Dive! Dive! Dive Planes… on Stock Cars?

TurbulentSmokeA 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.LaminarFlowOveraWing

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.

2013_FordFusion_CFDTopandSide

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).
divePlane_NateRyan

DivePlane_Joey

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.

Required vs. Recommended Tire Pressures

TirePressureIconRunning on underinflated tires can be dangerous.  Underinflated tires they create more friction and more heat, which leads to not only bad handling, but also can produce structural problems.

You may remember the late 1990’s, when Firestone tires had problems with tires blowing out or treads coming off.  The flat tires caused vehicles to roll over and there were more than numerous (Wikipedia says 250, but there’s no source for the stat)  fatalities and many more injuries. The majority (but not all) of the vehicles on which this happened were Ford Explorers. So there’s a rule now that any car made after 2007 has to have tire pressure sensors that warn the driver when a tire is significantly under-inflated. (Significantly means around 25% under pressure.)

Underinflated tires produce high stresses and temperatures. In a correctly inflated tire, the gas inside the tire supports most of the car’s weight. If the tire is underinflated, then there’s not enough gas pressure and the sidewall of the tire has to support the weight.  An underinflated tire flexes a lot as it rolls, which causes two major problems.  One is that it put more stress on the tire, and the second is that it produces more heat. (Graphic from http://www.tirebuyer.com/education/tire-pressure-and-performance.)TireUnderOverInflation

Underinflated tires heat up faster. You need some heat in the tire for it to work right; but there’s a Goldilocks situation here. Too cold and they don’t have any grip. Too hot and they fail. The pressure has to be just right.

Which makes you wonder, how hard can it be to just put the right pressure in the tires in the first place?

When it comes to racing, it’s because street tires are rarely called upon to sustain the extreme conditions race tires endure.  The issue is what we call tire pressure build – the increase in the tire pressure due to the heating of the tire and the gas inside the tire. At it’s most basic, heat is simply the motion of molecules. The faster the molecules are moving, the higher the temperature.

In a tire, the faster the molecules move (i.e. the hotter they get), the harder they hit the walls of the tire and the higher the tire pressure.  This is why your car’s owner manual tells you to measure the air pressure in your tires when the tires are cold. The pressure changes a lot with temperature. The video below, which I did a few years ago with the National Science Foundation, delves a little deeper into the specifics of tires and gas pressure. (And if the video doesn’t embed correctly, then try here.)

For the temperatures passenger car tires reach, a good rule of thumb is that every 10 degrees Fahrenheit corresponds to 1 psi (pounds per square inch) increase in pressure. NASCAR tires routinely change pressure by 20-40 psi from the tire sitting on the pit wall to the time it’s run a couple of laps. So the pressure you put into the tire is no where near the pressure you have three to five laps into the race. For a tire to have the right pressure during a long run, it has to have a much lower pressure when it first goes on the car. It takes a few laps for the tire to heat up, so the tire is really being stressed in the initial part of a run.

It’s a calculated risk teams take in how low they will start their tires.  Goodyear specifies a recommended pressure for each tire. NASCAR officials often check right front tire pressures in the pits.  The idea is that because the tires work in concert, if you the left front is way off, the car won’t be set up well. But there’s plenty of evidence that teams are getting around the recommendations and starting off with very low tire pressures.

Lee Spencer had an article this week suggesting that NASCAR might finally give in and require tire sensors as a way to get around the “shenanigans” (Her word, but one I love using). She uses the example of the left rear tire failures of the 48 car at New Hampshire a couple weeks ago. Johnson swore it wasn’t because of low tire pressure. Goodyear begged to differ.

TirePressureSensor_McLarenSo why not just put tire sensors on all four tires and let NASCAR officials monitor the tire pressure before the tires go on the car? Teams already use sensors during testing. They’re basically replacement caps for the tires that measure and transmit the tire pressure through an RF (radio frequency) link to a data logger on the car.  Places like McLaren (where the picture at right comes from) already have sensors for both inner and outer liners. The technology is pretty much already there to require sensors on the cars.

So why not just go ahead and put sensors on the cars?

Because NASCAR is very careful when it comes to opening cans of worms. Who will you allow to have access to the data?  The pit official? The team? Just before the tires go on the car, or will you allow access to that data during (or even after) the race? A lot of the same issues that arose when NASCAR made the transition to Electronic Fuel Injection will come into play here. How much technical data do we want the teams to have?

Sure enough, the first thing the husband said when I mentioned the idea to him was “That’d be great because then the teams would know when they had a tire going down.”

Do we want to go there? Are you encouraging crew chiefs to try edgier setups because they expect they’ll have a bit of a warning before they lose a tire? Do you want to take the driver’s ability to sense what’s happening with the car out of the picture?  NASCAR historically has had a de facto ‘no real-time telemetry’ rule for a very long time. The science and engineering in NASCAR go into the developing and preparing the car.  At race time, you turn over the majority of the control to the driver.

The other issue that always comes up is how teams will attempt to game the system. When NASCAR allowed for skew in the rear end of the cars, teams kept pushing the skew until NASCAR had to say “enough”. That we’re even talking about tires and tire sensors is because of teams ignoring the recommended pressures. Pushing the envelope is their job.

The discussions, according to Lee Spencer’s interview of Goodyear’s Greg Stucker, are ongoing, but NASCAR has a tendency to spend quite a bit of time thinking through issues like this before taking action.  The problem needs to be addressed, both to protect Goodyear’s reputation and for driver safety.