May 242013

Listen to SiriusXM NASCAR radio, or peruse any of the racing websites and you will find a lot of theories about how races should be changed to make them ‘more exciting’.  To try to amp up the All-Star Race, NASCAR went with four 20-lap segments, followed by a realignment (the cars were ordered in rank of average finish over the first four segments) and a 10-lap shootout.  With no series points on the line, that should have made for an exciting evening of hard driving and competitive racing.

Or not.

First, let’s acknowledge a fundamental sports fact.  Sometimes, competitions just aren’t exciting.  The blowout between two unmatched football teams, for example, isn’t going to be interesting unless you introduce profoundly contrived gimmicks like loosing live lions on the field during timeouts.  So let’s just accept that sometimes we’re going to see races that just aren’t very exciting.

That doesn’t mean there’s nothing to be done.

We want side-by-side racing with lots of passing.  We’re frustrated by long green-flag runs in which the car that starts out leading is never challenged for the lead.  The only chance to get track position is to take two tires, or no tires during a pitstop and hope that your lack of new rubber doesn’t hurt you too much when the cars that did take tires catch up to you.

We want to see changes in the running order during green flag runs.  So what naturally changes on the cars over that time?  The two most significant factors are:

  • The tires heat up and wear down.
  • The fuel burns off, making the car lighter.

A NASCAR car holds about 120 lbs of fuel, so as a green-flag run progresses, a car ought to get faster because it’s getting lighter.  But that’s not what happens.  Lap times increase over the course of a green-flag run because the tires wear down (which we call “tire fall-off”).

Grip is just another word for friction.  When tires run against a track, two types of friction are in play.  The first is abrasive friction, which is the same type of friction as a wood block rubbing against sandpaper.  This wears down the tread.  The second type of friction (which they never seem to teach in school) is called adhesive friction.  Imagine you have a piece of chewing gum on your shoe.  It’s a hot day and you’re walking down the sidewalk.  You walk over another piece of chewing gum.  The two pieces glom onto each other – that’s another type of friction.  As tires run on a track, they heat up and the first few atomic layers become very soft.  Those layers interact with the layers of rubber already on the track and increase adhesive friction.

The more friction, the faster the tires break down.  Those top layers of rubber are worn off – they get hot.  Tires can easily reach 250-325 degrees Fahrenheit.  The “marbles” — clumps of rubber, track dirt and dust show you how much rubber used to be on the tires.  The tires lose their tread, get warmer and thus lose their grip.

Consider a track like Atlanta Motor Speedway.  Tires last for about 40 laps and a typical lap time is around 30 seconds. At a 1.5 mile track like Atlanta, the typical lap time will be one second longer after the first 10-15 laps.  By the time you’ve completed 40 laps, you’ve added another second to your lap time.  That corresponds to decreasing your average speed by 10 mph over the course of the green flag run.

The problem is that tires wear pretty much the same for everyone.  If we all start with fresh tires, and we all run 20 laps, then (barring accidents and acts of stupidity), we all experience the same tire fall off and we’ll stay pretty much in the order we started in.  In cases where the tires really don’t fall off much, you’ll see teams stopping for fuel and not taking new tires because it doesn’t get them anything.

Back in the day, tires were a heck of a lot softer than they are now.  You might see lap times increase by a couple of seconds per lap.  That’s a situation in which strategy starts to become important.  It might make sense for me to pit and get new tires because the time I lose pitting could be made up on the track since my new tires make me significantly faster than the cars that didn’t pit.  A few extra laps on the tires made a huge difference in speed.

The problem is that very soft tires wear quickly (especially if teams use very aggressive setups) and can lead to tire failures.  Then you have, for example, Tony Stewart very publicly excoriating Goodyear for making “the worst tire I’ve ever been on”.  Goodyear has some pretty stiff challenges:  tracks are constantly being repaved and reconfigured, and there’s an entirely new car this year with different characteristics.

When a Goodyear tire fails, it damages the company’s reputation.  Goodyear is in the sport to get business.  When a driver announces that he or she doesn’t trust the tire, or that Goodyear is incompetent, it’s a major blow.  So Goodyear stays on the conservative side, producing harder tires that will withstand some level of abuse from overly aggressive setups and overly aggressive drivers.  Harder tires don’t fall off.  They also cause fewer accidents and less negative publicity.

If I ran Goodyear…

OK, scratch that.  If I ran Goodyear and had all the money in the world, here’s what I would develop:  Multilayered tires.

In the nano world, we make things like coatings using multiple layers of different materials.  In making a coating for, say, a drill bit, we may start with a material that sticks really well to steel, but isn’t superhard, and then follow that layer with a layer of a material that is superhard, but wouldn’t stick well to the steel.  The two layers together give you superior properties to either one.

