Michigan’s NASCAR Race Is A 200 MPH Dance On The Razor’s Edge: NASCAR Physics

We are so back, baby. While the Cup and Truck series got to play in Richmond last weekend, the Xfinity Series was still on our Olympic break. That all ends this weekend in the Irish Hills of Michigan just west of Detroit.

Michigan International Speedway was built in 1967 by Lawrence H. LoPatin, president of American Raceways at a cost of $5m ($47m in 2024). At the time, American Raceways controlled Atlanta Motor Speedway in Georgia, Trenton Speedway in New Jersey, Riverside International Raceway in California, and Texas World Speedway (obviously in Texas).

Gates were opened for the first time on October 13, 1968, for a 250-mile USAC Champ Car race won by Ronnie Buckman. The speedway was visited annually by USAC-turned-CART-turned-Indycar until 2007. In the track’s second year, it welcomed the NASCAR Cup Series for two races in addition to its USAC event. The Cup series visited Michigan International Speedway twice per year until 2021 when it was reduced to one race weekend and has been visited annually by the Xfinity Series since 1992.

Roger Penske purchased the speedway from American Raceways in 1972 for approximately $2m ($15.3m in 2024). Penske owned the facility until it was sold to International Speedway Corporation (ISC) in May of 1999 as part of a $700m merger that combined Penske Motorsports with ISC. At the time, Roger Penske also owned Nazareth Speedway in Pennsylvania, Rockingham Speedway in North Carolina, and the now-defunct Auto Club Speedway in California.

For much of the time that he owned the speedway, Penske Racing used Michigan International Speedway as a secret testing facility for their Indycar program. The noise of any racecar thundering around Indianapolis Motor Speedway would surely attract prying eyes, and Nazareth Speedway was within earshot of Mario Andretti’s home. Far removed from the rest of the Indycar world, Penske could test and develop their racecars with an assurance of privacy. These exploits are explored in detail in Jade Gurss’ novel “Beast” about the top-secret development of the Ilmor-Penske engine for the 1994 Indy 500. The engine, nicknamed “The Beast” dominated the 500 that May and was swiftly outlawed before it could be raced again.

If tales of Concorde jets overnighting parts, secret engine-dyno labs, and a full-fledged testing team running laps in Michigan while the rest of the team was in Indianapolis practicing for the 500 sounds interesting to you, I would highly recommend picking up a copy of “Beast.” It’s a fascinating tale of engineering, adventure, and millions of dollars being poured into an engine that would only be legal for one race.

For what it’s worth, Penske’s Indycar team won the 500 ten times while he owned this private testing grounds.

Finding That Perfect Balance

Over the years, teams have placed tremendous emphasis on performing well at Michigan International Speedway. This is a racetrack where it truly takes a complete package to find success. Teams spend extra time and money to put some “big steam” under the hood for Michigan. Even a marginal increase in horsepower is evident at the end of Michigan’s 3,600ft (1,100 m) front stretch and its 2,250ft (685 m) back straightaway. Drivers will be on the throttle for roughly 73% of the 2-mile (3.2 km) lap. At the end of each straightaway is a wide sweeping corner where the team’s aero department comes into play. With mid-corner speeds in excess of 180 mph (290 kph) every pound of downforce is needed, but finding the balance between downforce and drag is critical. For more information on low versus high downforce setups, you can check out my pieces on Pocono Raceway and Indianapolis Motor Speedway. Being located just outside of Detroit, MI, a stone’s throw away from America’s “Big Three” auto manufacturers, the higher-ups often find their way onto pit road to see what teams have produced with their millions of dollars.

The final piece of the puzzle is the driver. Qualifying at this racetrack, especially in the high-horsepower-low-downforce, era was a sight to behold. I can clearly remember my first visit to Michigan, standing at the end of pit road listening to see who could push it the furthest into Turn 1 before cracking the throttle

Once the car has settled into the corner, drivers will find themselves on a razor’s edge trying to balance the rear of the car as they apply throttle through the center and exit. You can see the throttle trace from Marcos Ambrose’s 2012 pole lap and watch this phenomenon.

Exiting Turn 2 he briefly goes back to full throttle, before rolling out of it momentarily. In Turns 3 and 4, he again tries to back to wide open and quickly lifts back to around 60% throttle. A driver must sail the car off into the corner and then let it take a set after landing into the banking. Once the car has settled into the middle of the corner, a driver will try to hold the wheel steady with constant pressure in their hands. They will squeeze down more and more onto the throttle until the resistance in the steering wheel gets slightly lighter, indicating that the car is beginning to rotate into a spin. Balancing this oversteer is the key to getting around the speedway quickly, and the ability to dance right up to the edge is what separates the good from the great.

