Keeping Racecars on the Racetrack

 Aerodynamics, Downforce, Drag, Lift-Off Speed, Roof Flaps, Ryan Newman, Talladega, Yaw  1 Response »
May 102013
 

Ryan Newman escaped NASCAR sanctions for his comments immediately after being discharged from the infield care center at Talladega.

“They can build safer racecars, they can build safer walls, but they can’t get their heads out of their asses far enough to keep them on the race track and that’s pretty disappointing, and I wanted to make sure I get that point across,” he said. “You all can figure out who ‘they’ is.”

You’ll hear people talk about aerogrip and mechanical grip.  Aerogrip comes from air pushing the car into the track, while mechanical grip is due to the weight of the car pushing the car into the track.  What pushes down can also push up, so it’s not surprising that the same two factors contribute to cars becoming airborne.

Aerodynamic Takeoff

When a car rotates (so that its side or its back is leading instead of its front), it looks an awful lot like an airplane wing — a shape that is optimized to generate lift.  The faster air flows over a surface, the lower the pressure.  In the figure below, the longer and greener the arrow is, the faster the air flows over the car.  This shows airflow from the front of the car, but the same areas where air flows quickly when the car is going straight are the same areas where air flows quickly when the car is yawed.

BernoulliCar

NASCAR race cars have roof flaps and hood flaps that are located where – surprise – the green arrows are in the figure above.  (Figure Credit: USA Today) .  In the Gen-6 car, the roof flaps are much larger and the previous “cowl flaps” were moved onto the hood proper.  The hood now extends up to the windshield and there is no cowl anymore.

Gen6_RoofandHoodFlaps

A roof flap or hood flap slows down the air passing over the car, thus forcing it to exert more pressure downward, which pushes the car back down to the ground.

Mechanical Take-Off

The other way a car’s wheels can leave the ground is due to mechanical forces that cause the car to roll over.  Torque is the application of a force that causes rotation.  If you look at some of the classic rolling accidents from Talladega, a number of them are caused by a car moving from the pavement to the grass or vice-versa.  There is almost always a step up or down at the transition from one surface to another and that step creates a torque that causes the car to roll.   In the diagram below, the car is skidding sideways, then hits a bump.  The torque created by the bump causes the car to rotate.

CG_Torques

A torque can be applied anywhere and could be caused by anything on the track — including another car.

Talladega

The incident last weekend at Talladega was combination of these factors.  The video shows that the 36 hits Busch (78) at the right rear quarter panel, turning the 78.  As the car rotates, one roof flap deploys, indicating that the pressure above the car became less than that inside the car; however, the critical factor looks to be the 36 getting under the right rear quarter panel and creating a torque that rolls the 78.  If you look back to the first diagram, you’ll see that these cars have a rake in the back. The upswing helps air move out from under the car and decreases lift; however, it’s also a place where the nose of one car can launch the other.  As the 78 is rolling, the 39 drives right under him and the 78 lands on the 38.

The Fix(es)

1.  Eliminate abrupt steps between the racing surface and the infield in the triovals.  Remove the grass in the infield of the trioval area so that there isn’t a transition.  You can paint asphalt as well as you can paint grass.  Having a continuous surface, with a very smooth transition from banking to flat (the abrupt change from banked to flat can also create a torque that flips cars) would eliminate a lot of problems.

2.  Decrease speed.  The ability of cars to take off when they rotate is dependent on the yaw angle and the speed they are traveling.  You can’t do anything to prevent the cars from rotating, so the only option is to decrease the speed of the cars.  You could do this by making the restrictor plate smaller (see below why that probably won’t help) or giving the cars much more drag so that they can’t go as fast.

3.  Stop Pack Racing.  The Big One happens because there are so many cars so close to each other traveling at high speed.  200 mph is a football field per second.  You literally have no time to react.  This is a direct consequence of restrictor plate racing, where the drivers are at full throttle all the time.  Many fans like pack racing; however, if you want pack racing, you’re going to have to accept that we’re going to have accidents like the ones we’ve seen this year during the Nationwide race in Daytona (in which fans were injured) and like we saw last weekend at Talladega.

4.  Have the drivers get their heads out of their @**$* and drive better.  The Talladega accident happened because drivers tried to make their cars do things the cars weren’t capable of doing.  Simple as that.  NASCAR cannot fabricate an idiot-proof car.  As long as drivers push their cars past their limits, there will be accidents.