You could do the same thing for tires, right?  Make the outer layer of the tread a soft, grippy, really fast rubber that wears down within the first quarter to third of a fuel run.   The tread layer below could be a harder layer that still gives reasonable grip, but is nowhere near as fast as the upper layer.  This would give the crew chiefs some serious options for strategy while still keeping the tires from being unsafe.  The disadvantages, of course, are that the tires would be more expensive and take a lot more time to develop.

Goodyear has a neat video up on how the NASCAR tires are made.  The video could stand to have some titles to tell you what’s happening at different spots – and it doesn’t really do justice to the fact that a significant portion of the work is done by hand.  Different tires are made for different tracks and Goodyear is constantly tweaking the recipe.  There’s also a nice article up on Autoweek talking about the NASCAR tire factory in Akron.

And here’s a re-run of a video in which some of the top crew chiefs and engineers from NASCAR argue for more tire fall off.

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.


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.


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.


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


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:

May 062013

When you were a kid, perhaps you locked yourself in the bathroom, turned out the lights, positioned yourself in front of the mirror and then turned on the lights to watch your pupils grow.  And if you’ve never done this, shame on you for not being curious.  Go do it.  Now.  Or maybe if you’re not the participatory type, you’ve noticed your cat lazing in the sunlight with her eyes narrowed down to nothing but vertical slits.  These are both examples of how an iris adjusts to control how much light enters an imaging device – in this case, a person’s (or animal’s) eye.

File:Schematic diagram of the human eye en.svgAs shown in the diagram, the iris (the colored part of your eye) encircles the pupil.  The pigmentation in the iris prevents light from coming through, so light gets in only through the exposed part of the pupil.   The iris is connected to a muscle that controls its size.  When it is dark out, the iris pulls back, exposing more of the pupil.  When it’s bright, the iris relaxes and gets smaller, decreasing how much light comes in. You can think of the iris as a sort of restrictor plate for the eye.

We see when light comes in through the cornea, through the pupil, and then is focused on the retina that lines the rear of the eye.    Cats have a reflective membrane behind the retina that focuses light passing through the retina back into the eye – one reason they see better in the dark.

There are limits to how big or how small the iris can get, which is why we simply can’t see when it’s really bright outside – then we use another type of light restrictor, like sunglesses.  There are also limits to how well we can see in the dark because we can collect only a small part of all the light that is out there.

IrisTelevision cameras as have an iris, to allow them to shoot in a wide variety of light.  Their irises are, as shown, mechanical in nature and are adjusted manually or automatically by the camera based on algorithms that try to optimize the quality of the image.  If you’ve used high quality cameras, you’re familiar with the term ‘f-stop’:  The larger the number, the smaller the iris.  The f-stop (the f is for “factor”) is the ratio between the lens opening and the focal length of the lens.  That’s why f-stops come in such goofy numbers.  The area of the is proportional to the square of its radius.  When you increase the  the aperture by a factor of 1.4, you double the light.  (F-stops are usually 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22)

The figure below gives you an idea of how the iris changes the amount of light let into the camera.  The numbers at the bottom are the f-stop values.


The iris on a professional television camera can open way, way up and make it look like it is a lot brighter than it actually is.  It’s simply gathering more light than it would if the iris were at a normal setting.

Twitter was abuzz during the Talladega race with people asking why the drivers were complaining about the lack of light because their television picture looked just fine.  You can’t judge the amount of light from a television picture because the television camera is always optimizing its settings to give you the clearest picture.  You have the same issues with still photographs – how light or dark it looks depends entirely on the setting on the camera or phone.

What I thought was odd were the varying evaluations from the reporters who were actually at the racetrack.  Some were saying there way plenty of light, while others were asking how NASCAR could even think of re-starting  the race given the darkness.  The folks I would really like to hear from are the spotters because if they can’t see their cars, that’s a disaster waiting to happen.

Just for the record, driver reports are similarly unreliable due to a psychological effect that makes you think it’s too dark if you’re leading the race.  If you’re not in P1, the light looks just fine.

Incidentally, installing lighting at Talladega is a tens of millions of dollars project.  Maybe not what a track is able to do given the struggling economy and sagging ticket sales.

NOTE:  As Allen Lee (@wxguy) points out, the CCD (Charge-Coupled Device) – the thing that acts like the retina in your eyeball – is also more sensitive than your retina.   You can read more about CCDs in an article I wrote for Cocktail Party Physics awhile back.

May 042013

Water is critical to the existence of human life.  Why do you think we spend so much time looking for it on other planets?

It is, however, less than desirable on a racetrack.  Water gets between the tires and the track, which decreases friction.  Decreased friction means lower speeds and higher probability of crashing.

Before you read any more, head over and read why it takes so long to dry racetracks.