Looking at Michigan International Speedway, with its wide and sweeping corners, you might be misled into believing that this track would be easier to get around than others. Making lap time around Michigan requires utmost precision. When paving a track this wide, the asphalt must be laid down in strips with a seam in between each ribbon. The racing surface is paved in five lanes, meaning that the surface has four seams. Some quick math tells us that if the racetrack is 75ft (23 m) wide, then each lane is 15ft (4.5 m) wide. The maximum tread width for an Xfinity Series car is 5.85ft (1.78 m) and for a Cup Series car it is 6.17ft (1.88 m). Taking the average tread width, if a driver centers up their car within a lane they will have 4.5ft (1.4 m) on either side of their car. The lanes and their proportions relative to car size can be seen below.

Can you believe how close this finish was?!@Chris_Buescher edged out @MartinTruex_Jr by 0.152 seconds, the third closest finish at our track in TWO DECADES!#FireKeepersCasino400 pic.twitter.com/EknYx9vF98

— Michigan International Speedway (@MISpeedway) August 17, 2023

You’re probably wondering why this is relevant, so let me explain. In between each lane is a roughly 5in (13 cm) wide strip of sealer, which provides much less traction than the asphalt on either side of it. The tread of a Cup Series tire is roughly 12.5in (31.7 cm) wide and an Xfinity Series tire is about 11in (28 cm). This means that when crossing over a seam, 40% of a Cup series and 45% of an Xfinity series tire will be producing less grip. Touching a seam with the left-side tires tends to produce oversteer and the opposite is true with the right-side tires. You can see below where Daniel Suarez loses control of his car when touching a seam at Michigan’s sister track Auto Club Speedway. As soon as his left rear tire touched the seam his fate was sealed.

At tracks like Michigan, drivers must pick a lane and stick to it to get through the corner. The preferred groove is traditionally in the second or third lane. This gets a bit tricky when racing in traffic. To keep clean air on the nose of their car and avoid aero wake from the leading car, a trailing driver will tend to offset their car to one side or the other and split the seams. You can see in this clip that Kevin Harvick uses this technique while trailing Martin Truex Jr. From the onboard shots it’s very obvious how the drivers will aim for a ribbon of asphalt and let the seams dictate their line.

Historically, Michigan International Speedway had been a multi-groove circuit, similar to its sister track in Fontana, Ca. After a 2012 repave, the groove was narrowed significantly, despite the geometry of the track remaining consistent. The balance point of distance-traveled versus speed-carried fell right in the second lane, with the left side tires just above the first seam. You can see Jeff Gordon utilizing this line in 2014 as he set the track record for a stock car with a lap time of 34.85 seconds at an average speed of 206.558mph (332.4 kph).

August 15, 2014: Jeff Gordon won the pole at Michigan with an average speed of 206.558 mph. It was the fastest qualifying lap in the past 35 years. pic.twitter.com/w0zQhhZuvX

— nascarman (@nascarman_rr) August 15, 2024

In an attempt to widen out the racing groove, NASCAR began treating the racing surface with a traction compound for the August 2019 race weekend. Initially, it was just applied to the top two lanes, leaving the bottom 3 untreated. After three rounds of practice, most of the drivers agreed that it was applied too high on the racing surface to be of much use.

“You can’t get to the PJ1, it’s too high. Hopefully when they get done with the truck race they lower that down three or four feet so that at least the middle-third lane has PJ1 in it, because right now it is really hard to get to.”

-Kevin Harvick, 6-time Michigan Winner

NASCAR did listen to the drivers and lowered the application zone by a couple of feet to cover some of the third lane for the race. While the additional grip in the upper lanes does give drivers some more options as far as driving lines go, it creates an interesting problem from a setup standpoint. With the track untreated, drivers would move their line up to help with rear security, in other words, to reduce oversteer. The key to getting around Michigan is to maximize on-throttle time. If the car feels too on edge in the lower grooves, the driver won’t have the confidence to carry throttle through the corner. Moving their line up increases the radius of the corner and reduces the cornering force required to get around it.

The cornering force (Fc) that a car can provide is a fixed quantity with a given setup. The tires can only provide so much grip. The mass of the car (m) is also fixed. The two variables in this equation, velocity (v) and corner radius (r), are both controlled by the driver.

