Will You Be Able to Tell if NASCAR Cars are Slower in 2019?

When is the answer to better racing slower cars? It just might be for NASCAR.

Note: This post analyzes the change in speed due to the new rules package. Please don’t interpret it as my defending the rules change. I won’t be making up my mind on whether this package ‘works’ until after Phoenix at the the very earliest. I share some of the drivers’ worries that instead of a level playing field, we’re going to get random winners. But I’m going into it with an open mind and we’ll see.

Why Slower?

For years, NASCAR has battled ‘the passing problem’. Teams have made huge gains in aerodynamic downforce over the years. These gains don’t just affect peak speed: If you have more grip, you can corner faster. Higher corner entry speeds narrow the racing groove, which makes it difficult to pass. The theory is that decreasing corner entry speeds will widen the groove and allow for more passing.

Aerodynamic downforce (Fd) depends on speed (v) squared.

F_d \propto v^2

The squared is important:

  • If you double the speed, you get four times the downforce.
  • Conversely (and more relevant to our purposes), to reduce the downforce by a factor of 2, you only have to reduce the speed by a factor of the square root of two (which is 1.41)

The graph below compares the mechanical grip of the car (i.e. its weight) with its aerodynamic force. The weight doesn’t change with speed, but the aerodynamic downforce does. The faster you go, the larger the fraction of your downforce that is aerodynamic.

A graph comparing mechanical and aerodynamic grip.

NOTE: These numbers are for an earlier rules package, but they give you an idea of how significant aerodynamic grip is.

The easiest way to slow down a racecar is to decrease its engine power. That’s why NASCAR is using differently sized tapered spacers at different tracks in 2019.

A bar graph showing how the effective horsepower changes with various sized tapered spacers and restrictor plates

So Why Give the Cars More Downforce?

It seems counterintuitive that, at the same time they took away engine power, they increased the spoiler and splitter sizes. That’s because the situation is much more complex that simply total downforce. Different corners of the car experience different forces.

  • When turning left, the mechanical load shifts to the outer wheels and the inner wheels lose grip.
  • When accelerating, the rear wheels gain grip and the front wheels lose it.
  • When braking, the front wheels gain grip and the rear wheels lose grip.
  • The aerodynamic downforce depends on the attitude of the car, so when the car turns, brakes, or accelerates, the aerogrip of each wheel changes.
  • Proximity to another car (or a wall) near a car also changes the aerodynamic forces on the car.

Remember that the goal isn’t simply lowering speeds: It’s lowering corner entry speeds. You don’t need to slow the cars down everywhere. Also, you want the cars to have enough stability to be able to race close to each other. That’s the only way they can pass.

So NASCAR took away horsepower, but made a number of other aerodynamic tweaks. NASCAR took from the engine, but gave back a little of what they took on the body.

How Much Slower?

All some NASCAR fans heard was that they were dropping 200 hp and there was a hew and cry from some NASCAR fans that this rules package would Ruin. NASCAR. Forever.

In reality, the change in average lap speed at the affected tracks is likely to be in the 7- 12 mph range. Some people still think that’s a big deal.

But can you even tell whether a car is going 200 mph or 190 mph?

CAVEAT: Record books record average lap speeds. If you slow corner speeds, average lap speed will slow, but we don’t know how much slower the cars will be on the straightaway — if they are slower at all. Everything from here on out is dealing with average lap speed.

CAVEAT 2: As I’ve discussed before, the speeds reported are not measured. Lap times are measured and the average lap speed is calculated using the standard track length. That doesn’t account for different lines around the track. I’m using speed here because that’s what people complain about, but it would be the exact same analysis if I used lap times.

Speed vs. Time

One of the very first equations you learn in physics is the equation that relates average speed (v) to distance (x) and time (t):


The television broadcasters like to show this formula when talking about pit road speed and, much to my consternation, someone gets it wrong at least once every year. So watch for it and inundate them on twitter if they do it again this year.

It’s not like this is a difficult formula to remember. Speed is measured in miles per hour. To get a distance (in miles) from miles per hour, you have to multiply by time. Scientists call this dimensional analysis. Regular people call it common sense.

Our perception of speed is really our perception of the time it take an object to traverse a distance. Since the distance the racecars travel doesn’t change, what we’re really talking about is how we perceive time.

What is Time, Anyway?