OK.  Welcome back.

The traditional way of removing water from a race track was to evaporate it using jet engines.  Evaporation is the process of turning a liquid into a vapor.  In the case of the jet dryer, this is done by heating the water until it evaporates.   The exhaust from a jet engine is somewhere in the 1000 F -1300 F temperature range.

The problem is that evaporation is a slow procedure, even when abetted by jet engine temperatures.  Only the molecules on the surface of the water can evaporate, so you have to evaporate the water layer by layer.  This is why it takes a couple of hours to dry an intermediate-size track.

The new system (Air Titan) relies on a little different approach.  The majority of the water will be blown off the track, leaving a thin residual layer that can then be evaporated by the jet dryers.

The leading edge of the system is a series of pickup trucks towing trailers and attached to another vehicle by giant hoses.  The trailers contain the equipment that pushes the water off the track.  There’s a really great picture of the water sheeting off in the New York Times article.

This is trickier to design than it sounds. Air has to be compressed to very high pressure, then blown onto the track in a controlled way.  If you just blow air on the track, the water will spatter in every direction and the net drying effect will be negligible. If you have jets of air that focus on discrete spots, the water in the area you miss is going to spread out and re-wet the track.  You have to have a continuous sheet of air, angled to push water exactly where you want it to go.  The compressors are huge – as you can see from the diagram above.  Three compressors are being towed on a semi truck on the apron, as the center of gravity of the semi is too high for it to be on the banking.

A series of pickup trucks will circle the track in a synchronized way:  the truck closest to the outside wall will blow the water there down to the next groove.  A pickup following behind in the that groove will push it down further and so on.  (One might imagine a blower powerful enough to blow air down the track from the outside groove to the apron.  But I bet you’d have a really hard time finding someone to volunteer to be the driver of the truck on the apron.)  There’s a nice animation of the process on YouTube.

NASCAR_AirTitanDiagramBlowing the water off the track is only the first part of the process.  A vacuum truck on the apron will suck up the water pushed down by the air jets.   The process finishes off with the traditional jet dryers – which now only have to evaporate the very thin layer of water that remains on the surface.

The system isn’t Juan Pablo-proof, but because of the hoses and the water sheeting down the track, racecars won’t be allowed on the track while the dryers are out there.  Reports are that it’s costing NASCAR mid-to-upper six figures to develop the system — which is probably a small price compared to losing valuable airtime or being shunted to a less-widely distributed cable station due to delays.

The idea is to be able to dry a big track like Talladega in less than an hour, but even Air Titan doesn’t solve all the problems.  The rain has to stop before the drying system can be used, so rain delays may be minimized… but they won’t be a thing of the past.

Not until NASCAR figures out how to control the weather.


May 042013

This is a major revision of a post that originally appeared on the now-defunct on 4/18/10.

Why does it takes so long for a track to dry?  Why does humid weather make track drying take even longer?

Air is a mix of gas molecules:  mostly (78%) nitrogen, about 21% oxygen, the rest misc. gases.  The composition is pretty uniform with the exception of how much water is in the air.   The absolute humidity is the amount of water in some chosen volume of air, for example, how much water vapor is in one cubic meter of air.  Air can only hold so much water vapor and that amount depends on the temperature and pressure.  Dry air would be no ounces of water in a cubic foot of air.  If the vapor is saturated at 30 degrees centigrade (86 degrees Fahrenheit), then the amount of water could be up to three one-hundredths of an ounce of water per cubic foot.

The mechanisms we use to get rid of water on the track are evaporation and possibly boiling.  Evaporation is the same mechanism we use to dry dishes, or even ourselves when we get out a pool and just let the sun dry us.  Evaporation is a liquid changing into a gas.  Boiling is also changing a liquid from a vapor to a gas, but there’s a difference.  Evaporation happens at the surface of a water drop.  Only the outermost few water molecules change from liquid to gas.  Boiling affects the bulk of the water drop.

Regardless of whether we’re talking evaporation or boiling, the water on the track doesn’t exist in a vacuum.  There’s that water vapor in the air.

Nature likes equilibrium.  Equilibrium is when things are equal and concentration is one property that can be equal.  If you pour a glass of red dye into a fish tank full of clear water, the red dye molecules will spread out and uniformly distribute themselves throughout the fish tank.  (Don’t try this if there are fish in the tank, please…)

So we have water molecules in the water drop – a lot of water molecules – and water molecules in the air.  The concentration of water molecules in the air is smaller than the concentration of molecules in the water droplet, but it can vary depending on how humid it is.  The picture below schematically shows three situations in which there are increasing amounts of water vapor in the air surrounding the water drop. The darker the green, the higher the concentration of water molecules.