About Newman’s Comments

It’s really easy to criticize abstract entities like “NASCAR” because you’re not really criticizing a person — it’s a faceless corporate entity.  When you know the people who are being criticized, you take the criticism differently.  I know a number of the people responsible for safety in NASCAR.  They have dedicated their lives to making racing safer and not just for  ‘stars’ like Mr. Newman.  Think about how many lives these folks have saved.  It is unfair  to characterize them the way Mr. Newman did.  I understand being mad, but it’s disappointing that Mr. Newman lacks the grace to express his frustration some way other than name calling.

I’m also disappointed by NASCAR’s stand on the issue, either.  I’m OK with not fining Mr. Newman for his comments.  They were made in the heat of the moment and I’d be pretty steamed if a car landed on me, too.   It seems to me that there’s a very fine line between criticizing officiating and criticizing people’s integrity.  The statement NASCAR put out, however, seems to imply that it’s OK to attack the integrity of people as long as you’re not attacking the “racing product”.  Seems to me there’s something wrong when you put product before people.

I’ve said it before, but it beats repeating:  there is no way to make racing 100% safe.  NASCAR has made enormous strides in safety, but there is — and always will be — the potential for serious injury and death.  If human beings never made mistakes, racing would be significantly safer than it is; however, the fact is that the human element of racing is perhaps the most important and that brings with it the likelihood of mistakes.

If you’re not willing to accept that, you should consider another line of work

Here’s an older video about aerodynamics and lift:

A Quick Post on Why Cars Go Airborne

 Aerodynamics, Downforce, Lift-Off Speed, Roof Flaps, Yaw  1 Response »
May 072013
 

A quick post for my friend, @TheOrangeCone that I’ll expand on later (I have theater tickets tonight!)

@TheOrangeCone asked why Kurt Busch went airborne in the Talladega crash.  The answer is the same for all the cars that end up in the air:  when a car rotates (so that its side or its back is leading instead of its front), it looks an awful lot like an airplane wing — a shape that is optimized to generate lift.

If you have an isolated car rotating, the roof flaps deploy and that’s usually enough to keep the car on the ground; however… if you look at the video from Talladega, you see that the 36 hit Busch at the right rear quarter panel.  That pushes air under the car and the combination of the mechanical lift and the aerolift sends the car up in the air.

How do you fix this?

1.  Get rid of restrictor plates so that the cars aren’t operating in such close proximity to each other and the drivers have more throttle response.  This would need a new engine design and would eliminate pack racing, so it’s not a popular idea with many fans or with the engine companies that would have to do something totally different.

2.  Drastically change the car’s shape to make it much draggier to slow the speeds and make the car look less like a wing when it rotates.  After having fans gush about the identity of the Gen-6 cars and how they look more like their street cousins, neither NASCAR nor the manufacturers are likely to be very excited about sticking something on the car that looks nothing like anything on a production car.

3.  Come up with an active device – the roof flaps are passive.  They work on a pressure differential  (explained in the video below) and they just fly up.  Something that would be more like the flaps that an airplane deploys when it is time to land could be used — but you have to figure out a mechanism for it to deploy at the right time and then be out of the way the rest of the time.

The true answer to “how do you keep cars from going airborne” is that you have to slow them down.

More on this before long.  I’m headed out to the theater…

From the Lab Notebook: Las Vegas and the Mysteriously Missing Oil Tank Cover

 Aerodynamics, Downforce, Drag, Edwards Carl, Jack Roush, Las Vegas Motor Speedway, Rules  No Responses »
Mar 062013
 

As we head for Las Vegas this weekend, I thought I’d repost on of my most popular posts from stockcarscience.com on 3/5/2008  since the redirects for the old stockcarscience.com site don’t work reliably.  The post is about Carl Edwards’ 2008 win at Las Vegas when the team was subsequently fined for having their oil tank cover lid askew at the end of the race.  I have edited the post extensively, adding some new information and better graphics.

Danny LaDue asks: Can you explain the location of a NASCAR oil tank reservoir and how the lack of one could improve aerodynamics?

Thanks for the question, Danny.

