How To Pass On A Banked Oval: NASCAR Physics

This weekend’s race at Kansas Speedway is likely going to be the most important race of the playoffs for both the Cup and Xfinity drivers. The following two races in this round are Talladega Superspeedway, where anything and everything can happen, and the Charlotte Roval, a tight and twisting road course with little room for error.

Both of those races are solidly in the “Wild Card” category. A victory in Kansas guarantees that driver and team a surefire ticket into the next round, which means they can sleep easy before the next two unpredictable races. In order to pull off a victory, a driver might have to take a trick from circle track racers and use the old Slide Job.

But before we explain how that works, a little on why Kansas ends up usually being a good race with a lot of passing.

Flat As A Pancake, Windy As A… Pancake That Has Air Rapidly Moving Over It

Kansas City Star

Kansas Speedway is a product of the late 1990s track construction boom, as the series and its promoters tried to cash in on the sport’s growing popularity. Between 1997 and 2001, seven speedways were built by International Speedway Corporation and Speedway Motorsports LLC across the United States. The list includes Texas Motor Speedway (1997), Las Vegas Motor Speedway (1998), Kansas Speedway (2001) as well as the now-defunct Auto Club Speedway (1997), Kentucky Speedway (2000) and Chicagoland Speedway (2001).

Kansas Speedway was constructed on a nearly empty plot of land just outside of Kansas City, KS for $228m, with the promise of adding a significant boost to the local economy.

In the decades since, the area surrounding the speedway has seen tremendous growth with the addition of the Legends Field minor league baseball stadium, Children’s Mercy Park major league soccer stadium, two shopping centers, a water park, the Village West apartment complex, and a multitude of restaurants and breweries. In 2009, the speedway added a large casino to the property just outside of Turn 2. According to the Kansas City Sports Commission, Kansas Speedway is responsible for contributing $247 million annually to the local economy and has driven much of the growth in the area.

With Kansas Speedway playing such a pivotal role in the playoffs let’s take a moment to talk about what it takes to make speed around this race track.

You Can Bank On Kansas

The pièce de résistance of Kansas Speedway is the progressive banking that was added to its corners during a 2012 repave. The bottom lane is banked at 17* and this slowly increases up to 20* of banking right up alone the fence. The goal of this progressive banking was to equalize the upper lanes with the bottom lane and open up more racing lines to the drivers.

Obviously, we know from watching races at Kansas Speedway that it worked marvelously, but let’s break it down mathematically. The arc length of the corners around the inside groove is roughly 1,500ft and at a track width of 55ft that gives the outer groove an arc length of 1,625ft or a path that is 8.3% further in distance. Theoretically, for the lanes to be equalized, the top groove would need to be able to maintain a mid-corner speed just over 8% higher than the bottom lane to cover the increased distance in the same amount of time.

Anyone who has taken an AP or above level physics course will be familiar with solving for the maximum speed of a car around a banked corner. If you like math I have attached a derivation of the generic formula below. Anyone else can skip ahead and I’ll work out the numbers for you.

Image: HyperPhysics Lab

Obviously, this is a very simplified look at what it takes to get a car around a banked corner, but the fundamental principles hold true. By adjusting the banking angle and corner radius to correspond with each lane you will see that the outer lane has a Vmax value that is about 9% higher than that of the inside lane.

Let’s look at some actual data and see if the theory translates to reality. Below you can see Justin Allgaier racing alone the bottom groove in his orange number 7 car while Cole Custer is up by the fence in his white number 00 car. John Hunter Nemechek, just out of frame, is leading the race and also running along the fence.

Now let’s compare that to some SMT data from that lap. I have removed all other data channels except for the speed trace from this screenshot, but you can see that Cole Custer is traveling at 157.1 mph and John Hunter Nemechek is travelling at 158.1 mph by the fence while Justin Allgaier is travelling at 147.1 mph along the bottom. An 8% increase in Allgaier’s speed would be 158.8 mph. The speed traces are color coded corresponding with the color in the bottom left next to their number. The math does indeed math here.

As a driver’s tires age over the course of a fuel run, the racing groove will move to be predominantly against the fence. We’ve explored this phenomenon in detail in other pieces but I will try to summarize it briefly here.

As a racecar moves travels through the air, the air is pushed to either side of the nose of the vehicle in much the same way as a boat creates a wake. Air pushed off to the right-hand side of the car’s nose will run into the wall and not be able to outwash. This air in effect becomes trapped between the side of the car and the wall and creates a sort of high-pressure bubble that holds the car off the wall and forces it into the corner. If you’re having trouble conceptualizing this, picture a person sitting and bouncing on an exercise ball. The floor represents the wall, the person’s butt represents the racecar, and the exercise ball represents the pocket of air preventing the two from crashing into one another.

NOTE: This scenario acts opposite to the Venturi Effect. In a Venturi scenario the objects (wall and car) would be stationary and the air would be flowing between them, thus reducing pressure. In this scenario the air and the wall are stationary and the bulk air mass is being moved aside by the racecar.

Running the wall is an art form, and there are a handful of drivers that consistently stand out at tracks where this becomes the preferred groove. It’s a high risk, mega-high reward racing line that takes incredible amounts of precision to pull off. Let’s take a ride around Kansas Speedway with Ross Chastain as he leads the 2020 Xfinity Series race and break down what he’s doing.