There’s a philosophical answer to that question and a physics answer. Let’s go with the physics answer first.

The second was originally defined at 1/86,400 th of a mean solar day. (‘Mean’ meaning ‘average’, not ‘cruel’.) This was slightly less than robust, so in 1960, the world agreed that:

One second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom when the cesium atom is ata temperature of 0 Kelvin.

National Institutes of Standards and Technology

Believe it or not, that is actually much clearer than the first definition. But for us, it doesn’t really matter because what’s important here is how we perceive time, not the actual time.

Time vs. Time Perception

While the second is a very precisely defined quantity, your perception of it differs if you are getting married or sitting in the dentist’s chair. They may both last the same time, but it sure doesn’t seem like it. And there are people who devote their lives to trying to understand why this is.

The study of time perception requires psychology, linguistics and neuroscience. It considers time, not the objective way we measure it, but the way human beings experience it. And human beings’ experience of time is highly subjective.

  • People tend to remember relatively recent events happening further back in time then they actually happened; conversely, events from the far past are often remembered as being more recent.
  • Vierodt’s law says that shorter intervals tend to be overestimated while longer intervals tend to be underestimated.
  • How long you perceive a task to take depends on how motivated you are to do it and whether you are focused on doing the task or just having it done.
  • Your emotions and mental state can change how you perceive time.
    • People who are depressed tend to more accurately estimate the passage of time.
    • Fear tends to make time seem longer.
    • Even something like very loud or very high pitched noises can skew our perception of time. You will perceive a loud car as being faster than a quiet car

Hence my husband’s suggestion that all NASCAR really needs to do is add 10 deciBels to the engine so everyone would think the cars were faster.

The Data

The only numbers we have with the new rules package so far come from the late-January test at Las Vegas Motor Speedway. So let’s use LVMS as our test track and take a look at historical pole speeds. (Data from the good folks at racing-reference.info, as always.)

The history of pole speeds as a function of time at Las Vegas Motor Speedway
The red square represents the 2018 fall race.

If we consider only data after the track was reconfigured in 2006, you can see that the pole speed increased almost 15 mph from 2008 (182.3 mph) to 2016 (196.3 mph). The most recent pole speed (fall 2018) was 188.1 mph. The pole speed that spring was 191.489 mph.

Now let’s look at the data from the January testing sessions. There were three test sessions and 50 total runs. These are the top speeds for each car from all three sessions.

A histogram of the top speeds for the January 2019 test at Las Vegas Motor Speedway

Of the 50 runs reported

  • 22 (44%) were between 177 mph and 178 mph.
  • Only 5 runs (10%) were above the median speed
  • This leaves 22 runs (44%) below the median range.
  • The highest speed in a team car was Jimmie Johnson on Thursday morning, at 178.885 mph.

OUTLIER EXCLUSION: Ross Chastain reached 183.16 mph on Thursday morning, but a) that was in the heavily instrumented Chevy Wheel Force car and b) No one else got anywhere near that speed during the entire test. I’m being conservative and going with Johnson’s speed.

That’s the Clock: What About Me?

What would you see if you were sitting in the stands at LVMS?

The front straightaway at Las Vegas is about 1800 feet. How long does it take to drive that distance?

The time it takes to go 1800 feet at different speeds from 178 to 188 mph.
  • If you drove at last year’s pole speed (188.1 mph), it would take 6.524 seconds to make it all the way from Turn 4 to Turn 1.
  • If you drove at the top speed in the testing session (178.885 mph), it would take you 6.861 seconds.

A difference in speed of 9.236 mph corresponds to a difference in time down the frontstretch of about a third of a second (337 milliseconds). The blink of an eye takes about 300-400 milliseconds.


The difference we’re talking about ‘slowing the cars down’ is

  • About a third of a second down the frontstretch
  • A little less than 2 seconds over a 1.5-mile lap
  • 8.9 minutes over the course of a 400-mile race.

Will I Notice That?

Yes — because you’ll be looking for it.

Unfortunately, we can’t do the experiment because you already know the cars will be slower. Your expectations impact what you see. To really test this, we’d need to have you at a track, have a driver make laps at two different speeds and ask you which one was faster and by how much. And we’d have to do it with maybe a thousand people to get a range of opinions.