NPR got this one wrong.  Frank DeFord in his usual Wednesday commentary made a comment that was essentially — look, the lid was still in the car, it didn’t give him a weight advantage, so NASCAR was wrong to penalize the team.  Don’t these folks known anything?

That’s the problem with aerodynamics — you can’t see it happening.

Unlike your car, the oil in a NASCAR car isn’t stored in the engine (called a wet sump system).  NASCAR uses a dry sump system, in which oil is

stored in an oil  reservoir. The oil reservoir is located behind the driver’s seat and is surrounded on the sides and top by sheet metal, which forms the oil tank.   The sheet metal minimizes heat radiating into the car, traps fumes from the hot oil, and serves as an additional firewall.  This function is so important that NASCAR doesn’t allow the top of the tank to be attached using quick connect fasteners. Some teams duct tape the lid on. The picture to the right shows the location of the oil tank with respect to the chassis. It doesn’t show the cover, which would sit on top of the tank.  The oil reservoir itself is closed and pressurized.

So if the oil tank cover plays such an important role, why would you leave it loose, much less leave it off?   The answer is aerodynamics.  The air exerts forces on the car in different directions.   Drag is the force air creates along the length of the car.  Air creates drag when it hits the front of the car,  but it also creates drag when it gets inside the car because there is no way for it to get out.   Drag always acts opposite the direction the car is trying to move, so you want to eliminate as much drag as possible.

Downforce and lift are the names for the forces pushing straight down or up on the car. Downforce pushes the tires harder into the track and provide grip, while lift pulls up on the car.  These two forces are in direct opposition to each other.  The bigger force wins.  You want to maximize downforce and minimize lift.

Downforce_oilTank1The oil tank is open to the bottom of the car. Air under the car creates lift.  Even though you try to keep the splitter close to the ground, there is always some air that gets under the car.  If the oil tank lid isn’t firmly tightened down, it creates a path for air to get out of the car, which reduces lift.

When the amount of lift decreases because of the loose oil tank cover, then the net downforce is larger because there is less air pushing upward. More downforce translates directly into more speed, as shown in the figure below.  Remember learning about ‘net force’ in physics?  Yep – it is actually useful.  The loose oil tank cover likely provided a little extra downforce — in a sport where races are won by thousandths of seconds, even “a little” advantage is important.

Downforce_oilTank2

One of Rusty Wallace’s cars originally penalized in the Nationwide series won its appeal on the basis that all of the bolts on the oil tank cover were engaged fully and the design of the oil reservoir was such that it led to the apparent opening. I can imagine (especially having seen graduate students overtighten bolts) that if you screwed down really hard on the bolts and the oil tank lid were on the thin side, you might be able to warp the cover on the oil tank lid a little and get some air escape.  The problem with this argument is that you can only use a ‘bad design’ argument once because NASCAR will make you redesign it.

The case of the No. 99 car’s oil reservoir lid is a little different, though, because the reports have been that the lid was entirely missing. In fords, the oil tank cover is held on by a single bolt.  Carl Edwards said on NASCAR This Week that a “bolt backed out”.  Jack Roush made the argument that the vibrations in the car caused vibration harmonics that caused the bolt to unscrew itself.  Even if that’s true (and I have to admit I’m a little skeptical about it), should you really have a safety feature held in place by a single bolt?

NASCAR fined the driver and the owner 100 points (old points scheme!), fined crew chief Bob Osborne (B.S. in Mechanical Engineering from Penn State) $100,000, 100 points and suspended him for six weeks.

Why Safety Takes Time

 Aerodynamics, Daytona, Lift-Off Speed, NASCAR, Safety  2 Responses »
Mar 012013
 

I suppose it’s really our own fault because of the way we teach science.

We give you labs constructed to get the right answer on the first try.   We have you measure things you already know the value of.   We tell you that things were invented by a single person on a specific date.

We give you an absolutely bogus model of the scientific method, putting it into a nice pretty, linear flowchart that is perfectly designed for multiple-choice tests. (Right, from HowStuffWorks.)

In reality, the scientific method looks a lot more like the figure below than the one to the right.  Visit the webpage to read the text on the flowchart.  It’s worth it because a lot of the boxes lead to explosions and/or lab fires.  It may be NSFW — depending on where you work. If you work in a lab, you’ve already said all those words today, I bet.

http://i2.wp.com/i.imgur.com/2Hrfu.png?resize=363%2C163This blatant misrepresentation of how science and engineering actually work is probably why the blogosphere and all electromagnetic radiation methods of communication (that would be radio and television) are full of people calling on NASCAR to ‘fix’ their ‘safety problem’ before we go back to Daytona and preferably before Phoenix.