The most important part of the corner to get correct for a driver running the fence is their entry. Drivers running the fence will lift early or “back up” their corner. What separates the good drivers from the great is their wall proximity in this portion of the corner. A driver that is not as confident at racing along the wall will turn into the corner early, pulling their car down slightly off the wall before floating back up towards it. There are two problems with this approach. If the driver carries slightly too much entry speed, the air won’t be enough to hold them off the wall.

They will pop their exercise ball and land square on their tail bone. If they don’t carry enough entry speed, they will be giving up valuable lap time. A great driver will enter the corner right on the wall, butt firmly attached to exercise ball, and they will have a consistent aero assist the entire way through the corner. They don’t have to guess how much speed to carry on entry because the cushion will be there the entire time. The ability to enter the corner with this level of confidence is what separates the good from the great.

In the onboard footage you should note that Chastain is not as secure with his entry into Turn 1 as he is Turn 3. You can see the nose edge ever so slightly away from the wall entering Turn 1 compared to the constant proximity in Turn 3.

Once a driver gets into the corner, they must carefully manage their wall proximity. A racecar goes through a corner with a certain yaw angle, meaning that the rear of the car is closer to the wall than the front of the car. The angle of the right side of the car has two effects. First, it forms a sort of funnel that gathers more and more air between the car and the wall. Secondly, the side of the car squeezes the air as it gets closer to the wall. This squeezing effect causes the pressure to increase more towards the rear of the car. The increased aero effect at the rear of the car causes a balance shift. As the driver get further away from the wall, the aero effect dissipates and the balance shifts towards oversteer. If the driver gets closer to wall the effect will increase and shift their balance towards understeer.

As you could hear in the onboard footage, Chastain is constantly rolling into and out of the throttle. If the TV broadcast ever showed an in-car camera that faced backwards towards the driver at a track like Kansas Speedway you would likely see the driver looking slightly up and to the right instead of to the left as you would expect. Most drivers who are skilled at running the fence will have a visual reference, typically one of the windshield braces, that they will line up with the top of the wall. They use this reference to guide their wall proximity in roughly the same way as someone uses a tennis ball hung from the garage ceiling to judge their parking. If the reference point dips below the top of the wall, the car is drifting away from the wall and slightly more throttle will bring it back up the track. If the reference point climbs above the top of the wall, the opposite applies.

Exiting the corner, especially Turn 2, requires a driver to pull slightly down off the wall to smooth out the transition from the curved wall to the straight wall. A slight lift of the throttle will bring their trajectory down slightly and allow them to exit onto the straightaway.

It’s Time To Do The Electric Slide

When the racing groove moves predominantly to the top, passing becomes trickier. A car running the bottom lane will pull ahead by the middle of the corner as they have taken a shorter radius, but when the top and bottom lanes converge at the beginning of the straightaway the car running the top lane will be carrying significantly more speed. This leads to a lot of crossover moves and what we refer to in circle track racing as slide jobs.

In a slide job scenario, the defending driver will be running the outside lane and the overtaking driver will be running the inside lane. The overtaking driver will overshoot their corner entry, carrying more speed than they normally would on the bottom lane. Because of the increased corner entry speed, they will not be able to hold the bottom lane once they get into the corner. Their goal is to be fast enough in the entry phase that they can clear the defending driver and then slide up the racetrack in front of them.

It is a high commitment type of move as once the driver has committed to carrying that much speed on corner entry there is no backing out of it. Too much speed and they will not be able to regain control of their car before colliding with the outside wall. Too little speed and they will not be able to clear the defending driver resulting in a failed overtake, or worse a crash.

This is a move that is fairly commonplace in dirt short track racing. You can see below in this dirt modified heat race how the drivers trade the lead back and forth. Because the overtaking driver is still attempting to regain complete control when they get to the high lane, they will be travelling at a lower speed than the defending driver. If the defending driver senses that this move is coming, they will slow down slightly in the center of the corner, allowing them to turn down off the wall and underneath the defending car. Ideally their momentum from the top lane should be enough to get the two drivers back side by side on the following straightaway. Suddenly, the drivers have reversed rolls and the formerly defending driver will find themselves on the inside of the next corner where they can attempt to repay the favor.

The below example may not be NASCAR at Kansas but it’s a perfect illustration of where the technique comes from.

The Great Plains region of the central United States is a notoriously windy place. Of the 50 states, Kansas is the second windiest, behind neighboring Nebraska and ahead of both the Dakotas and Iowa.

A tail wind will increase entry speed and decrease downforce. A head wind will decrease entry speed and increase downforce, particularly on the nose of the car. If the wind is fairly constant, drivers can easily compensate for this. In Kansas however, the wind is almost never constant and there are significant variations in gust speed. This can make the car feel unpredictable from one lap to another, further complicating matters for a driver who is trying to place their car within inches of the wall.

From the flags, you can see that drivers will have a tail wind into Turn 1 and a head wind into Turn 3 for most of the weekend. While cornering, the banking will shield the drivers from the wind in Turns 3&4 but in Turns 1&2 the left side of the car will be completely exposed. Gusting wind will try to push the cars up the track towards the wall. W

When racing in traffic, this wind will put the driver on the inside line at a significant disadvantage. The driver on the inside will already have less side force due to having a car on their outside, and wind pushing against the car’s left side will cause the car to want to oversteer.

I hope you’ll take all of this into consideration when watching the race. If it’s not an exciting race you can always try to pause your TV to grab the perfect shot:

Good luck to our contributors @AedanMcHugh and @pkligerman this weekend in Kansas! pic.twitter.com/HaAOOGF9NS

— The Autopian (@the_autopian) September 28, 2024