1969 Torino

[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 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.

Thanks for the kind words.

.

[carbuff]

Ron, another general question.

When taking the measurements to perform all of these calculations, what amount of "error" can you tolerate? There are so many variables here, and the easy answer would be none. But in the real world, that's just not possible. Even down to the clearances for bolts through holes, 'slop' in the various components or the suspension components, play in the steering rack, etc will add up to easing hundreths of an inch in some cases...

So I'm curious, with all of this great information and technical analysis, at the end of the day, how do you deal with those variables, and how much tolerance do you build in to your systems?

[Ron Sutton]

Originally Posted by carbuff

Ron, another general question.

When taking the measurements to perform all of these calculations, what amount of "error" can you tolerate? There are so many variables here, and the easy answer would be none. But in the real world, that's just not possible. Even down to the clearances for bolts through holes, 'slop' in the various components or the suspension components, play in the steering rack, etc will add up to easing hundreths of an inch in some cases...

So I'm curious, with all of this great information and technical analysis, at the end of the day, how do you deal with those variables, and how much tolerance do you build in to your systems?

I can't say we approach the measuring project with any tolerance in mind. When we're doing the process on an existing car, literally on the floor, we use tape measures, plumb bobs, levels, digital inclimeters & even lasers. We measure to the 1/32" where we can.

On our chassis jig, which has a surface within .003" ... and it's easy to get right at things, we use these really long 18" digital calipers, so we're measuring in the thousandths.

It's funny, you can be off on some measurements 1/4" ... and the roll center doesn't change. And on other measurements 1/16" makes a noticeable difference.

I always advise guys to be as accurate as you can, which requires patience more than anything. If you can get your measurements within a 1/16", you're doing good.

Also, When we're measuring a car we're running ... we want the real numbers ... warts & all ... because we have to deal with whatever is "out" or "off" about the geometry.

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[Track Junky]

So Rob, I'm wondering after all of this potent suspension intel are you still going to be runing 285's up front?

[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.

.