The Reality of Race Car Dynamics

The End of The Dynamics Controversy

I’ve presented information on race car dynamics and geometry many times in the past. This piece is unlike any other I’ve written or that you have ever seen anywhere. It is very different because this one puts it all together and explains the truth and reality of race car dynamics and what influences chassis dynamics. It’s not what you think.

The SAE (Society of Automotive Engineers) has what is known as papers that engineers present for publication on various subjects where the author wants to make a point or explain a concept. This is my paper, or thesis, or maybe a much better word, treatise.

A treatise is as defined by Merriam-Wester, “a systematic exposition or argument in writing including a methodical discussion of the facts and principles involved and conclusions reached”. That is exactly what this is.

The Scientific Method

Before we get fully into this discussion, we need to talk about the scientific method. This is a recognized way to approach the initial thought, development, and testing of a theory to gain knowledge of its validity. It involves: 1) first ask a question, 2) develop a hypothesis, 3) experiment and test the hypothesis, 4) observe and record the results of the testing, 5) analyze the results to draw conclusions, 6) share the results with the world.

In the racing world, we have a lot of theories about the many components on the race car. What we also have in abundance is a way to experiment, observe and record, and to analyze those results. We have race tracks.

A team gets an idea, builds or sets up the car a certain way, goes to the race track and then observes the performance in lap times to see if what they thought would make them faster worked or not. It is the scientific method at work.

Development of Knowledge

We have been developing knowledge about vehicle dynamics for some seventy years. The earliest attempt was made by automotive manufacturers, with a heavy influence by the General Motors group.

The primary thread of the analysis of vehicle dynamics back in the 1960’s involved a model of a vehicle that treated the body and frame as a single unit with a single center of gravity for the sprung mass. There were two roll centers, front and rear where a line was drawn connecting the two forming an roll axis. This was called the Roll Axis thread of technology. Each end of the car was calculated to have a given “roll resistance” percentage based on the spring rates and spring base among other factors.

This theory of vehicle dynamics never really panned out as useful for a number of reasons, the primary one being this. The model is only really valid for a production automobile with identical opposing suspension components and spring rates on each side. When circle track racers started putting different spring rates on each side of their cars, the roll axis approach did not deal with the resulting differences in dynamics that the spring split caused.

The Modern Era of Race Cars

As we moved into the modern era of race car design that started around 1990, we found that we had to conduct our development by trial and error, even in the upper echelons of racing circles. I know this because it was at this precise time that I became involved in racing as an engineer.

Wanting to better define vehicle dynamics and to be able to better predict what a race car wanted and to prepare the car for performance without trial and error, I developed the theory of roll angle analysis. I even got a patent for it, which didn’t really impress anyone, it just looked good on my resume’.

When I applied this roll angle theory to the car using a crude computer program I put together, it worked pretty well. I was onto something here I thought then. Little did I know, it would take another twenty years to fully understand what was going on and to come to some conclusions on how this vehicle dynamics thing really worked.

The overall roll angle theory was valid and is still valid today. The way I thought it works is different in some critical aspects than what I thought and preached for years. This is not unlike other theories where the authors come to believe theirs is the holy grail so to speak. I hope you recognize here that I am being totally honest with you about all of this.

What did happen was that I had tested this theory countless times on many different and varied race car designs and it always worked to improve the performance of those cars. You could say that I was following the scientific method with the development and testing of the theory. This is not unlike what happened with the next popular theory to come along.

The Current Theory of Race Car Dynamics

A third theory came along about ten years ago and is called the jacking force theory. I believe the primary early proponent of this theory was Mark Ortiz, a very talented engineer who understands a great deal about vehicle suspensions and applications.

In this theory, the primary influence on the dynamics of a race car using a double A-arm suspension comes from the jacking forces produced by the interaction of the instant centers with the tire contract patch when a lateral force is applied to the chassis. In the jacking force theory, the roll center is discounted entirely.

This, like the roll angle analysis theory was tested on the race track by countless teams and when applied, seemed to improve the performance of the car. That suspension system had more traction and the circle track cars with front double A-arm suspensions turned better.

Different Approaches

So now, today, we have two very different theories that are based on utilizing very different components, but each proving to have increased the performance of the double A-arm system. This is a conflict that must be resolved. They can’t both be right, or could they?

For the answer, we have to look at the parts and pieces that make up the two theories. In the roll angle theory, we have control arm angles that create a roll center. This roll center has influence on the dynamics of the suspension due to its height and lateral location as the theory has been explained. A roll center that is designed to be to the inside of the turn creates better performance.

With the jacking force theory, the control arm angles produce a instant centers that are used to create jacking forces that dictates the dynamics of the suspension from its interaction with the lateral force generated at the tires contact patch. Each side of the suspension generates its own jacking force as the theory has been explained.

Very Similar Geometry

Here is where it gets good. When the two theories, roll angle and jacking force, are applied and the optimum control arm angles are arrived at to produce the best performance, the angles of the upper and lower control arms for each theory are very similar. So close in fact, we could say there is no significant difference.

We then have two very different approaches to explain the dynamics of the double A-arm suspension that arrive at the same control arm angles. Could there be something at play here that both theories have in common, but is not taken into account.