Sorry, folks.  Science and engineering just don’t work that way.  We’re talking about research here – which means we’re dealing with things we don’t understand completely.  You can’t set a timetable for when you’re going to discover something.  It happens on its own schedule.

You may hear about R&D, which is research and development.  Research is figuring stuff out.  Development is making it happen.  Research is “what type of barrier do I need to stop a 200-mph car?”  Development is making the barrier that comes out of the research.

You can’t do ‘D’ before you do ‘R’.  Einstein once said “If we knew what we were doing, it wouldn’t be research”.  By definition, research is trying to answer questions for which we don’t already know the answer.

Who Does Motorsports Safety Research?

The car companies all do extensive safety research — but not at 180 mph.  Their results, which are on passenger cars, can’t be extrapolated to high speed racecars, so there’s not much useful information there for racetracks.

There are a couple university groups that do motorsports safety research, but you can count them on one hand.  There are a few consulting companies that are involved with motorsports safety research (sometimes as an offshoot of defense-department research) but again, it’s a small number.  The FIA is a centralized automotive organization focused on F1 that does extensive research in Europe for open- and closed-cockpit cars.

Safety research is inherently expensive.  Take barrier research.  Start by getting a racecar – which is an expensive proposition to start with – and then figure out how you’re going to accelerate a driverless race car into a wall at a very specific angle without a driver.  You have to set up high speed cameras, high speed and high strength data acquisition equipment and it all has to work right.  The companies that are looking at protective gear for soldiers can get funding from the Defense Department (at least before sequestration they could) and some of those results can be applied to motorsports.  The FIA dips into the coffers of the F1 enterprise.  It doesn’t make sense for auto manufacturers to pay for motorsports safety research, since little of it will impact safety in passenger cars.  That pretty much means that motorsports sanctioning bodies, tracks and teams have to foot the bill for safety research.  And it’s a really big bill.

It’s big in part because we not only have to do experiments, we have to design experiments.  Anyone got a spare F1 car we can crash into a barrier for testing?

I have a great story in my book, The Physics of NASCAR, from Dean Sicking, the inventor of the SAFER barriers about their first attempt to demonstrate to the Indycar people that they could crash an Indycar into a barrier.  It wasn’t pretty.  It also wasn’t successful — the first time.  They learned from their miscues and — years later — they had the first SAFER barrier design.  The SAFER barrier research started in 1998.  The first SAFER barrier was installed at Indy in 2002.  It took that long to figure out what the right design for the barrier was.

Research also never really ends.   They’re looking at how to make portable SAFER barriers for street circuits, SAFER barrier gates, cheaper SAFER barriers for road courses, and how to protect the ends of pit walls.  Even when one problem is mostly solved, there’s lots of fine tuning and specialization to be done.

The Problem with Multi-Car Accidents

With all the computer power we have to do simulations, shouldn’t we be able to use a computer to predict in advance what types of accidents are most likely?  Let’s say you want to create a computer program that would generate all the possible types of accidents.  Start with a single car.  Consider all the places on the track where an accident could happen and the types of accidents (hitting the front of the car first, hitting the rear of the car first, hitting  the pit road wall, getting airborne, etc.).  I bet you could list a hundred possibilities (at least) without having to think too hard.  When you get done with your list, let me know.

Now list all the possibilities for two cars.  It’s not two times the number of possibilities for one car because the two cars can interact in many different ways.

There were twelve cars involved in the Nationwide accident at Daytona last Saturday.  There is no way anyone could go through all of the possible outcomes of a twelve car pileup.  The problem with plate tracks is that they cars are running in a bunch and therefore the probability of a lot of cars being involved in an accident increases relative to, say, Martinsville, where most of the accidents are a few cars at most.

Here’s the other problem.  If I want to simulate a program that shows me how a fence will respond if a car hits it, I need real-world data on which to base my simulation.  So at some point, you’ve got to have a way to propel racecars into catchfences.

