1969 Torino

[Track Junky]

Originally Posted by 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?

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This is great stuff right hear Ron and please correct me if I'm wrong but it is darn near impossible to find IC with all the sheet metal in the way?........which in turn makes it impossible to find static RC. I'm getting ready to transform the front of my stock subframe from conventional spring and shock to coil over. Maybe I should p/u another stock subframe and start from there?

Also, when drawing the lines through the UCA & LCA's at the outside connection points of the arms are you drawing through the center of the connecting point at the top of spindle/ball joint?

Maybe I missed it somewhere but now that we know how to get roll center how do we find center of gravity?

[Flash68]

Gaetano's case for the suspension sticky gets better with every one of Ron in Norcal's recent posts.

[fleetus macmullitz]

Originally Posted by Flash68

Gaetano's case for the suspension sticky gets better with every one of Ron in Norcal's recent posts.

Keep tuning in to your favorite suspension forum.

https://lateral-g.net/forums/forumdisplay.php4?f=38

[Ron Sutton]

Ron

Starting with the first statement about looking at the car in two halves. Correct me if I am wrong but I'm assuming you are speaking about the need to tune each end of the cars roll resistance based on each ends independent RC and not that the effect at one end is independent of the other end of the car.

Rob,

Most of that statement is correct. Parts are not. One of my friends said, “Ron is a race car designer that did not go to engineering school, so he speaks car guy". Because I like to make complex things simple to understand, I’m going to explain this in layers, using car guy terms we all can relate to.

1. Each end of the car has a “mind of its own” about roll angle, due to differences front to rear … in track widths, CG heights, roll center heights, spring rates, shock valving & sway bar rates.

2. The ends of the car do “want to” roll differently … and each end would roll to its own desired roll angle … if there was a pivot in the middle of the car.

3. But there is not a pivot in the middle of the car … exactly. Chassis rigidity … specifically torsional rigidity … plays a role in how tied together the front & rear of the car are.

If the car with a 50/50 weight balance & had 100% chassis rigidity (it does not) … then the car would roll to an angle that averaged the roll angles of the two ends. In other words … if the front end “wants to” roll 1.5 degrees & the rear end “wants to” roll 2.5 degrees … if the car was 100% rigid & had a 50/50 weight balance, it would achieve an averaged roll angle of 2.0 degrees unilaterally.

But all car chassis flex or twist when the forces are different front & rear … including race car chassis. Stock production cars flex & twist a lot when high powered engines, grippy tires & performance suspensions are added … and drivers push them to their limits. Well designed race cars flex & twist very little … but they do flex & twist to a degree. Chassis designs differ …therefore so does the torsional rigidity differ between chassis brands & designs.

Without complicating the discussion by trying to quantify numbers … we could easily envision this example as a race car with a relatively stiff chassis achieving a 1.9 degree roll angle in the front & a 2.1 degree roll angle in the rear. This is just an example so everyone understands the concept. A more flexible car could & does achieve a higher degree of roll angle difference.

But … we still see the rear of this example car with a higher demand for roll angle … is “pulling” the front end with it. If we’re trying to get the car to run flatter … the solution is in tuning the rear suspension, not the front. So yes, we need to tune each end of the car independently … but so as to work together in harmony … because each end can & does affect the other end.

Did that make sense? Please feel free to ask about anything I didn’t state clearly enough.


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[Ron Sutton]

Part 2 of 3


Now would be a good time to discuss chassis rigidity. Like most complex things, it makes sense to peel the onion a layer at a time. So bear with me as cover several key points.

Key Note: In my explanation about chassis rigidity … I will refer to “energy loss.” Think of engine power as energy & speed as stored energy.

Rule of thumb on chassis rigidity:
The more rigid the chassis … the less energy loss the chassis experiences … the quicker & faster the car “can be” … if tuned optimally inside its handling “sweet spot.” But, the optimum handling sweet spot gets progressively smaller & smaller as chassis rigidity increases … as the car becomes more responsive to tuning changes with larger effects.

The less rigid the chassis … the more energy loss the chassis experiences … reducing how quick & fast the car can be … even when tuned optimally inside its handling “sweet spot.” But, the optimum handling sweet spot gets progressively larger & larger as chassis rigidity decreases … as the car becomes less responsive to tuning changes with smaller effects.