It would be like when you visit the eye doctor and do the ‘Which is clearer: 1 or 2’ thing. When we’re close, I can’t tell much of a difference. I bet you’d see the same thing with the track experiment.

In general, people aren’t very good at estimating speeds. A 2016 study from the Centre for Accident Research and Road Safety (in Australia) found that drivers did a miserable job estimating how fast trains were coming when approaching railroad crossing.

When the train was 350 m (382 yards) away

  • Drivers estimated a train going 130 km/h (~81 mph) was only going 90 km/h (56 mph) — a 29% error in judgement.
  • That means the drivers thought they’d have 14 seconds before the train got there when, in reality, they only had 10 seconds.

The farther away the train was, the worse their speed estimates were. When the train was 1100 m (1200 yards) away and going 130 km/h (~82 mph), people estimated the train was only going 75 km/h (47 mph) – a 44% error in judgement.

The lesson here might be that if you think you just have time to scoot past a train crossing before the train gets there, you probably don’t.

A 2009 paper in the journal Vision Research found that people are also very bad at estimating the speed of a flying object, such as a ball. And there are plenty of studies that show that people have a really, really hard time interpreting the motion of accelerating objects, even if those objects are moving slowly.

The takeaway: If you’re one of those people who’s already convinced that the cars are going to be slower and it’s going to ruin your enjoyment of the sport, it’s very likely that this is exactly what is going to happen.

But Is Racing Really Just Speed?

I’d argue no. So would NASCAR, because the entire point of this speed-reduction exercise better racing at intermediate tracks.

I love the people who call into radio shows and say that NASCAR should just use a rules package that is fast and allows for close racing. Believe me, if that were possible, they’d already have done it. That’s what they’d like, too. But the laws of physics say otherwise. And the laws of physics always win.

If all you care about is how fast the cars go, you’d probably find drag racing more your, uh, speed. If you want fast stock cars, follow the folks setting land speed records — like Bob Keselowski, who shattered the all-time stock car speed record by 30 mph at the Bonneville Salt Flats last September by running 271.84 mph.

The best part of this story is that this Dodge Charger was driven in a 2010 Martinsville race, crashed, and repaired. The second-best part is the headlines: “Retired NASCAR Cup Racer Makes 271.89-MPH Pass, Smashes Class Record at Bonneville” and “1989 ARCA Champ Keselowski Sets Speed Records in Utah“. That’s Bob Keselowski on the left and, I think, Bobby Gerhart on the right, who was there to earn his license.

NASCAR fans have been complaining about lack of passing for years. NASCAR has made a number of attempts to address it given the constraints of time and money. I guarantee you that all of this is being taken into account as they design the Gen-7 car.

For me, watching two cars run fender to fender as one tries to pass the other is the best part of racing. It’s good at Bristol at 100 mph and it’s good at Texas at 200 mph. And if I get a choice between Texas at 200 mph with minimal passing and Texas at 190 mph with more passing, I’ll take the latter, thanks.

In summary, the world is not fair. No one’s figured out a way to get high speeds and passing with the current car. Sure, you could solve the problem by throwing money at it, but that’s not a possibility in the current climate. What you see is highly influenced by what you expect to see. So I encourage you to go into Atlanta with an open mind and fingers crossed.

Thanks to @dmcgrew, who figure out I’d transposed the digits in the calculation of time and had 6.681 instead of 6.861. I can’t blame this one on Excel. I messed up typing! Thank you, Drew!

The 2019 Rules Package: The Tapered Spacer

NASCAR announced the 2019 rules package last week. Reactions from fans ranged from wait-and-see to despair to a surprising amount of anger.

The 2019 package is similar to the package was tested at the 2018 All-Star Race, but different in some very important ways. It further specializes the standard set up for different types of tracks. The larger spoiler and splitter and the underbody changes will be required at all tracks. Other changes (the tapered spacers and the aeroducts) will be different at different tracks.

So how much is really changing?

Tapered Spacers Aren’t New

A 1.170″ tapered spacer was introduced in 2015, reducing engine horsepower from about 850 hp to 725 hp. (Note that teams did quickly find ways to increase the horsepower a little, despite the new rules.)

That same spacer that was introduced in 2015 remains one of two spacers that will be used in 2019.  You’ll also note that the spoiler goes back to its 2014 height and the driver-adjustable track bar that was introduced in 2015 is being taken away.

Which Rules at Which Race?