Now, as I stated earlier, taking a wider radius around Michigan International Speedway in a stock car tends to add rear security, aka reduce oversteer or increase understeer. The traction compound also does the same thing. This effect is magnified if a driver is only running in the very bottom of the treated area, where their left side tires are on untreated asphalt. Drivers can use this as a crutch if their car has too much oversteer, but traditionally the racing groove tends to move right into the bottom areas of the traction compound. If this becomes the preferred lane, passes will have to be made in the bottom lanes. The problem presented here is that if a racecar is handling well and neutrally balanced in the preferred groove it will likely have significant oversteer when a driver dives to the bottom for an overtake.

Additionally, the traction compound typically takes some amount of laps to become “activated” and more so on colder or overcast days. The adhesive properties of the compounds are temperature-dependent and need to be heated by the cars running over them in order to work as intended. It typically takes until mid-way through the first run for this lane to fully come in for two reasons. First, as tires wear out drivers will begin to struggle with the balance of their racecar and can be forced into the higher lane as they search for a racing line that compliments their car. Second, as the leaders put cars a lap down it will force drivers into the treated areas of the racetrack.

The Cup Series is running its traditional intermediate package this weekend, and the Xfinity Series is using its semi-speedway package which was last seen at Indianapolis Motor Speedway before the Olympic break. If you somehow haven’t seen the finish to that race, it’s well worth a watch.

WE ARE SPEECHLESS. pic.twitter.com/ScHYYn8EIv

— NASCAR Xfinity (@NASCAR_Xfinity) July 20, 2024

One thing to take note of for the Cup Series is that with their rear diffuser and under-body focused aerodynamics these cars make maximum downforce when the rear of the car is as close to the racetrack as possible. To keep teams from getting too crazy, NASCAR regulates the length of all four dampers. Therefore, to get minimum rear ride height teams will often times bottom out the damper, essentially turning it into a strut rod at the minimum legal length. The problem with this approach is that now the only part of the suspension that can deflect is the tire sidewall which can lead to an instantaneous failure.

Going into Indianapolis Xfinity Series race weekend, a lot of people expected the reduced horsepower to put on a fairly boring race. Unfortunately, due to a first-lap incident, I ended up watching the race as a fan from an Applebee’s. No, Mike Harmon was not there. But we did get to see a surprisingly compelling race.

Hopefully, the same level of excitement comes with this package when we roll into Michigan. Even with the reduced horsepower, drivers still had to lift out of the throttle on corner entry at Indianapolis due to the low levels of downforce that an Xfinity car produces. The biggest question mark going into Michigan is how much off-throttle time will there be. Whichever team finds victory lane will be the one that best balanced downforce and drag. For those of you watching from home, an easy way to tell if a car is high downforce or low downforce is by the side skirts, particularly the exhaust skirt on the right rear. A longer exhaust skirt (more material) means that the back of the car will be higher as it goes around the racetrack, producing more downforce. If a car’s exhaust skirt is cut right below the tailpipes that means the back of the car will be traveled down more indicating low downforce.

I have a feeling that this race will play out somewhat similarly to an Indycar race and there are several things that will be interesting to watch. If you’ve ever watched an Indycar oval race, you will often hear the team instructing the driver to adjust their weight jackers (wedge) or anti-roll bar blades when they go from leading to following.

In a stock car, these tools don’t exist. When following another car, a driver will be riding directly in their wake which reduces front downforce and creates understeer. This is why you saw Kevin Harvick peaking his left front headlight below Martin Truex Jr in the earlier clip. If the following car were to swap positions with the leading car, all of that air would be hitting their nose, significantly increasing front downforce and shifting the balance towards oversteer. A car that is neutral in traffic will have significant oversteer when leading. A car that is neutral in the lead will have significant understeer when following. Ideally, the car would be a little edgy on the oversteer side out front and slightly snug on the understeer side in traffic. Essentially, the driver will never be 100% comfortable with the balance of their car. The trick is to find a way to make your driver the least uncomfortable.

Another interesting side effect of this lead-follow dynamic has to do with fuel mileage. The leading car will likely be able to carry much more throttle than the following cars due to increased downforce. More throttle means worse fuel mileage which means longer a longer pit stop. There is a good chance that being the leader will actually be a disadvantage during certain portions of the race Xfinity Series race this weekend.

It should be fun to watch!

Photo: Top Photo Daylon Barr/Big Machine Racing. This post contains an Amazon affiliate link and we might get a small commission if you buy some stuff.