If the chassis is too stiff, the sweet spot gets super small & the car is too sensitive to changing track conditions. I bought a race car once like this. We used to say, “It’s the fastest car on the track for 10 laps. But as rubber gets laid down over the course of a main event, the car’s handling changed too much. That car was great for winning poles & but difficult to win races with.

On the other hand, if the chassis is too soft … often termed a “flexi-flyer” … the sweet spot is super wide … because the car is not very responsive to tuning changes. I’ve been hired to help tune on these cars and the driver doesn’t feel a 50# spring change.

Let’s talk chassis rigidity … but instead of trying to use torsional rigidity formulas & numbers, I’ll use generalizations that will make the concept easier to understand & discuss amongst us car guys. To describe levels of torsional rigidity let’s keep it simple by using these general terms, like a scale of stiffness in this range:

Extremely Stiff
Very Stiff
Moderately Stiff
Intermediate Stiffness
Moderately Flexible
Very Flexible
Extremely Flexible

And let’s embrace these terms are relative to the application. Here is an example …

In drag racing at the IHRA Pro/Stock & Pro/Mod level in the 1980’s … which raced mountain motor cars, with cubic inches ranging from 615” to 672” the “standard” Pro/Stock chassis design flexed too much, and was hurting 60’ times & therefore overall ¼ mile times. Chassis builders strengthened the torsional rigidity of the chassis by building a narrow version of a dragster chassis in the transmission & driveshaft tunnel area … connecting the 4-link to the rear motor plate. It basically added upper frame rails down the center of the cockpit along with uprights & diagonal braces connecting the two existing lower center frames rails with these new upper frame rails. It looks like the front half of a dragster chassis in the middle of the car.

There were called “Double Frame Rail” cars & measurably improved the 60’ & ¼ mile times of Mountain Motor IHRA Pro/Stock & Pro/Mod drag cars. When NHRA Pro/Stock racers bought & raced them, they didn’t go any quicker. In fact, they didn’t like them, because they became more finicky to tune the suspension & and went away from that design. (I do not know what they run today, as I haven’t been to a drag race since the end of 1987.)

So why the difference? Major differences in torque at launch. The typical NHRA Pro/Stock car achieved just under 500 cubic inches with a maximum bore of 4.625” & strokes around 3.700”. The 672” Mountain motors used the same bore … but 5.000” strokes. Even though they only made 80-100 more hp, the torque difference was HUGE … and the chassis experiences that torque at the drop of the clutch.

For the NHRA Pro/Stockers the standard Pro/Stock chassis was “Very Stiff” with minimal energy loss. The Double Frame Rail chassis was “Extremely Stiff” and too sensitive to track & tuning changes with its narrow sweet spot.

For the IHRA Pro/Stock & Pro/Mod racers running Mountain Motors, the standard Pro/Stock chassis was only “Moderately Stiff” … and while easier to tune with the wider sweet spot … suffered from some energy loss. The Double Frame Rail chassis was “Very Stiff” reducing energy loss, and making the cars quicker … but brought the sweet spot back to normal.

Just understand the car’s application … power, speed, car weight, etc, play a role in the desired chassis rigidity. There are many goals in designing a chassis, of which one key goal is designing the structure to minimize chassis flex & twist to a high degree … but not so much as to make the chassis hard to keep in the sweet spot of its suspension tuning window.

Here is an example of this …

Bob East of Beast Race Cars in Indy designs & builds some of the winningest cars in open wheel oval track racing. His Midget chassis have won a gazillion races. When I designed our Gen 2 Midget chassis for my race team, we studied Bob’s design. We initially kept his frame & cage layout, but moved most of the suspension points.

The cars were fast & had a wide sweet spot. We won races with it. After running it for a season, with our Engineers running data acquisition on our 4-6 race cars every outing, we could see where the chassis was flexing & how much. Frankly it was/is a great chassis for most racers, because the relatively wide sweet spot makes it easy to tune, & harder to “go off the range” as I call it. Because the sweet spot was wide … the chassis didn’t require a lot of tuning as track conditions changed. I suspect Bob received less customer complaints & had more happy customers due to this.

I was clear we were leaving some performance on the table through energy loss of the chassis flex. As a veteran tuner, I wasn't worried about our ability to "keep up" with the track changes. So in the off season, we redesigned it & built new chassis that stiffened the chassis in two key areas … and the car responded. It produced faster corner speeds & quicker lap times. The drivers noticed small tuning changes more. The sweet spot did narrow, but we didn’t go so far as to make the car “finicky.” But enough so, that we needed to tune the car every round to keep up with changing track conditions.

