A persistent motorsports issue (and not only with stock cars) is the aerodynamic passing problem. You can’t pass without grip. Grip is a direct result of downforce. Downforce comes from two places: the weight of the car (mechanical grip) and the billions and billions of air molecules hitting the car (a.k.a aerogrip).
Racecars are designed to take advantage of aerodynamic downforce. Everything from their shape to the aerodynamic appendages added to the car are all optimized to produce downforce. You can play around with mechanical grip some by adjusting the weight on each corner of the car and trying to control how the weight changes as the car turns, brakes and accelerates. Aerodynamic grip is even more subtle.
And complicated.
Aerodynamicists think about fluid flow (fluid meaning liquid or gas) in terms of two extremes. Laminar flow is when the air (or water) moves predictably over a surface in nice, uniform sheets with relatively little variation from sheet to sheet. In the diagram below explaining how a wing works (an airplane wing; turn your computer upside down if you want to see how a car wing works) the air is represented by nice, neat lines that very politely crowd each other as they work their way around the wing. Changes in pressure and velocity happen gradually.
The other extreme is turbulence – when the air (or water) flows in swirls that are not at all well behaved. Turbulence is chaotic – large differences in pressure and velocity that change quickly. Turbulence is very difficult to describe mathematically because it’s just so darn complicated.
It’s easier to see experimentally. Pour some cream into your morning coffee and stir it with the back of your spoon. The spoon moves the cream out of the way, creating a gap. The cream swirls around the back of the spoon and fills the gap, forming a lovely swirling pattern. You can see the same thing in the wake of boat – the water flows back in to fill the gap the front of the boat made. Smoke rising from a cigarette is turbulent as it mixes with the air. Breaking waves are turbulent.
In turbulent flow, the air molecules end up going in all different directions. If you’ve ever driven very close to the back of a semi on the expressway, you’ll feel your car buffeted from different directions – that’s the turbulence.
Laminar and turbulent flow are both evident in the aerodynamics of racecars. The front of the car is smooth and sloped. The cross section of the car (what you’d get if you took a slide of the car perpendicular to the direction the car’s traveling) gets larger and larger as you get further from the front fascia. The car keeps pushing a bigger and bigger hole in the air.
Things change when you reach the B-post. Now the car needs to push away less air because it’s sloping down. Its cross section is getting smaller. The air starts swirling in around the rear window, becoming turbulent. The wake of a racecar is similar to the wake of a boat. The water’s going in all directions, trying to fill the hole made by the front part of the boat.
A technique called computational fluid dynamics lets engineers visualize the airflow. The diagram here is from Ford Racing and shows the turbulence on the 2013 Ford Fusion. This visualization shows you where there are big changes in the airflow. You can see the giant wake behind the car. It’s strongest the closest to the rear of the car, but note that the wake extends almost two car lengths behind the car.
If you want to learn more about Ford’s CFD calculations and the role they play in designing racecars, check out their YouTube video – it’s worth a gander.
The wake creates drag on your car and slows it down just a little, but as the driver of said car, it’s not really a big concern. For the guy running behind me, however, my wake is a really big problem. Laminar air makes downforce. Turbulent air doesn’t.
And that’s the origin of the passing problem. A fast car catches up with the car ahead of it. As the trailing car research the leading car’s back end, the turbulence from the wake of the first car makes the flow over the front of the trailing car turbulent, which means the trailing car loses downforce or becomes ‘aeroloose’. And you can see from the CFD calculation that you don’t have to get so close for aeropush to become a problem.
Right after the Michigan race last week, NASCAR ran a big test (10 teams) to try out some options for possible rules changes for the 2015 season. In case you think this is a simple problem to solve, they had two approaches: more downforce and less downforce.
On the increase downforce side, the first change was to a bigger splitter – nine inches tall. The problem with increasing the splitter is that it unbalances the car. One of they key principles in racing is that you can only go as fast as your least grippy tire. Grip is proportional to downforce. If you increase the rear downforce without making a commensurate change in the front downforce, you get a really tight car. Lots of grip in the back, but the front tire – the ones that turn the car – don’t have enough grip.
Increasing the front splitter has its own challenges, so NASCAR turned to dive planes. Dive planes have been used for a long time on sports cars. They’re simply small, curved pieces of metal or carbon fiber composite. NASCAR used two dive planes – one above the other – and put one set on each side of the car. The dive planes started at the front fascia and swoop upward, ending at the front fender. The pictures below are from the twitter feeds of @nateryan (top) and @2spotter (Joey Meier, bottom).
The principle behind the dive plane is that it takes the turbulent air coming onto the front of the car and funnels it to make the flow more laminar. More laminar flow should translate to more downforce.
NASCAR made the point that the dive planes may not be part of the final rules package; however, having the dive planes allowed them another little benefit – they could put pressure sensors on the dive planes and measure how the downforce changed for the different configurations.
The ‘prime rules package’ that was tested consisted of the larger spoiler, a lower rear differential gear, and decreased horsepower. They tested 850hp, 800 hp and 750hp. The second test package was actually a lower downforce package, in which they went with a smaller spoiler and they removed an underbody piece that had been new this year. The estimate is that these changes decreased the overall downforce by 28-30%.
