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Dr. Diandra: What’s a diffuser?

Most NASCAR fans got their first close look at the underwing -- what NASCAR calls the bottom part of the car -- while safety crews extricated Chris Buescher from his car after he barrel rolled at Charlotte Motor Speedway.

The car’s underside used to be an an aerodynamic nightmare. The mish-mosh of pipes, tubes and parts often trapped and slowed air. Teams spent a lot of money optimizing the area, but there also was a safety issue. Too much air going too slowly under a car generates lift. The rear ends of stock cars are raked to help move air out from under the car faster.

The 2022 Ford Next Gen Mustang showing the rake in the rear to help air get out from under the car.

CONCORD, NORTH CAROLINA - APRIL 22: The NASCAR Next Gen Chevrolet Camaro is previewed at NASCAR R&D Center on April 22, 2021 in Concord, North Carolina. (Photo by Jared C. Tilton/Getty Images)

“Most people don’t understand how much lift the body actually makes,” Dr. Eric Jacuzzi, NASCAR’s managing director of aerodynamics/vehicle performance said. “Other than the deck lid and the spoiler, the greenhouse and the hood and everything, all of that is trying to lift the car up.”

Part of NASCAR’s design philosophy was increasing downforce without involving the body. That allows for a symmetric shape, and lets manufacturers add styling details that better evoke the street models.

The Gen-6 car started to address the under-car issue with radiator pans -- metal sheets under the front part of the car that presented a smooth surface to air moving under the car. Because balancing front and rear downforce is so important, NASCAR chose a full-length underwing, from splitter to diffuser, for the Next Gen car.

The Next Gen splitter

The biggest change in the splitter relative to the Gen-6 car is that the Next Gen splitter is stepped. The old splitter was straight all the way across. I’ve outlined the splitter profile in this photo so you can see it a little better.

A Next Gen car from the 2020 test at Homestead with the stepped splitter outlined

HOMESTEAD, FLORIDA - JANUARY 15: Erik Jones tests the Next Gen car at Homestead-Miami Speedway on January 15, 2020 in Homestead, Florida. (Photo by Michael Reaves/Getty Images)

The graphic below shows the top and bottom of the splitter. The red lines in the top graphic show where teams are allowed to add tape. A contrasting color helps the crew better see the position of the splitter relative to the racetrack.

The splitter, top and bottom view, with pieces labeled

NASCAR Rule Book

The bottom view, which shows you the surface the air sees, shows splitter wear blocks in turquoise. They’re hard pieces that erode when the splitter hits the track. They can only wear to half their original thickness before NASCAR mandates replacement.

The red pieces (call ‘splitter stuffers’) determine how air enters the underwing. Superspeedways require smaller splitter stuffers.

The stepped splitter makes it impossible to seal the car to the ground. That’s by design. Instead of trying to prevent air from getting under the car, the Next Gen underwing manipulates air under the car to prevent lift and create downforce.

The inverted scoop in the middle of the splitter and the splitter stuffers direct the air along the outside of the underbody. The rocker box assemblies, which replace side skirts, corral the air until it gets just ahead of the rear axle.

That’s where the diffuser takes over.

The diffuser

While a smooth underbody helps air flow faster, a diffuser tailors that flow. Since diffusers are hard to see, I’ve highlighted the part on the rear view of the Camaro below.

A rear view of the diffuser on the NextGen Camaro

CONCORD, NORTH CAROLINA - APRIL 22: The NASCAR Next Gen Chevrolet Camaro is previewed at NASCAR R&D Center on April 22, 2021 in Concord, North Carolina. (Photo by Jared C. Tilton/Getty Images)

I’ve done the same on the 3/4 rear view below.

A 3/4 rear photo of the chevy camaro showing the diffuser

GM Motorsports

NASCAR has a really slick 3D model you can rotate to see the diffuser from all directions.

The diffuser is a sheet of carbon-fiber-composite with seven vertical blades called strakes. The outer two strakes form a three-sided box open to the ground. Two additional strakes run along each side of the diffuser. A smaller center strake is hard to see because it’s set back a bit.

The diffuser starts just ahead of the rear wheels. It’s narrower there, then widens toward the rear of the car. At its simplest, a diffuser is a venturi, which are also used in carburetors and paint sprayers. When a fluid (like air) flows through a narrow section of a duct into a wider section, the pressure on the wider side is lower and the air moves faster.

The strakes create a swirling motion that not only hurries out air already entrained in the flow, but also draws in additional air. The outer tunnels pull in the wake from the front tires. The way the air exits from underneath the car and re-joins with the air from above and on the sides of the car forms a smaller, more well-behaved wake. That means less dirty air and easier passing.

The flap over diffusers

Stock cars can become airborne when turned around at high speed. To keep the Next Gen car on the ground, NASCAR added a diffuser flap. I showed it in green in the photos above and made it just a little larger than actual size so you could see it.

Like roof and cowl flaps, the diffuser flap is a piece of metal that lies horizontally under normal conditions. When deployed, the diffuser flap falls to about 70 degrees from its initial position, blocking the diffuser’s central tunnel. The flap increases the lift-off speed -- how fast the car can go before leaving the ground -- by 10 to 20 percent when the car is almost backward.

Roof and cowl flaps are purely mechanical devices. Magnets hold the flaps in place until the pressure above gets low enough to basically suck the flap off the magnet. NASCAR struggled to use the same idea for the diffuser flap.

“And then somebody had the idea,” Jacuzzi said, “that the roof flaps are gonna go up (if there’s a problem). So why don’t we connect them? Buy a cable and have it just drop?”

If you see a car’s right roof flap lift, you know that the diffuser flap has deployed. The flap must be reset before the car can continue racing.

“That diffuser flap is as significant in terms of lift off as the roof flaps,” Jacuzzi said. “In the (wind) tunnel, it shows a bigger gain in certain positions than the roof flaps themselves. So it is a pretty big innovation for this car.”

The underwing isn’t only for safety. In the mid 2010s, when there were no limits on wind tunnel or computational fluid dynamics time, Jacuzzi noted that the aerodynamic forces on cars could vary by hundreds of pounds.

The thing (the underwing) has done,” Jacuzzi noted, “is put everybody on the same playing field. Before, they all had different underbodies. They had oil pans that were shaped a certain way, and they would build the chassis to do all the things that the Next Gen floor does. But everyone didn’t have that equally.”

They do now.