"A sign of brilliance is being able to take a complex subject and make it simple. And while driving the car on the edge of the friction circle is not easy, Donohue made the concept simple to understand."
The Friction Circle
by Jerry Austin, Ross Bentley, Satch Carlson, Peter Krause, Chris Sneed and Ingrid Steffensen
One of the most gratifying experiences in publishing this niche magazine for track day enthusiasts has been the opportunity to meet with and talk to the most accomplished drivers who happen to also be very generous in sharing with us their expertise. Each of these experts enthusiastically jumped onboard and agreed to contribute to our driving technique series. In fact, when we asked Satch Carlson to write about threshold braking, he said “Oh, HELL yes... I'll be happy to maunder on about the joys of sheer terror, segueing into total blind panic!” We know you are hungry for technique related articles. So periodically, we will be surveying our panel of subject matter experts and combine their input into one single article covering a specific driving technique from multiple perspectives. We are very excited to share with you our assembled experts for this second go-around: Jerry Austin of Austin Motorsports, specializing in data analysis and data logging equipment; Ross Bentley of Speed Secrets and driving coach; Satch Carlson, Editor, Roundel Magazine; Peter Krause of Peter Krause and Associates and driving coach; Chris Sneed, owner of Sneed’s Speed Shop, instructor for PCA, BMW CCA and NASA, and professional racer in the Pirelli World Challenge series; and Ingrid Steffensen, BMW CCA instructor, professor, author and contributor to Bentley's Speed Secrets.
Full disclosure here: when I was a novice and the classroom instructor put the graphic of the Almighty Friction Circle up on the screen, I used to look at it in utter bafflement while the rest of the class nodded sagely, as if agreeing: oh, yeah, that explains everything. As a result, I don’t like that graphic very much. Instead, I find it helps to picture the car like an overweight cat: think Garfield on wheels. His four paws are the total amount of contact he has with the floor, which for argument’s sake we’ll say is wood and has recently been waxed. So he’s galloping full tilt at Odie when suddenly he needs to turn because he’s just spotted his pan of lasagna. All the grip his paws have had in carrying him forwards towards Odie have been in a straight line. If he has to turn his big ol’ butt, then the momentum of his weight shifts away from the lasagna, and the grip he had going forward is compromised by that sideways pull—therefore, he has less grip if he is trying to move forward and turn, as opposed to just moving forward. Or let’s say he has to come to a stop. Because he’s so fat, his weight wants to keep moving forward, but his paws can only grip the floor so hard. Things get even trickier if he has to balance slowing down with turning. The diagram of the friction circle just outlines where on that continuum from moving forward in a straight line, to turning and braking, the grip is, and understanding that the total amount of grip remains the same, and you can only distribute what you’ve got amongst those various forces at work on the car (or cat), is pretty much all you need to know when the pan of lasagna beckons.
Chris Sneed of Sneed’s Speed Shop, Instructor and Pirelli World Challenge Racer
The Friction Circle is a way of stating what a tire can do in a given scenario. Tires can only give 100% at a time, so if for example you want to turn and slow down at the same time the tire can do both of these things but only to a combination of 100%. This means the tire will stop the car faster and in a shorter distance if it’s going straight because it can give 100% to stopping but if you turn the tire slightly then the tire can only do 95% stopping and now must do 5% turning so your stopping distance is slightly longer than true straight. As the turning input increases, stopping distance extends, if you try to increase turning or decrease stopping distance and over run the 100% then the tire loses traction and slides. Anytime the tire is sliding you are outside of the 100% of that tire’s friction circle and you must decrease turning and/or braking to get the tire back into its 100% area of traction to regain control of the car. Just remember if the tires start to slide unwind the wheel and decrease brake pressure until they stop sliding. Once control is regained you can reduce steering angle to reduce braking distance or increase steering angle and reduce brake pressure to turn sharper, either way will get the tire back into the friction circle.
Peter Krause of Peter Krause and Associates
There are many measures that define how and how well a track day driver is using the capability of their car, but few that combine several individual, important measures into one, powerful and easy to visualize graphical display AND a REAL number!
The Friction Circle was engineer Mark Donohue's enduring concept of how best to show the forces acting on the car in the two axis (vector forces) that we, as track day drivers and club racers, are SO obsessed with. Lateral (cornering, or side-to-side) and longitudinal (in line, or acceleration and braking) forces are not only determined by how quickly we decide to go through a long, steady state corner or how hard we stand on the gas or brake pedal, but are ultimately defined (read: limited) by the total amount of grip generated by the tire contact patch. This concept is often conveyed by the experienced, wise instructors at Skip Barber, Bob Bondurant and Bertil Roos' schools, among others, as the idea that, in your 3000 lb high performance street car, "the only connection you have with the earth" is through four tire contact patches, each not much larger than two normal size palm prints tightly together. Put in that perspective, the importance of this concept is heightened!