The Most Important Data Comes About When Something Doesn’t Work

You don’t learn anything if you don’t get data.  You don’t get a lot of useful data in motorsports safety research when everything works perfectly.  The only time you get real data is when something goes wrong.

The number of accidents at any one track is actually pretty small, so the amount of data we have is very limited.  NASCAR has an accident database where they’ve collected every bit of video, data from crash recorders and anything else they can get their hands on.  Every bit of data gives them a little more information on which to base future work.  But the most valuable data, unfortunately, comes when something doesn’t work.  This is true for every type of safety equipment:  barriers, HANS devices, seats, roof flaps, and yes, catchfences.

Priorities:  Ends and Openings

Most people in motorsports safety will tell you that the most challenging problem right now is building protective gear that is also required to move.  Window nets, for example, need to keep speeding car parts out of the car, but must be rigged so that they driver can lower the window net very quickly if necessary.  Ends and openings are the most challenging aspects of track design. One of the challenges to putting SAFER barriers on the inside of tracks is that there have to be openings for emergency vehicles to get on the track.  This requires SAFER barriers on hinges that can’t be damaged if the car happens to hit the hinges.

Saturday’s accident involved a pedestrian crossing gate – gates are the literal weak links in the catchfence design.  I suspect that if you had the exact same accident happen on a different point of the track, the results would have been very different.  It was the unfortunately perfect storm.  I’ve addressed most of the simple suggestions for catchfences in a previous blog post and those arguments apply here.

Where We Need to Focus

If we gear our efforts to simply preventing the type of accident that happened Saturday, we’ve missed the whole point.  The odds of that exact same type of accident are very small.  Then a different kind of accident happens and now we all run to prevent that specific type of accident.  Remember that a racetrack is a system:  it’s the cars, it’s the drivers’ safety equipment, the configuration of pit road, the track shape and size, and the catchfence.  All of these things work in concert with each other and focusing on any one of them misses the point.  It’s a complicated system and it takes time to figure it all out.

I wish I could say that, at some point, motorsports will be perfectly safe – for the drivers and the fans.   But it’s unrealistic.  There will never be any guarantees.  We can just do the best we can.

Related Articles

  • My hometown newspaper, the Milwaukee Journal-Sentinel, has a nice article on safety by Dave Kallman
  • Catchfence science after the Las Vegas Dan Wheldon accident
  • A HuffPost Live discussion on the Daytona incident, as well as some interesting comments about who owns footage from a sporting event and when a sporting event becomes news.

 

 

 

 

 

 

Brief Thoughts on the Nationwide Accident at Daytona

 Collisions, Daytona, Lift-Off Speed, Safety  4 Responses »
Feb 242013
 

We’ve been painting the new house.  I was straining to hear EPSN’s commentary over the swoosh-swish of the paint roller as the race came to a close – but it was all too easy to hear the change the tone of Allen Bestwick’s voice.  We heard it in Marty Reid’s voice in Vegas not too long ago.   I remember the first-hand feeling sitting about 50 yards from Michael McDowell’s wreck during qualifying at Texas.  A track full of race fans – all quiet – is one of the worst sounds in all of sports.

The first reaction to things like this is often blame.  Blame NASCAR for blocking videos, blame ESPN for showing too many replays — or not showing enough replays.  Blame reporters for not having information fast enough.  Blame reporters for information reported in good faith that turns out to be incorrect.  Blame drivers for blocking.  Blame drivers for trying to pass.  Blame NASCAR for letting the cars go too fast.

I was at the NASCAR R&D Center a couple of weeks ago talking with the head of NASCAR’s safety effort.  There is no one more committed to safety or doing more to make racing safe — for fans and for drivers.  There is no one at NASCAR who thinks the show is more important than safety.

In talking to folks at NASCAR and people like Dean Sicking, inventor of the SAFER barriers and one of the foremost motorsports safety experts in the world, the same themes emerge.  We have solved 99% of the safety issues.  The fact that Kyle Larson emerged from what was left of his car uninjured is a miracle.  The fact that the engine did, in fact, get stopped by the fence, saved lives.

The safety issues that remain are the ones that are hardest to solve:  The ends and the breaks in pit road walls.  SAFER barriers for inner walls that can still open quickly — even when damaged — to let emergency vehicles onto the track.  Gates in catchfences that open, but retain the same strength as a solid piece of catchfence.