  • The Daytona 500 will be the only race that will use a restrictor plate.
  • A 1.170″ tapered spacer will be used at all tracks less than 1.33 miles and will (still) result in about 725 hp* overall engine power.
  • A 0.922″ tapered spacer will be used at all oval tracks 1.33 miles and above. which will decrease engine horsepower to about 550 hp*.
    • At five of those larger tracks (both Pocono races, Darlington, Atlanta and Homestead), the aero ducts won’t be used.
    • Aero ducts will be used at the rest of them.

*Note: NASCAR regulates the spacer/restrictor plate, not the horsepower. 

Since different horsepower means different mileage, NASCAR will require teams to block part of the fuel cell so that the number of laps run between full fuel runs should be the same.

Let’s see if we can’t make this clearer with a graphic.

You may wonder about my choice in how I listed the tracks. They’re listed in order of most recent pole speeds for each track. Most pole speeds came from 2018, but some qualifying sessions were rained out, so I went to 2017 and, in one case, 2016.

I put Sonoma at 2.52 miles because they’re changing it for next year, even though the pole speed was for the shorter track.
  • Tracks with the 1.170″ tapered spacer and no aero ducts have pole speeds ~160 mph and below
  • Track with the 0.922 tapered spacer and no aero ducts have pole speeds 173-184 mph
  • Tracks with the 0.922 tapered spacer and aero ducts have pole speeds 185 mph and up.
  • Chicagoland looks like an exception, but the pole speed there this year was anomalously low, so I understand why they put it with the last group of tracks.

Lemma: Doesn’t Pole Speed Scale with Track Size?

Sort of. In the graph below, the three configurations are represented by the same colors as I gave them in the table above. You could make an argument that speed is somewhat linear for the short tracks. (I did use five or six speeds for each track and they’re pretty consistent.)

But our 1.5-mile tracks range from 173 mph to 201 mph, and the superspeedways are already artificially restricted.

Tapered Spacer vs. Restrictor Plate

The All-Star Race package used restrictor plates to throttle the engine back to around 400 horsepower. Most fans liked the All-Star Race, but a few drivers (notably Brad Keselowski) objected, saying that the pack racing that resulted put less control in the drivers’ hands. 

It’s hard to compare All-Star Race data because they change the rules every year. Some years require a pit stop during qualifying. However, in 2013, 2014, 2015 and 2017 (2016 was rained out), the pole speed was between 144 mph and 147 mph. The pole speed for last year’s All-Star race was 127.644 mph.

How Plates/Spacers Work

A restrictor plate (or the tapered spacer) slows cars down by preventing them from bring air into the engine as quickly as without the plate or spacer. Remember that aerodynamic forces go like the square of the speed: double the speed and you quadruple the force. 

When cars are so dependent on aerodynamics, however, it’s hard to pass. Lower speeds, along with the aero ducts creating bigger wakes behind the car is theorized to improve passing. 

The Chemistry of Fast

Combustion is the chemical reaction between fuel and oxygen that releases energy. It’s very similar to another chemical process called respiration, which is how your body converts food to energy.

For example, to combust two octane molecules, you need 25 oxygen molecules. Not 24, not 26, but exactly 25.


So if reduce how many air molecules you have in the cylinder, you have to reduce how much gas you put in the cylinder. Otherwise, you’re just wasting gas.

But tapered spacers and restrictor plates work differently when you get down to the details. Below, on the left is a restrictor plate. It is 1/8″ thick and really nothing more than a plate with four holes on it. On the right is a tapered spacer, which is on the order of an inch thick. It also has four holes, but the holes are conical.

In fluid or aero-dynamics terms, the restrictor plate is a set of orifices, while the tapered spacer is a set of nozzles.

Both fit over the spot where the carburetor used to be.

Some Differences

There are a couple minor differences between these two parts. Restrictor plates are stamped: A big die comes down and punches out the hole, leaving a bit of a chamfer on the side the air enters and a bit of a burr on the side the air leaves. This significantly affects the way the air travels into the engine. One engine builder told me that the four outer cylinders get about ten times less air than the central four cylinders with restrictor plates. Tapered spacers, on the other hand, are machined parts and provide for much more even distribution of air.

Because the restrictor plate is so thin, any imperfection makes a big difference. A scratch can mean more horsepower. Tapered spacers are much less sensitive.

