NASCAR’s Goldilocks Problem: How much Downforce is “Just Right”?

The Problem

The current racecar handles badly when its aerodynamics are disturbed by another car. This makes it very difficult to pass. The advantage of having clean air become insurmountable, which makes for less exciting racing.

Why? A word once relegated to engineers, but now you hear everywhere :downforce

There are two types of downforce

  • Mechanical downforce is the weight of the car pushing the tires into the track
  • Aerodynamic downforce is air passing over the car and pushing the tires into the track

The total downforce is a combination of the mechanical and the aerodynamic downforces — but there’s a big difference in how these two types of downforce change while racing

  • Mechanical downforce is constant
  • Mechanical downforce doesn’t change with speed (Steady state; there is load transfer during changes in speed or direction)
  • Mechanical downforce doesn’t change when you get close to another car
  • Aerodynamic downforce changes
  • Aerodynamic downforce increases with the square of the speed
  • When the airflow over the car changes, the aerodynamic downforce changes

The graph below illustrates the mechanical grip (blue) and the aerodynamic grip (purple) as functions of speed

The most important thing you should see from this chart is that the faster you go, the larger the percentage of your total grip is due to aerodynamics.

  • At 90 mph, about 12% of the grip is aerodynamic
  • At 205 mph, about 40% of the grip is aerodynamic

That means that you rely more and more on a type of grip that is subject to change depending on speed and where you are in traffic.

When you get in someone else’s wake, your aerodynamic downforce decreases — while the guy you’re trying to pass doesn’t experience much change to his or her car’s aerogrip. They have more total grip than you do.

So you’re running great, you come up on a car to pass it… and all of a sudden, you’ve lost grip. You can either slow down or crash — but passing becomes very difficult. This is why ‘clean air’ is such a big deal — the car you’re trying to pass keeps all its grip while you lose some of yours.

NASCAR’s Experimental Method

NASCAR has become very systematic in their validation of new rules packages. The Michigan race is the third step of a five-step validation plan for the new rules package.

BSPEED_AerodynamicsRules_NASCARRulesRevisionFlowChart

Everyone is thrilled NASCAR is making progress on the passing problem — but the crew chiefs would really like more time to implement rules changes. They’d especially like the 2017 rules package to be announced much earlier than this years’ package was announced.

But even if this package produces the best racing you’ve ever seen in all your life… you probably won’t see any changes until 2017. While everyone would like to see the rules changed, they don’t want it done mid-stream because one company usually manages to hit on something that takes advantage of the new rules faster than the others. You don’t want your champion determined by how fast engineers can experiment.

Fixing the Passing Problem

Hypothesis

The hypothesis NASCAR has been working on for the last three years is:

  • If you decrease the overall aerodynamic downforce, you
  • Lessen the importance of clean air (meaning the lead car doesn’t have as big of an advantage by simply being in the lead.
  • Decrease the fraction of a car’s grip due to aerodynamic forces
  • Make it easier for cars to pass
  • And the result is that you get more passing and better racing.

Let’s acknowledge that they work under some constraints, too.

  • Any changes to the car cannot compromise safety
  • Changes can’t be inordinately expensive
  • They can’t risk making a change that turns a race into a disaster
  • They can’t work too slowly because fans are mighty vocal when they don’t like something.

So far, I think you have to admit that NASCAR is doing a pretty good job.

A Note about Measuring Downforce

Specifying mechanical downforce is easy. It’s the weight of the car plus driver. So NASCAR controls that simply by mandating a minimum weight.

Gene Stefanyshyn, NASCAR’s Senior Vice President of Innovation and Racing Development stated recently that NASCAR had a goal of 1500 pounds of aerodynamic downforce.

But if downforce depends on speed, what does that number mean?

Brandon Thomas, Head of NASCAR Sprint Cup Series Vehicle Development at RCR told me that (most of the time) when you hear downforce numbers, they’re talking about the number at 200 mph. This is one of those things that is taken for granted by the people who work in the field and not even on the radar of most people who don’t. So it doesn’t get said. But when you hear a number being bandied about, it’s probably the aerodynamic downforce at 200 mph.

Decreasing Downforce

Aerodynamic downforce has been steadily marching down since 2014, as shown in the graphic below (Sources: 2006, 2015–2016). You’ll note that the target downforce number is pretty similar to the number from ten years ago and a little more than half (okay, it’s 55.5%) the downforce the cars had last year. So if you’re one of the people who have been complaining that NASCAR isn’t doing much just taking a couple inches away here and there, stop looking at the physical dimensions of the parts and pay attention to the downforce numbers.

The other interesting thing is the increase from February to May 2016. You don’t remember a rules change? There wasn’t one. That’s just how brilliant the team engineers are. Stefanyshyn said that teams have gained back about 300 lbs of downforce since the beginning of the year.