The Friction Circle is a graphical representation of forces that act on a moving vehicle. Picture a pair of fuzzy dice or an event lanyard hanging from the rear view mirror in your car on the street. When you accelerate, the dice move straight back. When you brake, the dice move straight forward. Turn right, and the dice move left. Turn left, and the dice move right. Easy, right? The harder you actuate each pedal, the more distance from the center, static position the dice move. If there was a pen drawing the position of the dice on the piece of paper, you would, after a few laps around the block making sure to turn left, right and fully accelerate and brake, have drawn a basic friction circle.
The reasons why the friction circle is SO important to the track day driver are two-fold.
The first is because this is one of the few measures that does not require a plethora (or even any) sensors other than a basic two-axis accelerometer. The Traqmate Basic, VBOX Sport or an AiM Solo, even an iPhone with Harry's Lap Timer, are ALL capable, without ANY connection to the car other than being securely mounted, can collect this crucial information and display it so drivers can make intelligent decisions about where there is opportunity left, and where they dare not tread! This can be represented not as just a simple Lateral G or Longitudinal G measure, but a COMBINED G (or GSum) measure that is a REAL number that shows, down to decimal places and in fine resolution, EXACTLY what is going on and how hard those tires are being "worked."
The second is because it is one of the best, simplest ways to identify VERY targeted opportunities for improvement for the track day driver. The reason for this is because the greatest performance differential between novice, intermediate, advanced and professionally practiced drivers is their ability to FULLY exploit the Friction Circle. How do you do this and what are you looking for, you ask?
Any driver can, with practice, think and execute relatively easily and with increasing confidence and capabilities, basic fore-aft axis control inputs. Going to power, FULL power. Crisply, cleanly and without equivocation. Then, as they get more comfortable and especially in a straight line, they get better (and sometimes a little too good) at pushing down the brake pedal, HARD! As they get more comfortable and confident with the car's ability to go and stop, then they can shift their focus to sustained cornering force, basically a result of following a particular radius around a corner with progressively more and more speed. Most drivers can sustain near one G (one hundred percent of their weight, forced laterally and in the opposite direction that they are turning), but this is the easy part. "What," you say? "I thought THAT was the exercise?" No... The hard part is what's NEXT...
The tire's (and to a lesser extent, the car's) ultimate performance capability is easily measured when simple skill executions are mastered. It's not uncommon and a measured fact that in a car with a properly functioning brake system, that the car should develop 90-95% deceleration G force on a flat, level surface than the maximum measured, sustained cornering G. Most track day drivers come nowhere close to that benchmark. If you, as a track day driver, are unable to develop at least 90% of the maximum sustained cornering G's when braking in a straight line over a long distance (more than 250-350 feet), than BRAKING is what you should focus on. Quick application, high initial pressure and deceleration, trailing off gently throughout the braking zone.
We've already talked about the fact that most accomplished track day drivers can attain high lateral (cornering) G numbers, but the area that IS the exercise (and the ultimate challenge) is the TRANSITION between the end of braking g's and the assumption of maximum cornering. The Friction Circle shows an unalloyed representation of how well drivers "blend" these forces together, with the sole goal of keeping the "ball" indicating the forces acting on the car in the Friction Circle as FAR out from the center as possible. SO often, and especially among the novice and intermediate track day driver, does the the "ball" spike correctly under braking, but ebb TOO fast (brake pressure released because they KNOW they braked too soon), then spike a little again (as they realize they're not QUITE going slow enough to get the car to begin to turn) and then ebb more before heading out sideways as the cornering G's come up.
The BEST drivers "walk" the ball around the circumference of the available grip of the tire, as evidenced by the concentric "rings" of the friction circle. They brake quickly, hard and even throughout the braking zone, often maintaining brake pressure to keep weight on the nose of the car PAST turn in. Not necessarily to "rotate" the car, but to insure that it begins to turn at the proper rate. As the car begins to turn and assume lateral loading, the progression to acceleration, hopefully ahead of the slowing resistance of the "scrub" of the turning front tires so the car doesn't slow further after brake release, begins to occur. Then, maximum cornering occurs under acceleration as the radius widens at exit, until finally, the "ball" moves into the forward thrust field of the friction circle and comes back to center as the car get straight.
A fascinating question and the crux of how fast drivers go faster!
More math: John Block, Auto-Ware,http://www.auto-ware.com/setup/fc1.htm
"Analysis and Effective Strategies Combining the Art and Science of Driving Fast!"