I’ve got some general information about catchfences that was written after the Dan Wheldon accident, and I expect I will reiterate my thoughts that there needs to be a concerted effort to figure out how to fund the very expensive research necessary for preventing what happened Saturday night from happening again.

Right now, my thoughts are with the people who were injured and my best wishes go to them for speedy, complete recoveries.

What’s a NACA Duct?

 Gridlock, NACA Ducts, Uncategorized  2 Responses »
Dec 212012
 

Once upon a time, during a period in which our country — and the rest of the world — was in turmoil, the President of the United States wanted to create a governmental body that would be charged with coordinating critical scientific work being done by government with similar work carried out in industry and at universities.  Congress repeatedly refused to authorize the legislation.  The body wasn’t approved until three years after it was originally proposed – and then, it passed only because its creation was tacked onto the Naval Appropriation Bill and no one really noticed.

Sounds like our current Congress, doesn’t it?

NACALogoThis actually happened during World War I and the governmental body that was finally approved was the the National Advisory Committee for Aeronautics.  Its task was coordinating war-related projects between the government, industry and universities.  President Taft started the process in 1912, but it wasn’t until 1915 that President Wilson finally signed the legislation directing  that NACA (which was pronounced with each letter spoken, not as a single word)  would “supervised and direct the scientific study of the problems of flight with a view to their practical solution”.   One of the first appointees was Orville Wright.  (Yes, the logo — shown at left — is a little lame, but the government had a couple sort of important things going on at the time.)

NACA addressed their mission and much more – they were responsible for new, high-speed wind tunnels, engine test facilities, and lots of fundamental discoveries that facilitated commercial and military planes, including breaking the ‘sound barrier’.  Although the organization no longer exists (having morphed into the National Aeronautics and Space Administration in 1951), many of its discoveries and breakthroughs are still used today.

NACA Ducts

NACA Ducts

The words “NACA Duct” usually go by so fast that they sound like “Nackaduck”, but a NACA duct is really just a specialized type of duct.  Ducts are hoses or other devices used to move air from one place to another.  brings air into (in our case) a racecar in a very special way.   Clear NACA ducts can be seen in the rear windows of NASCAR cars – there are usually two or three in the window.  Carbon-fiber NACA ducts sit in the front of the car.

Aerodynamic forces — like drag — increase like the speed squared.  If you go twice as fast, you get four times as much drag.   If you think a race car going 200 mph experiences a lot of drag, think about an airplane at the speed of sound — which is 768 mph.  If a NASCAR race car could go the speed of sound, it would experience 15 times more drag than when it was going 200 mph.

The reason we don’t like drag, of course, is because drag always acts in the direction opposite the direction the car is moving.  No matter what direction you’re moving, drag is always trying to slow you down.  If you don’t care about being able to go faster, you still want to decrease drag because you need to use energy (gasoline) to overcome drag.  (I’m going to have an interesting article coming up on some nanotechnology being used to decrease drag in airplanes — that might be an option for NASCAR teams for Daytona and Talladega.)

Air must be brought into the car  to cool the brakes and to cool the driver. If you just pop out the rear window, you get a lot of air into the car — but you can’t direct where it’s going and you’ve just created a whole lot of drag.  The NACA duct was designed to bring air into a vehicle with minimal increase in drag.  (NASA has the original paper on the origin of the design on their website.)

How It Works

NACADuctCarbonFiber

Some current road cars have air scoops sticking up out of the hood of the car to bring air into the engine – the problem with something poking out of the car is that it increases drag.  NACA ducts are submerged ducts – they are installed flat.  The picture at left is a closeup of a carbon fiber NACA duct.  The air comes into the narrow end and a hose is attached to the round part so that you can direct the air to the brakes or the driver’s airbox.

Drag is literally friction between air molecules and the surface of the car.  The air molecules closest to the car move at about the same speed as the car, but the air molecules further from the car’s surface move faster.  The boundary layer is the thin layer of air nearest the car’s surface.  The boundary layer can be thick or thin and the flow can be laminar (meaning in straight, smooth lines) or turbulent.  NASCAR aerodynamicists want  a thin boundary layer with laminar flow.  The thicker and more turbulent the boundary layer, the more drag.  The length and shape of the NACA duct creates counter-rotating vortices that deflect the boundary layer away from the intake, but draw in the faster-moving air above it, thus allowing you to get air into the car without creating a lot of extra drag.

