# Using Drag and Downforce to Tailor Stock Cars to Different Tracks

Aerodynamics is complicated. Let’s just get that out of the way. But it’s not so complicated that we can’t understand what’s going on with just a little patience.

## Why 3D?

Every wonder why they call it three dimensions? The reason it’s three is because I (or you) can denote any point in space with only three numbers. For example: a latitude, a longitude and an altitude. Since we’re dealing with much more limited spaces, a simple Cartesian Coordinate system, like this one, usually suffices.

The line that goes out to the point P is a three-dimensional vector. It’s got parts going in the x, y and z directions. By specifying that there is so much in the x-direction, so much in the y-direction and so much in the z-direction, I’ve told you everything you need to reconstruct that vector.

## Forces in 3D

A force (a push or a pull) can act in any direction, but in order to understand the effect of that force, it’s easier if we break it down into its components – how much of the force acts in the x-direction, how much acts in the y-direction, etc.

When we do this on a race car, we give the different directions their own fancy names – just to make us seem extra clever. Basically, any force that acts in the opposite direction the car is going is called drag. Any force that pushes the car into the track is called downforce.

When the force acts up instead of down, it’s called lift. Yes, I know it should be called ‘upforce’, but the people who study aeroplanes named it.

Not shown in the picture below is side force, which would be in or out of the page.

Let’s think about the air acting on the spoiler. Because the spoiler is at an angle, the force on the spoiler is at the same angle (it’s perpendicular to the surface). So some of the force on the spoiler points down and some of the force points horizontally.

Which means some of the air hitting the spoiler creates downforce, and some of the air hitting the spoiler creates drag.

The more area there is for the air molecules to hit, the larger the force. A tall spoiler creates  more force than a short spoiler – but because of what I said up above, the angle of the spoiler is absolutely critical.

The more upright the spoiler, the more of the force is drag and the less of the force is downforce.  If the spoiler were horizontal, you’d get all downforce. If the spoiler were perfectly upright (vertical), all the force would be drag.

## Why a Different Package for Indy vs. Kentucky?

In Kentucky, NASCAR went with a shorter spoiler to reduce the downforce. Passing has been a persistent problem at 1.5 mile tracks and the idea was that if the cars weren’t quite so dependent on aerodynamic forces, then the loss of those forces when you get close to another car wouldn’t have such a great impact.

And that strategy seems to have paid off well.

But Indianapolis and Michigan are very different kinds of tracks. At 2.5 miles and 2 miles respectively, they  are closer to superspeedways than they are to 1.5 mile tracks. At Indy and Michigan, the cars get going very fast down the straightaways, which lets the leading car get away from its pursuers. And it’s pretty tough to pass a car if you’re two lengths behind it going into the corner.

So the goal at these almost-superspeedway tracks is to slow down the maximum speeds along the straightaways so that a car can’t get away so easily. This is a little different than the goal at the intermediate tracks.

There’s a couple of ways to slow down a car: the two most obvious are

• Decreasing horsepower
• Increasing drag.

Decreasing horsepower introduces its own challenges, as we know from restrictor plate racing, so NASCAR is using the increased drag approach at Indy. And they’re doing that by setting the spoiler height at a pretty astounding 9 inches tall. At Kentucky, the spoiler had been reduced to 3.25 inches.

The best way to understand how much of a difference this is comes from a tweet from JGR Racing, which actually shows you the difference. Extra points for having gotten the product placement in there!

That’s a pretty big honking spoiler, eh?

But, you’re thinking (at least I hope you’re thinking) wait a moment… If they increase the spoiler height to increase the drag, aren’t they also increasing the downforce?

Yep. They are. It would be lovely to have a knob that you could turn and independently change the amount of front and rear downforce, and the amount of drag. But real life isn’t that simple.

## Those Poor Engineers… NOT

The spoiler isn’t the only thing that’s changed. The changes in toto are…

• 9″ spoiler
• 1″ wickerbill (aka Gurney flap)
• 2″ splitter
• 43″ radiator pan width
• speedway extension on the quarterpanels and rear bumper – the same ones run at the superspeedways.

So you’re thinking – my goodness, pity the engineers. All these changes.

Lemme tell you – the engineers are not upset. They love the opportunity to get ahead of the other teams by being smarter and figuring stuff out before someone else does. This is a chance for a team to get a win simply by understanding the set ups better than anyone else.

And something else to think about. In my column about Kentucky, I showed the changes in the spoiler and radiator pan sizes as a function of time. Well, I’ve updated those.

The radiator pan is the exact same size at Indy as it was in 2014. The spoiler is only one inch taller than it was in 2014. And the teams have plenty of experience with the rear aerodynamic extensions from years of racing at Talladega and Daytona.

Yes, it does mean that they have to put those disparate elements together – which they haven’t done before – but the teams with the strongest technical staffs will be in the best position to take advantage of these just-in-time adjustments.

Personally, I’m psyched about track-specific packages. It gives the teams much more of a box to work in, which means they have that much more room to be creative. Looking forward to Indy!

1. Engineer-X says:

FUN FACT: Actually the Indy package does not increase downforce. The lower bumper kills most of the underbody aerodynamic effect, which is quite large these days. That part creates drag while decreasing downforce. As for why the cars are getting loose while in traffic, that has yet to be seen.

• I’ve talked to one engineer who claims the downforce is a little lower (Kyle Busch also said this) and one who says its a little higher. I suspect the answer is that there’s not a heck of a lot of change when you look at it net-force wise. The rear-quarter and end panels that are run at superspeedways were added for a reason, right?

• Engineer-X says:

Sort of. The speedway package is a whole different animal. The rear extension (same used at IMS) is/was there to increase drag and stop the tandem. That in conjunction with the “mail slot” grill opening on the speedway cars means there’s no way to tuck super tight behind the car in front of you, or you totally remove all air flowing into the radiator. They also don’t have a radiator pan on those cars, just a very small splitter, and Nascar mandates things like shocks and springs so they have to run in packs. (and this is why teams really dislike speedway races). I think the aero balance actually shifts on speedway cars, but that could be due to the “get less drag” rather than the “get more downforce”.

2. Of course, it’s a huge oversimplification treating the car like a point particle and not differentiating between front and rear downforce. But people tend to get glassy eyed when you talk about force moments!

3. bob emmons says:

You might want to do a blog on how they taylor engines to different tracks. That cost is much more than changing the configuration of the car.

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