What NASCAR taketh away, engineers try to taketh back

So even if we see a rules package for 2017 that is right at the target downforce number, believe that NASCAR will have to make some minor tweaks throughout the year as teams find ways to wriggle around within the rules.

This reminds me so much of when I first started teaching. You’d give an exam and then you’d realize that people were cheating. So you’d re-design the exam to make it impossible for them to cheat that way. Darned if they didn’t find other ways to cheat. So you’d have to change the exam again to prevent that way of cheating. Just remember: It’s a moving target and it will always be moving.

The Michigan Package

This week, the teams are running a package that is likely to be (or be close to) the 2017 rules package. Teams lose another 500 lbs of downforce on top of the 900 lbs taken away between 2015 and the start of the 2016 series. The Michigan package (which will be used in Kentucky, too) includes:

  • A shorter and narrower spoiler (with deck fin smaller to match the spoiler)
  • A narrower radiator pan (See here for pictures of the radiator pan)
  • Less side splitter

To meet the constraint of keeping costs down and giving them maximum flexibility, NASCAR is doing as much aerodynamic modification as they can with bolt-on pieces — as opposed to making body changes. Changing the body introduces all kinds of new problems

  • Body parts are much more expensive than splitters and spoilers
  • Body changes make a huge number of fairly existing parts useless, costing teams money
  • It takes a lot of time to settle on three different body shapes that are true to the manufacturer and give equivalent aerodynamics numbers
  • It take much more time to change a body, so if you’re wrong, it’s a big investment of time and money wasted
  • NASCAR has to re-do their inspection parts and pieces

Spoiler Alert

The most visible change is the spoiler, which is not only being decreased in height, it’s being decreased in length as well. Just for kicks, I compared it to the plate track package spoiler, too.

Aerodynamic downforce is created by air molecules hitting the spoiler. The more area on your spoiler (i.e. the longer and taller it is), the more air molecules can push down and the more total downforce produced.

Note also that a smaller spoiler decreases drag. The Michigan spoiler is the same width as the spoiler used at plate tracks, but it’s two inches shorter.

NASCAR’s experimented a lot with spoiler heights. If you consider smallest to tallest, that’s a 320% change over the last three years.

 

If we summarize the change to the spoiler over the last three years just according to the base rules package — and let’s assume for now that the Michigan package is what we’re likely to end up with for 2017 — we can see how they’ve been steadily shrinking the spoiler.

The Hitch: Balance

If you’ve been paying attention, you will notice that every change in spoiler height is accompanied by a change in the front of the car. If you take away rear downforce and keep front downforce, you get one heck of a very loose race car. While some drivers like loose race cars, there’s a point at which they’re just impossible to handle and that’s going to make the racing worse instead of better.

 

The front downforce modifications have been made by changing the radiator pan and the splitter. Because NASCAR’s experimented a lot with spoiler height, they’ve had to make corresponding radiator pan changes over the last few years.

 

If we eliminate all the experiments (I don’t know too many people who would claim last year’s Michigan rules package was a win) and just look at radiator pan widths in the base rules package for a year…

This is interesting because the pan width is likely going UP again — which seems counter the way it has been going. That may have to do with the fact that the aerodynamics on the underside of the car also affect how air moves out from under the car, which in turn impacts safety. Air under a racecar is a bad thing because it provides lift. Lift is a great thing for airplanes. Not so much for race cars.

But another thing to look at is the changes in the other front aerodynamic element: the splitter. In 2015, two inches of splitter projection was allowed. The 2016 rules package, with its smaller spoiler, required this be decreased, so this year, it’s only a quarter inch.

The problem is that you don’t have anywhere to go from there, really. You have a similar issue because there’s only so much you can change the radiator pan — there’s not a lot of room under the car.

Something that hasn’t been talked about much (or has been talked about WRONG) is that NASCAR is modifying the SIDES of the splitters. The 2016 rules allow 5 inches of splitter on the sides. For Michigan, that’s been reduced to 2 inches.

 

But even with specifying the front and the sides differently, NASCAR is running out of room to make changes in the splitter — unless they want to move into making changes to the way the splitter is positioned on the car.

But I bet they have to do that in the future.

A Note on Speed

When people talk about what NASCAR is trying to do with all these changes, they talk about “slowing down the cars”. But NASCAR is actually after something a little more subtle than that.

If it were merely about slowing down the cars, they could just weld an 8″ plate across the roofs and increase the drag enough to achieve the desired speed. That’s not what they want.

They want lower corner speeds. They want drivers to have to brake going into the corners, which greatly increases opportunities to pass. Slower corner speeds mean that it takes drivers longer coming out of the corner to reach their maximum straightaway speed, which again increases opportunities to pass. If I’m willing to get on the gas sooner in the corner to beat out someone with better torque, I’ll have that option.

The results from the Michigan test:

 

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