The Big Difference

While the distinction between the two seems minuscule, air molecules see two very different things. The next two graphics show the airflow through an orifice and a nozzle. The air comes in from the left.

The animation I took these clips from shows the motion, but you can still see that there’s much more turbulence with an orifice than a nozzle. Some of the arrows to the right of the orifice point away from the orifice, while others point toward the orifice. The flow lines are messier, also.

Compare that to the nozzle (below). See how much cleaner the flow lines are? And all the arrows point away from the nozzle. There is much less turbulence with a nozzle than with an orifice — meaning that the tapered spacer provides air to the engine in a very different way than a restrictor plate does.

When an air molecule passes through an orifice, it essentially has to take a right-angle turn. The flow of the air makes the effective diameter of the orifice smaller than the actual hole diameter.

The walls of the tapered spacer NASCAR uses are machined to be at a 7 degree angle. That’s the maximum angle at which air can travel without separating from the surface. This minimizes turbulence.

Simulations are useful, but there is nothing like being able to see fluid flow. Luckily for us, air and water are both fluids and a five-gallon water bottle provides the perfect illustration of the differences between a tapered spacer and a restrictor plate. 

A hole — the same size as the hole at the top — is cut into the bottom of the five-gallon jug. The hole in the bottom is like a restrictor plate and the top is like a tapered spacer. 

Now watch how the water empties differently.

Why is NASCAR Reducing Horsepower?

NASCAR is all about speed, but high speeds mean high dependence on aerodynamics (difficulty passing) and requires harder, stiffer tires (less opportunity to engineer fall-off into the tire).

More Tire Falloff.

When I talked with Goodyear’s Greg Stucker, he noted that race length doesn’t really factor into tire design because a tire only has to last for a fuel run. That forces teams to change tires (or chance not changing tires).

Corner entry speeds have become so high that Goodyear has to make relatively hard tires — which means they don’t wear as quickly. Lower corner entry speeds will allow Goodyear to go to softer compounds and different constructions. That will allow for more strategy.

The Future of the NASCAR Engine

Formula E, in its sixth season, has eleven manufacturers. The overwhelming interest in what is currently a niche series can be attributes to manufacturers looking to remain relevant in the future.

NASCAR’s been very clear that the eight-cylinder eight-hundred+ horsepower engine is a barrier to entrance for other manufacturers because it’s so far removed from current production cars. 

“it gives us the option to be more relevant. It gives us that option to look at new technology in the future and our current package doesn’t do that.

Steve O’Donnell (via NBC Sport)

So why not just mandate a new engine and not worry about tapered spacers? Change costs money and takes time. There are parts inventories to be considered, especially as teams struggle for sponsorship. There’s an additional issue for engine companies because NASCAR is requiring engines to be used for multiple races. That means fewer engines built.

NASCAR is the proverbial aircraft carrier trying to turn. The inertia is huge.

Aerodynamics and Passing 

Every engineer I’ve consulted says that their simulations tell us to expect pack racing at most of the 1.5 mile tracks next year. They expect drivers will need to be full-on the throttle around the track, although that may not be the case with less-banked corners and/or if tires have a lot of fall off. 

Pack racing gives you passing, but it’s a different kind of passing. If the inside line passes the outside line on the frontstretch, then the outside line passes the inside line on the backstretch, that seems to me to be a wash. 

So Will It Work?

Most drivers are taking a wait-and-see approach to the new package. Even Kyle Busch has been restrained in his remarks after the new package was announced. Everyone agrees that NASCAR is trying to improve the racing, even if they don’t agree with the precise way they’re doing it. 

The aerodynamic changes and the horsepower changes together are a pretty big change. There’s a test with the new package at Charlotte on October 23rd. We’ll see what happens, both on the track, and with driver reactions.

And there’s still time for changes. Goodyear can tweak tires and teams have time to experiment with different set ups. It’s all part of progress. You try things and you see if they work. This is the nature of research.

I’m along with Joey Logano on the change for the moment:

“You make change, and not every change is good, but you learn from every change. If you just sit still, you never make any progress forward. You don’t learn what’s wrong, you don’t learn what’s right, you’re just there… 

we will learn from this decision one way or the other, and I think as a society we need to be open to do that, not just in our sport, but in life. It’s a good thing for us. It’s healthy.”

Joey Logano (via NBC Sports)