Does Less Downforce Mean More Lift?

Last Tuesday, NASCAR announced aerodynamic modifications to be implemented for the Kentucky Speedway Sprint Cup race on July 11th. While the changes are (right now) only for that race, there’s every expectation that if they help reduce the dreaded ‘aero push’ problem, they may be extended (or modified) for other 1.5 mile tracks.

The changes are fairly straightforward to make, which is why NASCAR can mandate them without much lead notice. All the parts are bolt-ons, as opposed to changes in the body panels, for example. Here they are:

1. The rear spoiler will be shortened from 6 inches to 3.5 inches
2. The front splitter will be shortened by 1-3/4 inches
3. The radiator panel will be narrowed from 38 inches to 25 inches.

These changes continue in the vein of the changes made at the beginning of the 2015 season. At the start of the season, the spoiler was decreased from 7-1/4 to 6 inches and the radiator pan was narrowed down to 38 inches. So the changes are designed for the same goal: decrease the dependence of the car on aerodynamic forces so that passing isn’t quite as hard as it is now. This translates to decreasing the downforce on the car – depending less on aerodynamic grip than on mechanical grip.

The question I’ve heard the most this week is some variation on being concerned that taking downforce away from the car might lead to the cars being more likely to become airborne. I’ve discussed how cars can take to the air. Let’s look at how aerodynamic forces are generated so we can understand how big a problem this is — or isn’t.

We need to start with Bernoulli’s Principle. It’s a rather complicated principle, but we only need one part of it to apply it to a race car.

Here’s how I remember which way it goes. The roof flaps on the car are there because, when the car goes fast, the air moves quickly over the roof of the car and the pressure on the roof decreases. If the car spins, the roof flaps open. That slows down the air, and increases the pressure, keeping the car on the ground. (I’ve also done a pretty complete discussion of roof flaps in a previous blog.)

At right is a computational fluid dynamics diagram generated by Michael Waltrip Racing shown at right. Their color scheme is that the slowest moving air (the highest pressure) is red and the fastest moving (lowest pressure) air is in blue. (It’s a rainbow, except without indigo and violet.)

If you look at the front of the car, you’ll notice that the splitter area is a place where the air slams up against the car and slows way down. And, in fact, that’s why the splitter is there. The splitter generates front downforce. If we looked at the rear of the car, you’d see another area of higher pressure by the spoiler, which is responsible for rear downforce. This diagram also shows you the low-pressure area at the top of the car.

Net Force: Lift vs. Downforce

The classic example of how Bernoulli’s law is harnessed in vehicles is the airplane wing. The wing is shaped such that air moves faster over the top of the wing than under the wing. This means that the force pushing up is greater than the force pushing down, and that creates a net lift. (And if you want to see something really cool, take a look at this new NASA plane that changes wing shape in real time.)

 

If you want downforce instead of lift, you shape the wing differently.

 Splitting Airs

The splitter is so named because it literally splits the air coming at the car into two parts: air that goes over the top or around the car and air that goes under the car. A splitter is a much more tunable device than the old air dam, which provided a vertical surface to direct air around the car, but had no horizontal component.

The air running into the car is slowed down, which creates downforce and the faster moving air gets underneath and creates lift.

The splitter is designed so that you can vary how much of it juts out from the car. This is important because the force you get from any surface is the pressure times the (perpendicular) area of the surface. Having less splitter exposed means less area, which translates into less downforce. But it also means less lift, because the same air stream is creating both the up and the down forces.

At the same time, they’ve made the radiator pan (which is poorly named, because it has nothing whatsoever to do with the radiator) smaller. If the underside of the car is nice and smooth, air flows quickly under the car. The radiator pan is really just a flat panel that covers the pipes, ducting and other complicated shapes and presents the air with a smooth surface.

The larger the radiator pan, the more of a ‘sucking effect’ you get, where the lower pressure actually acts like a vacuum, pulling the car down to the track. NASCAR made the pan even narrower than before, which slows down the air.

I would be very surprised if there were any greater incidence of vehicles going airborne – especially given the lower horsepower we have now relative to 2014. Remember that aerodynamic forces go like the speed squared, so if you’re going slower, you’re not making as much lift or downforce.