Because the chassis was more rigid, with less torsional flex & twist, the car required more tuning … but because it was more responsive, the tuning changes needed were small. We got the suspension set-ups so dialed in that tuning changes of 1/2 pound in tire pressure, 1/8” in roll center … OR … .050” in sway bar size difference was all that was needed. And we won a LOT more races.

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[Ron Sutton]

Part 3 of 3

(Continuation of Rob's questions)
A well built car should have a stiff chassis that allows the suspension to do it's work without the added variable of the frame flexing. But that stiff frame translates load from one end to the other.
Yes, but all chassis flex to a degree, as outlined above.

Looking at you diagram of roll axis it seems obvious that increasing the length of the moment arm at either end would introduce some additional roll at the opposite end as the frame resists twisting.
Yes, as outlined above.

If that line was a piece of pipe running through the car as I twisted one end clockwise the other end would follow in a clockwise direction.
Yes, as outlined above.

Is it the front that unloads the inner rear tire on corner entry and loads it on corner exit?
Yes & no. Under braking & cornering the car/chassis is “rocking” diagonally … meaning the outside front corner is compressing the most … therefore unloading the inside rear tire the most. But the rear is rolling too … actually at a slightly higher angle … so the inside rear tire loading is transferring to BOTH the outside rear tire and the outside front tire.

Achieving this to the correct degree is key to achieving a car with good turning ability. If you don’t do this enough, the car is tight or pushy on corner entry & middle. If you do it too much, the car gets loose on corner entry.

If the balance is correct, would work something like.

The front have enough roll resistance going into the corner to keep the front flat and load both tires.
The rear also have the amount needed to keep it flat as the front first turns in.
The front roll translated through the axis provides enough added force to unload the inner rear tire which helps prevent overloading the outside front tire.
As you unwind the front on exit and that translated roll force diminishes the inner rear tire in again loaded for better corner exit.

You’re really close. Some of your terms are not completely accurate, so I’ll reword it slightly to provide a “tad” more clarity.

The front needs enough roll resistance going into the corner to keep the front flatter to better work both front tires.
The rear also needs enough roll resistance, but slightly less than the front as the car first turns into the corner.
The front & rear roll angles translated through the axis work together to unload the inner rear tire which helps the car turn better & properly loading the outside front tire.
As you unwind the steering on corner exit and that translated roll force diminishes the inner rear tire is again loaded for better corner exit traction.


In you statement about tuning the rear I think what you are doing is tuning the rear so that is have just enough of a moment arm to be overcome by the force from the front but not so much where it is overloading the front.
I apologize, but I’m not clear on what you’re saying.

When I am designing a baseline suspension set-up, I use FLLD calculations. FLLD stands for Front Lateral Load Distribution. But hey … we’re car guys. I like to think of FLLD percentage calculations as simply a way of quantifying a car’s front roll resistance.

There is of course a RLLD for the rear … as a way of quantifying a car’s rear roll resistance. Remember, more roll resistance = less roll angle.

Please don’t confuse these terms … FLLD, RLLD or Lateral Load Distribution with “roll couple.” They are similar in meaning but different in accuracy. FLLD/RLLD calculations are more accurate in determining the front & rear roll angles of cars … simply because they take into account all the factors that “roll couple” does not.

A quick primer …
FLLD/RLLD is stated in percentages, not pounds. The two always add up to 100% as they are comparing front to rear roll resistance split. Knowing the percentages alone, will not provide clarity as to how much the car will roll … just how the front & rear roll in comparison to each other. The term “Total Roll Stiffness” is expressed in foot-pounds per degree of roll angle … and it does guide us on how much the car will roll.

So you pick suspension spring rates, sway bar rates & shock valving* … and choose geometry settings like track width & roll center to achieve:
Less roll resistance for the end of the car you want to roll more.
More roll resistance for the end of the car you want to roll less.
* Shocks do not factor into FLLD or RLLD, but do play a real world role in the rate the car rolls.

We typically want slightly more roll angle in the rear & less roll angle in the front of the car … and therefore need lower roll resistance in the rear & higher roll resistance in the front, but you need to account for the car’s front to rear weight bias. When I design a car, my baseline is to have 4-5% higher FLLD% than the car’s front weight percentage.

