by Dana Comolli

The term “Friction Circle” is one that anyone bitten by the DE bug is bound to run into as part of his or her quest to become a better driver.  It is discussed in many books on performance driving and is often a topic of discussion during novice classes at DE events; as those of you who have attended a DE event at Blackhawk Farms, Raceway, you already know that I cover it in our BHF classes.

As a refresher, the Friction Circle is a graphical way to describe how the trade-offs work as tire grip is allocated between braking, acceleration, and cornering, at any given point on the driving surface.

Figure 1 – Friction Circle

The edge of the circle represents the limit of grip (friction) that the tires can provide.  The concept was developed by Mark Donohue in the late 1960s as part of seminars he gave at the Road Racing Drivers Club, where he was advocating for the use of a concept called trail-braking, which at the time was highly controversial.  The Friction Circle was used to explain how and why you could safely brake and turn at the same time.

Figure 2 – Limit of Grip

Recall that I discussed the Speed channel of your data software in my last article and how it can be used to improve your driving (read it here).  In this article, I’ll discuss how to use the Lateral and Longitudinal acceleration channels of your data software to implement the equivalent of a Friction Circle, and use it to show where you may not be taking advantage of all the grip that’s available to you. 

Once again, I’ll be using AIM’s SoloDL and their Race Studio Analysis software, although any similar device and application will work.  The data is from the same two laps at our 2019 Blackhawk Oktoberfest event that were analyzed in the previous article.

I’ll be using an X/Y Plot to display Longitudinal Acceleration (Braking/Acceleration) versus Lateral Acceleration (Turning).  Figure 3 shows how this should be configured in the Race Studio Analysis software.

Figure 3 – X/Y Plot Configuration

Because this is a graph of acceleration from the point of view of the car, rather than friction, the graph is inverted from a Friction Circle as shown in the labeled version of the chart in Figure 4.  I’ve identified the zero point (the black circle) for reference.  The unit of measure is g-forces (1g = the force of gravity at sea level, 32.2 feet/sec2), which translates to 21.95 mph per second. 

Figure 4 – X/Y Acceleration Chart

What can we learn from the chart in Figure 4?  First, you can slow down a lot faster than you can accelerate.  This makes sense, especially since this is a chart of a lap in the middle of the session.  Going back to the Friction Circle, the idea is to keep the forces as close to the edge of the circle as possible.  You can see on the lower portion (braking and turning) that in general, this is the case.  Nevertheless, it’s clear that there is some room for improvement, since the shape of the friction “circle” is more of an oval.  For reference, the left cross-hair in the diagram above is positioned immediately after a late apex at Turn 5.  The peak to the left of the left cross-hair is the late apex.

Let’s overlay the two laps from the last article and see how they compare (Figure 5).

Figure 5 – Overlaid Laps

The first thing we can see in Figure 5 is that the blue lap (the slower one) is using far less of the available grip.  The blue dots do not extend as far down in the lower left and right sections of the chart, nor do they extend as far left or right.  A prime example is Turn 7.  The difference is around 0.3g between the two laps.  An even greater difference is at Turn 3a.  These represent a significant difference in the use of available grip.  Looking at the other highlighted turns shows the same types of differences.

Zooming in on the handling of Turn 6 (Figure 6) is especially educational.  You may want to go back to the previous article for more context.

Figure 6 – Focus on Turn Six

As you can see, the entry to Turn 6 shows a big difference in transitioning from braking to acceleration.  It also shows a lack of smoothness at the first apex for the blue lap and a consistently greater use of both lateral and longitudinal acceleration in the turn through the second apex for the red lap.   This is consistent with what the speed trace showed.

Far from being simply a theoretical teaching concept, the friction circle can be used to identify those areas on the track where we are not using available grip—which translates to speed—that can be easily identified through the use of telemetry systems widely available to all of us drivers. 

By using this technology, each of us can get the most out of our cars, and become better and safer drivers.