Radiator Pan

The radiator pan is really just a flat piece of metal that (on a stock car) has nothing to do with the radiator except that it’s located in the general area in which the radiator is located. Its sole purpose is smoothing out the surface underneath the car.

The picture is from a NBC NASCAR America video in which Steve Letarte summarizes the 2015 rules changes. It’s the best overview of the changes and the one I use for reference all the time.

The underside of a race car is sort of a mess. Pipes, tubes, ductwork all snake their way through. Aerodynamically, that rough surface, with all its dips and peaks, slows the air down as it moves under the car.  Putting a smooth sheet of metal on the underside of the car decreases the drag and allows the air to flow more smoothly (which means faster) under the car.

And since faster means less pressure, a smooth undercar surface tends to ‘suck’ the car down to the track, giving you more downforce. There’s another effect, however. If a car does start to get airborne, a huge flat plate provides a really nice surface to generate lift off. Decreasing the size of the plate makes the underside of the car much rougher and would decrease lift in case a car does get airborne.

The plate was narrowed by 10% from 2014 to 2015 and now it’s being narrowed again from 38 inches to 25 inches. If the last change was 10%, then the reduction was by about 5 inches. This makes the radiator pan about 58% the width it was in 2014.

Spoilers

Moving to the  rear of the car, the spoiler is going to be a mere 3.5 inches tall. As in the front of the car, the larger the area, the larger the force. A smaller spoiler doesn’t let as many air molecules bang down on the car, so you get less rear downforce. But there’s more to it than that. The spoiler also produces a stream of ‘dirty air’ at the rear of the car. That turbulent flow is part of what makes it so hard for the trailing car to get close enough to the leading car to pass it. Lowering the spoiler decreases the wake behind the car, which should (in principle) help passing.

Even without the benefits of cleaning up the car’s wake, they pretty much have to adjust the rear of the car if they’re going to adjust the front. If they decrease front downforce and don’t do anything to the back, you’ve got a car that is going to be tight no matter what you do to it. The rear wheels will stick better that the front, so the car moves forward, just not necessarily in the direction you wanted it to go. It’s really all a matter of balance.

There’s another consequence of shortening the spoiler, and that’s decreasing the drag. While the spoiler supplies rear downforce, it also presents an impediment to the air molecules, creating a force opposite the direction the car is moving

Where Do We Go From Here?

It’s been a bit flip floppy around here, right? First NASCAR said they might use 2016 rules for the All-Star race. Then they decided not only not to try that experiment, but maybe there wouldn’t be any aerodynamics changes. Then we get an announcement that we’re changing the rules for one race in two weeks.

It seems like short notice, but the race teams had pretty good indication of what was coming long before we did. NASCAR did make the point that they didn’t want to give the teams six weeks to camp out at wind tunnels researching the changes. NASCAR has enough experience with the Gen-6 car that they’re pretty certain that the changes they’re making won’t be a disaster (plus, they’ve told the teams they need to be ready to revert back to the ‘old’ package at any time during the Kentucky weekend. If any problems crop up, they’ve got a good fallback.)

NASCAR made a big point that “this is a race, not an experiment”. They have to be cautious about what they say because people tend to jump on things and give disproportionate weight to them. Then if NASCAR changes something, they’re accused of not being honest or trying to mess with fans.

One of the hypotheses going around is that NASCAR would have different aerodynamic specifications for each track. This would reverse the trend toward trying to use the same car at every track, but it would give NASCAR a much better way of keeping the racing exciting by minimizing aeropush. And for those people who think it would be terribly confusing for the teams, trust me. These are really smart people. They’re good at keeping track of things.

You know what I’d like to see? I’d like them to run this same experiment at some of the other 1.5 mile tracks, and then come up with an intermediate track “box” – a range of values for splitter and spoiler so that the teams can have a little play in splitter and spoiler configurations. This would let them tune the car for different drivers much better than they can now – and it’s no secret that some drivers have not had an easy time adapting to the current configuration.

The one unknown in all this, however, is going to be tires. Goodyear already tailors tires to different tracks, so it’s not asking them to do a lot more, but it does mean that they have another round of development to do as rules shift.