Time To Experiment

What started me going down this road that I would have never otherwise have traveled, was being challenged as to the validity of my theory. The challenger was none other than Mark Ortiz himself. I set out to experiment and define exactly what influenced the dynamics of the double A-arm suspension to either prove or disprove the two theories.

So, I called Mark and proposed that I build a model to test our theories. He asked that I send him a copy of the plans for the test model and so I did. He agreed that the model design was valid and would indeed prove, or disprove the two theories.

When I finished the model, I began testing immediately. I had designed this model so that I could change the upper and lower control arm angles in order to create different roll center locations and different jacking force magnitudes as per the theory objectives.

I started with the jacking force first. I had designed this model so that the forces were applied to the contact patch of each tire individually. For a left turning car, a greater force was applied to the outside tire. I believe I had 65% of the lateral force applied to the right front and 35% to the left front. I recorded the roll angle of the sprung and weighted chassis and it was 6.0 degrees.

I then applied 100% of the lateral force to the right front tire and no force to the left front tire and recorded the same roll angle of 6.0 degrees. Nothing had changed from a major change in force on the tires.

I then applied 100% of the lateral force on the left front tire and no force to the right front tire and recorded a roll angle of 5.9, about the same as the other two conditions. Nothing was changing when the jacking force theory dictated there would have been a significant change in the dynamics and ultimately the roll angles between the three scenarios.

For all intents and purposes, I had disproved the jacking force theory. But we are still left with the results of on track testing that proved an increase in performance due to the “optimum” control arm angles created under the jacking force theory.

It was time to test the roll angle theory. I went through a series of tests whereby I created different control arm angles to produce different locations for the roll center. This would in theory change the moment arm and alter the dynamics of the double A-arm suspension and result in different roll angles based on where the roll center was located laterally. It did not.

In my testing, the lateral location of the roll center did not significantly affect the measured roll angle. The only thing that did affect the roll angle, and in a very predictable way, was the roll center height. As the roll center moved up, the roll angles became less (a shorter moment arm). As the roll center moved down, the roll angle increased (a longer moment arm), regardless of the lateral location. Surprise!

What Now?

As I studied the results of these tests it occurred to me that something else must be at play in the double A-arm suspension that created the increased performance that each group of theorists saw with on track testing.

To find the answer, I had to rely on all of my past experience and knowledge related to race car dynamics and geometry. When I finally saw the light, it all made perfect sense. The answer had been there all along, we just hadn’t recognized it.

And without the development of the roll angle and jacking force theories and the testing I had done, we might never have discovered it. The answer lies in the most basic of understandings of race car performance, the development of traction.

The Truth Comes Out

Through all of this, the positive is that we have learned something by disproving the two most common “truths”. The roll angle theory is valid as a concept of balancing the two suspension systems front and rear, but how it dealt with the double A-arm suspension was not correct.

The common denominator between the roll angle theory and the jacking force theory is the control arm angles. They ended up being nearly the same. Could that have something to do with the increase in performance? It does, and that is the answer.

In early race car engineering circles, it has been stated time and time again that all of the engineering we do on a race car involves working to increase the traction at the four contact patches. Now we have to define what creates better traction.

We know that the more load we put on a tire, the more grip it will have. The other component that has been mostly ignored over time is the contact patch. The size of the contact patch is important to gaining the most grip from the loading on the tire.

A tire with a fixed load of X will generate more grip with a greater contact patch area. So, if a tire had a contact patch area of say 20 square inches it would produce Y amount of grip. If we could increase the contact patch area to say 30 square inches, the amount of grip that tire would provide would increase to something more than Y with the very same loading. The creation of optimum control arm angles allows this to happen.

It’s All in the Angles

What both the roll angle and jacking force theories did was produce control arm angles that served to create the greatest contact patch area. That in and of itself created more traction for that suspension system and the cars worked better. If we change the control arm angles to produce less jacking force as the theory goes, or a less efficient moment arm in the roll angle theory, the contact patch becomes less as a result and the system is less efficient and has less overall grip.

Applying the scientific method approach while thinking about earlier testing I have done with a number of different race car designs, it becomes very apparent that much of the gains in performance from double A-arm suspensions comes from controlling camber change to create a larger contact patch.

Conclusion

The pot of gold at the end of this rainbow is the fact that we don’t have to do anything any differently than we have been doing. The jacking force crowd can still believe in their theory and the roll angle crowd can still put the roll center to the inside on a circle track car, and as long as the camber change is ideal and the resulting contact patch is as large as can be developed, we will have the best performance.

For those of you who don’t subscribe to either theory, you now have enough information to setup and design your race car to have the best performance by optimizing the camber change of the tires in the double A-arm suspension. That will in turn provide the largest contact patch. And don’t forget the balance so that the loading on the tires will be ideal.

Tire loading and contact patch optimization are the keys to performance for any car be it a Formula One car or a Street Stock. End of story.

Sources:

DRP Performance Products
www.drpperformance.com
888-399-6074

Gale Force Suspension
www.galeforcesuspension.com
251-583-9748

Intercomp Racing
www.intercompracing.com
800-328-3336

Longacre Racing Products
www.longacreracing.com
800-423-3110

RE Suspension
704-664-2277
www.resuspension.com

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