The speeds at Pocono were high, but the speeds at newly repaved Michigan are even higher. Nine drivers posted lap times over 200 mph on Thursday, leading to media and driver hysteria about the high speeds. What is the lift-off speed at Michigan?
Determining the Lif
t-Off Speed
It is a fact that, under certain conditions, a stock car can behave very much like an airplane wing. Legend has it that some of the first tests investigating this phenomenon were done using the exhaust from the NASCAR corporate jet. Later, more careful wind tunnel measurements confirmed that stock cars could indeed “lift off”. The measurements and how they led to the development of the roof flaps are detailed in my book, The Physics of NASCAR.
This issue has arisen again because of the fast speeds at Michigan. Radio and television commentators have repeatedly referred to “lift off speed”, as though this were something one could simply look up in a table somewhere. The common knowledge seem to be that if the lap speed passes 200 mph, cars will go airborne.
It is impossible to identify a single “lift-off speed”. Whether a car will lift off depends on multiple variables. The two most important variables are speed and the angle of the car with respect to its direction of travel. These two variables are dependent on each other and not in a simple way. A car coming down the frontstretch at 218 mph is just fine. A car going backward (or near-backward) at 218 mph is almost certain to take off.
But what about a car going at a 120 degree angle at 201 mph? Can’t say for sure. How grippy are the tires? How fast is the car rotating (delta theta/delta time)? Are there other cars in the proximity that can affect the way the air flows over the rotating car?
Despite the many advances of their R&D Center, NASCAR has not yet perfected a crystal ball that predicts the future. They use basic science and engineering along with what is called in the trade a “safety factor“. If I build a bridge and I know for sure it can hold 10,000 tons, I may assume a safety factor of two and rate it for 5,000 tons. Although it ought to hold twice that, I’m confident that it will definitely, under all circumstances, hold 5,000 tons, so I say that the bridge’s limit is 5,000 tons.
NASCAR operates on the conservative side and they will implement speed control measures if they think they are getting close to where there could be a problem. In the same vein, they will always call a caution when they see something on the track. It may turn out to be roll bar padding and the fans will scoff at a ‘fake caution’.
But what if it isn’t? What if they ignored something on the track or delayed calling a caution to try to see what it was? In the meantime, it flew through a radiator and caused a major wreck. This isn’t like baseball or basketball where a bad call will screw up the game: A bad call in racing can mean someone getting hurt or killed. If NASCAR says they aren’t worried about lift-off, I believe that, based on all the data they have, they don’t believe they are close to speed being a problem.
It is possible they are wrong. The ‘perfect storm’ of events is always possible, but I believe NASCAR when they say that they don’t think speed is an issue at Michigan. Yet.
Communication
Then again, I understand a lot more of the science behind the issue than the average fan. NASCAR does itself no favors in terms of PR with its unwillingness to explain things. I get this – sort of. The folks in the NASCAR R&D Center, especially people like Robin Pemberton and John Darby, understand that this complicated issue can’t be summed up in a ten-second sound bit. They could hold a ten-minute seminar on the details, but a minority of the press corps is going to be willing to make the effort toa) understand what they are saying and b) interpret it correctly for the average fan.
We face the exact same thing in science: I have personally experienced the frustration of picking up a paper and seeing my quotes taken out of context and my work explained incorrectly.
There are two ways to prevent this happening for scientists and for NASCAR. The first is to spend a lot of time with reporters and go over everything to make sure they actually understand the issue. This is time consuming, frustrating and cannot work unless you have reporters who really want to get it right and are willing to spend the time to do so. Some people in the NASCAR media corps are willing to put in the time and energy to do that. Many are not.
The other way is to simply state the end result tht you want to see in the press simply and straightforwardly enough that it can be quoted directly.
Series director John Darby said NASCAR has a speed at which liftoff is a concern as is the case at Daytona and Talladega, and “we’re not there yet.”
“Two hundred and one at Bristol might get us a little excited,” Darby jokingly added.
This latter strategy ensures that you won’t be misquoted, but comes across the same way “because I’m the parent” comes across to a teenager.
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Thanks for the great article. As an engineer who supports a technical sales organization, I sympathize with you and, at least in this instance, with NASCAR. I estimate 90% of the verbiage in my technical correspondance with salesmen is to ensure nothing I say can be misconstrued…..
Love your articles, keep ’em coming, please ma’am!
A NASCAR Cup car has a bulk density of about 10 lbs. per cubic foot, roughly that of very low density soft pine, or Bass Wood, and the aero shape that approximates a streamlined brick.
We’ve done CFD studies of rotating a Cup shaped car at 5 degree increments of yaw and 1 degree increments of attack angle over a range of speeds and heights above ground. There is a very large envelope of conditions which will allow a Cup car to take flight, some at surprisingly low speeds, e.g., 120 mph. Moreover, a Cup car once in flight will stay in the air for several seconds.
Other car series, notably IndyCar and Le Mans LMP 1 and 2, have under-car plates which act as an inverted wing and they keep the cars on the ground,… usually. Unless you run over the wheels of another car.
Prof pi
Hi Dr D,
You said, “NASCAR does itself no favors in terms of PR with its unwillingness to explain things”. That’s why we read your articles. We don’t need them to explain anything. We leave that to you. Like Joe said. keep your articles coming. I affectionately refer to your columns as “nascar Science For Dummies”. No offense to your reader base, I’m talking about people like me.
I noticed the example showed the COT with the old “wing.” While this was a huge issue with said wing, most of that was eliminated with the spoiler coming aboard in mid-2010.
Instead of having a wing with a gap that can help create lift when the car is moving backwards, you now have a spoiler without any air gap. This would essentially create drag instead, slowing the car down.
I used an old picture, but that has nothing to do with the physics of why cars become airborne. Regardless of whether there is a spoiler or a wing on the car, at stockcar can become airborne if it rotates. The issue has more to do with air getting under the car and flowing rapidly over the roof, which decreases the pressure on top of the car and increases the pressure below the car. This produces a net force up and the car becomes airborne. Roof flaps work by slowing down the air going over the roof, this increasing the pressure and creating force down. Although there were some theories that the wing exacerbated the problem when the car was traveling backward, this is a minor contributor to the car getting airborne.