How to Counteract Lift | Racecar Aerodynamics

Tom
By Tom Suddard
Mar 30, 2021 | aerodymanics | Posted in Shop Work , Features | Never miss an article

When racers talk about aerodynamics, the conversation usually focuses on one thing—downforce—and it’s easy to see why. Wings, splitters, diffusers and all the other fancy aerodynamic devices meant for race cars have one broad goal, and that’s to increase downforce.

Downforce is great and all, but there’s another side of the coin, one you should think about whenever you’re wing shopping: aerodynamic lift. It’s a nasty phenomenon that’s present on the majority of stock cars, GRM favorites included.

The topic came up on a recent tech call with Nine Lives Racing founder Johnny Cichowski, and he was more than happy to walk us through the basics. What does Johnny know about lift? As one of amateur racing’s more prolific producers of aerodynamic devices, Johnny spends all day, every day, thinking about lift and the best ways to counteract it. And has the data to back up his assertion, thanks to NLR's frequent consultations with Morlind Engineering, a firm that specializes in race car computational fluid dynamics.

What Is Lift?

What is aerodynamic lift? It’s the opposite of downforce, and the same thing that makes planes fly. The physics are fairly simple: Negative air pressure above your car sucks it into the air.

Flip lift upside down and it becomes downforce, where negative air pressure below your car sucks it onto the track. And while you may not have ever thought of lift, you’ve definitely experienced it.

Most drivers describe it as a light or floaty sensation at highway speeds and above. That feeling isn’t just a result of going fast; it’s caused by lift removing weight from your tires, limiting traction and making the steering feel light. That’s not good when you’re on track and still need to control your car’s momentum in the corners.

Why Do Most Cars Make Lift?

How could the world’s car manufacturers let this evil force creep into their products? Again, it’s simple physics, and the best way to illustrate it, Johnny says, is to think of an airplane wing.

Airplane wings work by having a curved (convex) top side to speed up airflow. As that air speeds up, it creates a low-pressure area above the wing. The underside of the wing is usually flat or concave, which slows down the air relative to the air above the wing. This creates a higher-pressure area. That difference in pressure between the air on top and below the wing results in a force that “lifts” the wing.

Most car makers are more concerned with factors like wind noise and drag than aerodynamic lift. For more road-going cars, the smoothest route for air to travel is usually over the top, resulting in a vehicle that has low drag but also some lift. That lift isn’t an issue at typical street speeds, but it can be extremely disconcerting at track speeds.

How much lift do normal cars make? Every model is different, but Johnny shared a few data points from Nine Lives’ own testing: A stock C5 Corvette makes 80 pounds of lift at 80 mph and 280 pounds of lift at 150 mph; a Miata makes approximately 65 pounds of lift at 80 mph and 200 pounds of lift at 150 mph.

Most cars make more lift over the roof, with a high-pressure area—negative lift or downforce—forming at the base of the windscreen or hood. This can lead to some rear lift at higher speeds, though it depends on the shape of the car.

How Can You Remove Lift?

We’ve established that lift is bad and that most cars make it. What can you do about it?

Johnny, of course, pointed right to his company’s trademark product, the Big Wang. Just remember, you don’t actually have to eliminate lift; just counteracting it is perfectly fine. And to do that, you can turn to the usual racer playbook: Air dams, splitters, wings, diffusers, canards, tunnels, hood ducts and more can all be part of a successful recipe to fight lift—even overpower it to the point of creating net downforce on your race car.

How do you decide which of the downforce tools to employ? Johnny says balance is more important than all-out downforce, explaining, “If you don’t notice the downforce, that means it’s balanced correctly.”

What’s that mean in practice? Simple: You wouldn’t want to have a 200-pound medicine ball rolling around your cockpit on track, as it would be super unpredictable to have 200 pounds on the rear through one corner, then somewhere else through the next.

Unbalanced downforce can act just like that theoretical medicine ball, drastically changing the car’s weight balance as the speed (and the aerodynamic elements’ effectiveness) changes. Johnny says the goal is for aerodynamic balance to have 3-5% more rear bias than the overall weight distribution, similar to the car’s mechanical setup.

Somewhat like aerodynamic balance, the goal for lateral load transfer distribution is around 5% more bias than weight distribution. In this case, though, that bias is more forward. As an example, if the weight distribution of a Corvette is 48/52, you should be aiming for an LLTD of 50-54% front with an aerodynamic balance of 43-45% front. Any less than 40% bias can make driving at higher speeds particularly difficult.

Of course, the balance question opens up a whole new can of worms: How much downforce are your aerodynamic elements actually making? Nine Lives Racing and some other vendors publish data for popular platforms and combinations. Nine Lives also has a strategic partnership with CFD firm Morlind Engineering—which has helped the company develop a few products, including splitter ramps and downforce packages for certain applications—if you’re interested in more advanced modeling and testing of your specific car.

What if you don’t have published data or an engineering firm to help you balance your setup? It’s possible to do your own aerodynamic analysis at home, but that’s a topic for another article. If you’re curious, we’ve written extensively about DIY aerodynamic testing in the past, and we host a whole forum dedicated to aerodynamics. Otherwise, the answer is testing, testing and more testing. As long as you remember lift—and the ways to fight it—you’ll make your car faster and easier to drive.

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Comments
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Johnny_at_NineLives
Johnny_at_NineLives New Reader
3/25/21 8:08 p.m.

He sounds like a jerk 

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