During qualifying, you may see teams taping over the NACA ducts – in a sport where hundredths of a second count, eliminating even the small amount of drag the NACA ducts create can make a difference in where you start the race.  Like the engine opening, you can run a couple of laps without brake cooling or driver cooling, but you can’t run an entire race.

Although NACA ducts were originally designed for high-speed aircraft, they can’t produce large airflow or high-pressure airflow, which means that they really aren’t used anymore in the jet engine applications for which they were originally designed.  They are, however, ideal for cutting down the drag on racecars.

 

Recent Restrictor Plate Posts

 Aerodynamics, Drafting, Electronic Fuel Injection, Engines, radiators  No Responses »
Jul 062012
 

Radiator Temperature vs. Pressure Setting of Pop-Off Valve

Why Radiators are so Important at Plate Tracks – The radiator pressure rules seem to change a lot at plate tracks:  here’s why…

Why Bump Drafting Seems Harder in 2012  A summary of why bump drafting done incorrectly causes so many problems

What’s a Pop-Off Valve?  One of the easiest last-minute changes for NASCAR to make is the pressure relief valve (a.k.a. the “pop-off valve”).  Here’s everything you need to know about how that affects drivers’ ability to draft.

Rules Changes…Again

 Aerodynamics, Downforce, Roll Bar, Rules, Yaw  8 Responses »
Jun 212012
 

Note added 6/27/12:  Kevin Harvick made the point in an interview that one team had figured out a way to use the sway bar for things it was not intended to do.  This gave that team (according to the engineers I’ve spoken with) a pretty good advantage on mile-and-a-half tracks.  Will we see a difference?  We might – who is to say that the team didn’t come with something new that hasn’t been obviated by a rule change yet?

NASCAR issues rules change on Wednesday – one of which extends an earlier rule change to try to improve aerodynamics at intermediate tracks.  The other is designed to cut off an entirely new direction of research before it starts.

Many of the problems with passing at 1.5-2 miles tracks are due to aerodynamics.  Toward the end of last May, NASCAR revised their rules about the height of the side skirts on the cars to try to change the aerodynamics.  NASCAR issued another set of rules changes Wednesday that continues in the same direction.

Left side skirts went from being a minimum of 4 inches from the ground and a maximum of 4.5 inches from the ground to 4.5 inches min/5 inches max from the ground.  The right-side skirts went from 4.5 inches/5 inches to 5 inches/5.5 inches.  The primary effect of raising the skirts is to make it harder for teams to get the car to seal to the track.  This lets more air under the car.  That air pushes upward on the car, which decreases the total grip.  Additionally, the asymmetry between left/right helps air escape from under the car in the case of a spin.  (Thanks to Dennis for noting that I had two left-side skirts there!)

The other rules change is a little more interesting and has to do with the sway bar, a part that people often talk about, but that is rarely seen.  Sway bars can be used in the front or the rear of the car:  this rule addresses the rear sway bar.  The rules change mandates that right- and left-side rear sway bar links must be perpendicular to the ground, as viewed from any direction when the car is at ride height.

Here’s a sway bar assembly.  The sway bar itself is the long cylindrical piece of metal running horizontally across the figure.  The arms are the two pieces of metal that come out either side.  (as Scott points out in the comments, there are no links in this picture.  I couldn’t find a decent picture that showed the links, unfortunately.)

Here’s an arm from another angle. One of the arms is attached to the left-side wheel assembly and the other is attached to the right-side wheel assembly.  This video from Jeff Hammond shows you the sway bar on a cutaway car.

Using the top picture, imagine that each wheel is raised by the same amount.  Both arms move up the same amount and the sway bar simple moves upward with the wheels.

 Now imagine that only the right wheel moves upward.  The right arm transmits a force to the right side of the sway bar and that force tries to twist the bar.  Bars are manufactured with varying amounts of resistance to twisting.  A stiffer bar requires more force to move one wheel with respect to the other.

The roll bar is used to minimize body lean – the shifting of the body from left to right when the car turns left.  When properly selected, the roll bar helps the car roll through the corner; however, there are a number of secondary things roll bars can do.

