they’re the ticket for car guys that want to get a good set-up & just drive it.

As a tuner, I like to have a full tuning “tool box” at my disposal, and a panhard bar (or track bar, depending on where you came from) is more tunable. With the right adjustable mounts, I can move the RC 1/16” if I wish. I can make it super low … super high … or anywhere in between.

If … for any variety of reasons … the car is working the rear tires different in RH corners versus LH corners … I can put a little “tilt” in the panhard bar … while keeping the same RC height … and even out how the car works the rear tires in RH & LH corners, making a more balanced, faster track car. Just lower the bar down on the side you want to work the tire more … and raise the bar up on the opposite side the same amount. If I’m at a road course where I need a “lil sumthin” extra in one corner, I can achieve that with a little panhard bar tilt.

As far as keeping the rear end centered … with the newer technology low roll suspensions … if the outside rear tire is compressing much more than an inch during cornering … I’ve got bigger problems than the rear end shifting a few thousandths off center.

Direction … regardless if you’re using a Watt’s link or panhard bar:
a. Raising the RC, places it closer to the CG, reducing the CG leverage, reducing roll angle … and working the rear tires less.
b. Lowering the RC, places it farther from the CG, increasing the CG leverage, increasing roll angle … and working the rear tires more.
c. With low roll suspensions utilizing mean stiff rear springs … or medium springs & significant rear sway bar … for faster corners found at big road courses I’ve found the happy window to be 9” to 12” … and 7” to 10” for tight AutoX events.
d. Softer sprung, higher roll rear suspensions run higher rear RC’s.

*KEY NOTE: 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 tire to roll over so much that the car rolls diagonally … and unloads the inside front tire.

Old school, conventional, soft sprung rear suspensions achieve this with higher rear RC’s. Modern low roll suspensions achieve this with stiff rear springs (or or medium springs & significant rear sway bar) to control the roll angle, while utilizing a lower RC to work the outside tire more. In other words, modern suspensions don’t use the rear RC as the primary tool to control the car’s roll angle.

For the hot rod I’m designing & building for myself, I have two rear baseline RC locations. For AutoX it is 8” & for road racing & high speed events it is 11”. Because both sides adjust easy, it is about a 2 minute change.

Please don’t use these numbers for your car. Suspension set-up is all about the total package and not piece meal a set-up together.

---------------------------------------------------------------------------------------------------------

Explaining what roll centers are, how to calculate them & how to tune with them ... front & rear ... was a little complex but very important.

This post got into the detail of roll centers (RC) ... but I think it's helpful for car guys & tuners to take several steps back & look at the big picture of handling ... to better understand the role of the RC with the rest of the car.

Total weight ... weight distribution front to rear ... and height of this weight (CG) act like a lever over the roll center. 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. BUT ... and this is KEY ... modern day tuners do not use the RC height as the primary means of controlling roll angle. They use the suspension tuning items as their primary tools & the RC height as a secondary tool.

So to achieve the optimum balance of roll angle & working the 4 tires optimally ... this all has to work with your suspension ... springs, anti-roll bars & shocks ... and track width ... to end up at the optimum roll angle for your car & track application.

Hopefully, this brief overview, helps clarify roll centers place in the tuning picture.


.

[Track Junky]

Ron you are the BOMB!! Cant thank you enough for sharing this info. I'm going to have to spend some time confirming my prior notions but I'm pretty sure if I drop the front of my car another inch I should be able to achieve the dynamic camber I am looking for through my control arms without putting so much static camber into it. I'll be testing at T-Hill on August 11th so I'll keep you posted.