Satch Carlson, Editor, Roundel Magazine
The friction circle is one of those ethereal concepts having to do with coefficient of friction and vector forces going in various directions. The theory makes sense to those who spend most of their times with charts and graphs, but it isn't very useful to those of us who cut class to go experiment in the Asphalt Lab instead of sitting through another lecture on vector quantities.
Basically, if you run out of the circle in any direction you lose traction, so by adding side force in a corner (a lateral vector) to accelerating or braking (fore and aft vectors), you run out of traction and start to slide. Well, duh!
Rally drivers spend most of their time trying get back INSIDE the damn circle. Or waiting for some traction, ANY traction, to catch up and change the direction of the car before we slide into the snowbanks/bushes/weeds or the Yawning Pit Of Death.
A more useful discussion has to do with weight transfer and how it affects the contact patch, with subsequent effects on handling. But that's a topic for another day!
Ross Bentley of Speed Secrets
Back in 1998, when I was driving for the factory BMW team in the ALMS and USRRC series, I met a fellow who had been one of Mark Donohue’s best friends. You know who Donohue is, don’t you? Along with Paul van Valkenburgh, he’s the guy who “invented” the friction circle. Well, he defined what really fast drivers do using an X-Y graph, which is what the friction circle is. Oh, and he’s the guy who won Trans-Am and Can-Am championships, the Indy 500, a NASCAR race, and drove in Formula One races for another guy you might have heard of: Roger Penske. To this day, Donohue’s book, The Unfair Advantage, is one of my favorites – and one of the best books you’ll ever read.
Well, the guy I met when driving for BMW (Remember him? Let’s call him Bill because I’m not sure he should have done what he did) knew how much I respected Donohue, and that I was a “student of the sport.” Okay, he knew I was a geek about the art and science of driving and cars going fast. So he showed up at a race with a stack of papers. They were photocopies of Mark Donohue’s original handwritten notes and illustrations depicting the friction circle. It was a bit like finding da Vinci’s doodles of the Mona Lisa.
Every once in a while I pull those pages out and study them, looking for more in them. And what’s so cool is that, as confusing as the friction circle can be, it’s really very simple. What it says is that a driver must overlap braking, cornering, and acceleration forces to be fast. That’s it.
So, as you approach the turn-in point for a corner, you gradually ease off the brakes, trading braking traction for cornering traction, and then unwinding the wheel to trade off cornering traction for acceleration traction. It’s the blending of braking, cornering, and acceleration that Donohue had figured out, both on paper and behind the wheel, that made him so brilliant.
A sign of brilliance is being able to take a complex subject and make it simple. And while driving the car on the edge of the friction circle is not easy, Donohue made the concept simple to understand.
Now, excuse me. I’m going back to da Vinci’s doodles again.
Jerry Austin of Austin Motorsports. LLC
I’ll start by saying that the Friction Circle isn’t a circle at all. It sounds good to call it a circle and makes the concept a bit easier to understand. In fact, the friction circle is the shape of the perimeter of the data points in figure 1. The reason for this is that all well engineered race cars (with little downforce) can corner at higher G loads than they can obtain in braking. In addition, few cars can accelerate at the G loads equal to cornering or braking.
I work mostly with Porsche GT 3 Cup cars and typically they can corner at 1.6 to 1.8Gs. The 997 Cup cars (used in this example) do not have ABS and can brake at 1.0 to 1.2 G’s. The car in this example had a 4.0 L engine and accelerated at 0.5 to 0.6 G. These factors define the data perimeter. The data is then analyzed using 4 quadrants. See Figure 2.
The value of looking at data is that you can get some idea about the effectiveness of the driver. Figure 3 highlights two vacant zones inside the circle.
If the diver is being effective in making the transition from braking to turning, these two areas will have very few data points, meaning that the driver has kept the tires loaded during the transition from braking to turning. In addition, Figure 3 shows a nice rounded transition at maximum braking G’s when turning left and right.
Less experienced drivers will do all of the braking before turning and that will create many dots in the vacant zones shown above. Figure 4 shows such an example from the same track, different car and driver. You can see that the transition off the brakes tend to go back toward 0 G (much more pointed bottom of the data points) and the dots are all over the place that means that the driver is not very consistent.
The theoretical friction circle is shown as a trace of vectors and looks like Figure 5
In real life the circle trace from real vector plotting looks like Figure 6
I hope this was somewhat helpful to the many people who use data but might not have evaluated their “Friction Circle” data using the X-Y plot feature. If you want to dig deeper into this, feel free to e-mail me at Jerryaustin@verizon.net, I’ll be happy to help.
Thanks Coaches! (Ed.)