Remember the year of yaw?  When everyone was trying to get the rear of the car jutted to the side so badly that some of the car looked like they were coming down the frontstretch sideways?

The gossip in the garage is that one team had figured out a way to attach the rear roll bar so that, when the car transferred load, the roll bar would push the rear of the car askew.  This is a really clever trick that relies on having very compliant bushings.  Anything you can figure out how to do so that the car behaves differently, but only when the car is at speed on the track, is significant, because that makes it much harder for the other teams (or the sanctioning body) to catch the change.  Apparently, mounting the roll bar this way requires you to cant the sway bar arms, which is what made the tweak visible to inspectors.

The roll bar is allowed on the car as a tuning device for load transfer – using it to shift the rear end housing opens up a whole new range of possibilities that NASCAR decided they wanted to cut off before things got out of hand.

Why 200 mph Laps at Michigan are not like 200-mph Laps at Daytona or Talladega

 Aerodynamics, Daytona, Lift-Off Speed, Michigan International Speedway, Talladega  2 Responses »
Jun 172012
 

That fact that people are even talking about restrictor plates for Cup racing at Michigan International Speedway indicates a lack of understanding of the issues that give rise to concerns about cars getting airborne.

I touched on the difference between average and instantaneous quantities last week with the pit road speeding issue at Pocono.  Instantaneous speed is the speed you are going at some particular instant.  A radar gun measures instantaneous speed.

Average speed takes into account that speed varies over time and gives you one number that represents your speed over a number of different points or times.  Speeding loops measure average speed.

For example, let’s say I look at my speedometer five times over the course of an hour.  I note that I’m going 55 mph, 62 mph, 67 mph, 52 mph and 64 mph.  Adding those numbers up comes to 300 mph.  Since there were five measurements, divide by five and my average speed is 60 mph.  The measurements could vary much more widely and still produce the same average.  Any five speeds that add up to 300 mph would give you an average speed of 60 mph.

Michigan

I’ve diagrammed to the right a rough sketch of speed vs. time (or distance).  A diagram of Michigan International Speedway is in the upper left-hand corner of the picture, with corners and straights labeled.  Those positions correspond to the positions shown in the graph.

The maximum speeds of 218 mph being reported happen generally toward the end of the straightaways.  You’re accelerating all the way down the straight, reaching maximum speed just before you have to brake to enter the turn.  You slow down going into the turn and speed up coming out of it.  (I drew a symmetric graph only because I have limited time and even more limited drawing skills.)

Pre new left-side tire, cars were reaching a maximum of 218 mph with average lap speeds a little more than 200 mph.  This tells you there must be a significant number of places on the track where cars are going slower than 200 mph.  Those places would be the turns, where they are slowing down to a little over 190 mph (Thank you @chrisneville84 and @DRodmanNASCAR.)  The solid line in my graph shows the instantaneous speed, while the average speed is shown as a dashed line.

Does that mean we don’t have to worry about lift-off at Michigan?

As I mentioned in my last post, the big concern is that the car becomes unstable against lift off when it a) reaches a high enough speed and b) spins.  Cars are more likely to spin in the turns, but the cars aren’t going 218 mph in the turns at Michigan – they’re going much more slowly.

The issue remains that two cars could hit on the straightaway, spinning one or both cars.  This concern is heightened because of the possibility that a spinning car can hook a tire on the asphalt/grass transition in the frontstretch.

Daytona or Talladega

A corresponding picture of speed vs. time (or distance) at a plate track would look like the picture to the left.  Cars are flat out all the way around the track.  The speed doesn’t change very much through an entire lap.  At Daytona, they are taking the corners near 200 mph.  That’s a far different situation than at Michigan, where the speeds vary considerably around the track.  The propensity for getting that perfect storm of speed and angle of the car with respect to the air increases under those conditions.

Why isn’t this a concern for Nationwide?

We’ve heard all week that the Nationwide cars are running wide open all the way around the track, just like they do at Daytona.  Their average lap times, however, are in the 190 mph range, which puts the safety concern squarely in the dangers of pack racing rather than cars going airborne.

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The Myth of the 200-mph “Lift-Off Speed”

 Aerodynamics, Lift-Off Speed, Michigan International Speedway, Racing, Rules, Safety  6 Responses »
Jun 152012
 

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 Lift-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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Don’t believe the speeds at Michigan

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