And (of course) the drivers were not very enthusiastic about the prime rules package. They liked the lower downforce better. Reporting from the track suggested that the prime rules package gave rise to in-line racing, while the lower downforce package got drivers really excited about possibilities for passing.
Unfortunately, there’s no time for another test because NASCAR really needs to have the 2015 rules finalized pretty darn quick so the teams have time to prepare for next season. Right now, a bunch of NASCAR engineers are sitting back at the R&D Center, trying to make sense of the gigabytes of data they collected during the test. I’m sort of glad I’m not the one who has to make this decision!!
I didn’t mention one of the big changes in this blog post – the ability for the driver to modify the trackbar position from inside the car, but I will comment on that in the near future.
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Dive planes & ability of drivers to adjust track bar, on the fly, both are great conceptions. King Richard, and the like, must be in amazement of just how far the sport has come….what’s next ?…WOOOSH !
Interesting article. Aero is something difficult to explain to the layperson, so well done. However again they miss the real problem. While “parity” is an interesting idea, it produces exactly what they already have. If you mandate equal cars, which by definition means equal speed, how does anybody pass anybody? They may be moving the bar but the result will still be the same.
Hi Russ: The only way to mandate ‘equal’ cars is to mandate every part and every set up. Even within the rather narrow box NASCAR sets out for the teams, there are many ways teams have to individualize the cars with the shocks, springs, track and sway bars, weight split, etc. And there’s the additional issue of the driver, because not all drivers like a car set up the same way. The idea of parity is equivalence, not equality. That’s one of the things that took so long with the Gen-6, which was getting the different manufacturers to agree to parameters. The cars have difference shapes, but similar drag and downforce numbers. Don’t worry too much about parity – we’re still a long ways from IROC, thankfully!
How does anybody pass anybody…It’s called “experience”, I guess, only speak’n from a lay persons view.
Less downforce and better racing. Putting more car control in the hands of the drivers. DUH!!! It finally got through to all the engineers at NASCAR. They have been breaking track records for years and the racing has gotten worse. Fans do not sit in the stands with stop watches. They want to see cars passing each other. Most of the tracks with the current configuration of the cars require no use of brakes. No wonder they can’t pass one another. Now we will find out if they really care about the fans.
Yep. Duh. The fact that the drivers were ecstatic about the lower downforce setup ought to tell you something. But we both know that when you’ve been working in one direction for a long time, sometimes it’s hard to do a 180, especially when you’ve got huge amounts of inertia that have to come along. (Something about turning an aircraft carrier here, eh?) Good to hear from you, Bob!
Please Bob, Please don’t say “fans”, as in “all fans”, don’t sit in stands with stop watches. Please tell me your only say’n “In general”. I’m try’n, I’m really try’n.
Question: downforce is critical in pushing down the front of the car to maximize the contact patch of the tires. But if the front splitter is already on the ground, what effect does additional downforce have? The front can’t go any lower, can it?
Hello! Great question. If they can get more downforce, they’ll set up the car so that the front splitter starts out higher and the additional downforce lets them take advantage of that extra grip without the splitter hitting the track. That’s one of the complications in setting up a car – there are many variables, and they are all interrelated. And that’s why the crew chiefs get the big bucks! (And the wrath of the drivers often!) Thanks for the question.
Excellent point Tom, With dive planes, the nose would be practically touchings. Do like the idea of the in car adjustable bar, (slide on the fly), compensation if “only a brush” with barrier/car. Hay, What the heck, lets give them more to do, why not. They ain’t gut nut’n to do @ tracks like Bristol anyway right, Besides 41 other cards, avg. speed well over 100, sometimes bumper ta bumper, crew chief
talk’n to ya….why not, you want more control of ya car…here ya go
Couldn’t agree with u more…If the splitter was forced down any more, with or without assistance, it would b eat’n up the carbon fiber.
I find it simple. The bigger the hole in the air the car in front makes, the more turbulent the air behind the car (especially with the spoiler, which is not a very efficient aerodynamic device in terms of drag vs. downforce). The car trailing is always going to be at a disadvantage no mater which package you put in, but for example if a lead car has 500 points of downforce, and the car behind loses 30% following a car, meaning they lose 150 points of downforce. If a car only produces 300 points of downforce and loses 30% following a car, they lose only 90 points of downforce. To me, it’s the net difference that counts. They need less aero dependence, not more.
Hey David – great way of explaining it. I’ve made this point a number of times, but people seem to be stuck on more is always better – more speed and more downforce. I don’t want to watch a race where the cars are so stuck the ground that any good racer could be competitive. Thanks for the comment!
Hello David, I was under the impression that the reason NASCAR mandated height of spoiler was, to some degree, keep the cars from become’n airborne. They did use ta look very impressive fly’n through the air though. I don’t know how the drivers felt though when it was happen’n to them. Oh well, that’s why they make big bucks, right…
JR Did not mean all fans don’t have stopwatches, but from my experience most fans cannot tell the difference between 175 MPH and 200 MPH unless Larry Mac tells them. For someone that is a fan there is nothing better then Diandra’s blog and her book to be able to learn what is going on scientifically.
I am a dinosaur. To me, if you do not have to use brakes, you are taking much of the driver out of the equation. But that is just me.