So, for these cars:
NASCAR Modified with 48.5% front weight, we start with 52.5-53.5% front roll resistance (FLLD).
If a car was truly 50/50 weight bias, I’d start at 54-55% front roll resistance (FLLD).
Remember, I worked out your Torino suspension set-up based on a 51.5% front weight bias, and therefore have yours at 56.5% as a baseline to tune from.

Now that’s a starting point. As a tuning guide only, because there are several “exceptions” … so TYPICALLY:
a. Decreasing the front roll resistance (FLLD) … increases the front roll angle … and loosens the car during corner entry & middle.

b. Increasing the rear roll resistance (RLLD) … decreases the rear roll angle … and loosens the car during corner entry & middle.

c. Increasing the front roll resistance (FLLD) … decreases the front roll angle … and tightens the car during corner entry & middle.

d. Decreasing the rear roll resistance (RLLD) … increases the rear roll angle … and tightens the car during corner entry & middle.


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Rob, as a side note:
Moving your rear panhard bar one inch changes the roll resistance by 1.8% … which is significant … and why I am such a fan of having it adjustable. Raising it achieves “b” … and lowering it achieves “d” in the guide above.

I'll wait on the answer to this to ensure I fully understand it before I ask what effect wheelbase would have on this relationship.
Whenever you’re ready.

Let me go find my hat I may be wearing it soon.
No hat needed. You are grasping this very well. Sometimes I don’t explain things clearly or use the wrong terms, but I’m working on improving that.

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[carbuff]

Originally Posted by Ron Sutton

No hat needed. You are grasping this very well. Sometimes I don’t explain things clearly or use the wrong terms, but I’m working on improving that.

Ron,

I don't know why you think that, but you are doing an excellent job of explaining all of this in my opinion! You should collect all of this information and publish it in a book! (after you've finished writing it all here for us to read first, of course! ) I've read a few chassis books in the past, but none have helped me understand this all the way your few posts here have...

[Sieg]

X's 2

Ron you've done an exceptional job of explaining a complex component system in comprehendible layman's terminology. It takes a fair amount of time just to compose your posts in that manner and it is truly appreciated.

[Ron Sutton]

Originally Posted by Track Junky

This is great stuff right hear Ron and please correct me if I'm wrong but it is darn near impossible to find IC with all the sheet metal in the way?........which in turn makes it impossible to find static RC.
Well ... we're not trying to touch the I/C location, nor are we running the lines to intersect physically on the car.

We're doing it in software. You have to measure x, y & z of all the pivot points of the UCA's & LCA's along with locating the centers of the 4 ball joints.

If your car has the inner fenders, it can tedious work & challenging to get accurate measurements ... but it's very doable. Lance Hamilton just did it (with my guidance) on his '85 Monte Carlo, so I could work out his front suspension geometry, roll center, camber gain, etc. He had access to a drive on lift that was fairly level. That made it much easier for him.

It won't be difficult for me at all ... because I'll be sitting at home in the air conditioning. It will be tougher for you under the car.


I'm getting ready to transform the front of my stock subframe from conventional spring and shock to coil over. Maybe I should p/u another stock subframe and start from there?
I find they vary too much. Lance's was off 1/4" at one point that shot his roll center over 9" to the left. That's a big deal.


Also, when drawing the lines through the UCA & LCA's at the outside connection points of the arms are you drawing through the center of the connecting point at the top of spindle/ball joint?
No. The line goes from the center of the control arm pivot ... on the frame ... through the center of the ball joint pivot. Look at the photo below. if you decide to do this, I'll be happy to provide you with a game plan & instructions.

Maybe I missed it somewhere but now that we know how to get roll center how do we find center of gravity?

I haven't outlined how to yet. I can at some point. Maybe when we start the "General Chassis Tuning" threads ... next week.

Frankly, of the two ... RC or CG ... finding RC is way more important, because your CG is where it is. But your RC is a bigger variable ... and you can make significant tuning changes by moving the roll centers.

.

[Ron Sutton]

Originally Posted by carbuff

Ron,

I don't know why you think that, but you are doing an excellent job of explaining all of this in my opinion! You should collect all of this information and publish it in a book! (after you've finished writing it all here for us to read first, of course! ) I've read a few chassis books in the past, but none have helped me understand this all the way your few posts here have...

That's a good idea. Thanks for the compliment.

.