Nature likes equilibrium, so it would like to have the same concentration of water molecules everywhere.  The rate at which it can move water molecules from the water drop to the air is proportional to the difference in concentrations.

If you have really dry air, there is a big difference in concentrations, and the water from the droplet moves into the air faster.  Have you ever hung your swimsuit out to dry on the balcony of a Florida hotel in July?  It takes forever to dry because the air is so moist.  There isn’t a huge difference between the concentration of water in the air and the concentration of water in the water drop.  If it were relatively dry and we had a rainfall, the track would dry much more quickly than it would with the current conditions:  the humid air is already pretty saturated – relative humidity is how close we are to totally saturated and the numbers have been around 90%.  100% relative humidity means that you absolutely can’t put any more water vapor in the air, so it would take a very, very long time to dry the track.

Jet dryers are literally jet engines that speed up evaporation by just heating the crap out of the water sitting on the track.  The temperature of the combustion fuel is on the order of 1100 degrees F, but it cools pretty quickly as it leaves the dryer (that’s why the jets are so close to the track surface.)  If you have eight jet dryers, each operating for 50 minutes on 175 gallons of fuel and it takes 150 minutes to dry the track, we’re talking about 4200 gallons of jet fuel.

In my next post, I’ll explain how the Air Titan system works and why it should be a huge improvement over jet dryers.
Apr 262013

We’d been hearing rumors of penalties stemming from Kansas and everyone expected them to be announced Tuesday.  Since penalties usually have some scientific component, I was sort of hoping for some new material.  Tuesday came and went.  Nothing.  Wednesday, all heck broke loose as penalties were announced for the No 20 JGR car (engine issues) and the No 98 ThorSport truck.

The JGR issue isn’t that complicated — or interesting.  Someone screwed up and connecting rod that was too light got into an engine.  It’s beyond surprising that Toyota said that they don’t have the personnel to check all the engine parts – given that everyone knows that penalties for engine violations are huge, why would you risk something like this happening?  The penalty was pretty severe and NASCAR has to crack down.  I get taking away points, leveling fines and such — but the tradition of holding the crew chief responsible for the engine when the engine comes from a supplier who doesn’t let the team touch it is out-of-touch with the realities of NASCAR today.  That was a valid punishment when everything came from the team, but there is no reason Jason  Ratliff should be suspended over an issue that was entirely JGR.  I have no problems with the other penalties, but that one is pretty unfair in my opinion.

The more interesting — and less discussed — penalty is the ThorSport/Johnny Sauter one.  (It was a tough week for Wisconsin drivers).  The team was docked 25 points, which is pretty huge for the Truck Series and the crew chief fined $10,000.  (I realize that seems small when compared to the Sprint Cup Series penalties, but the Truck Series has correspondingly lower purses and salaries.)  Here’s the official NASCAR statement.

“The No. 98 truck was found to have violated Sections 12-1 (actions detrimental to stock car racing); 12-4K (if in the judgment of NASCAR Officials, race equipment that has been previously verified or previously approved and/or sealed by NASCAR for use in an event, pursuant to sub-section 8-6 and/or 8-12, has been altered, modified, repaired, or changed in any manner); 20B-16 (once a fuel cell or fuel cell components have been certified, modifications of any kind will not be permitted to the fuel cell or fuel cell components); and 20B-16.1B (standard black, safety foam with minimum free-standing height of eight (8) inches, acceptable to NASCAR Officials, and used as provided by an approved fuel cell manufacturer, must be used: Fuel cell safety foam modification.”

A fuel cell is slightly different than a fuel tank.  A fuel tank is pretty much an empty container, which leaves open the possibility of raging fires and/or explosions.  Fuel cells, which provide additional levels of safety, became standard after the death of Fireball Roberts from burns received during a car fire at Charlotte in 1964.

Engines_FuelCell_FuelSafeAs shown below, a fuel cell is a metal can (minimum thickness of 18 gage, which is 0.047″) fitted with a flexible bladder shaped to fit the can.  The bladder contains the fuel, but it’s not just a bag of fuel in a metal case.

The most important part of the fuel cell is the item labeled ’1′ in the picture above – the safety foam baffling.  In the picture below, the black thing on the left is the bladder and the yellow stuff is the foam.  The foam is a cross between a nerf ball and the white styrofoam you use to make models of the solar system in elementary school.  That is to say that the foam is sort of coarse, like the white styrofoam, but it’s not rigid – it gives a little when you squish it.  It’s also gasoline resistant.

Foam is mostly air.  There are even special foams called aerogels, in which 99 percent of the volume of the foam is air.  (Those foams are fragile and would be destroyed by gasoline.)  The foam takes up a small volume of the fuel cell, so it doesn’t change the capacity of the fuel cell by very much; however, it plays two very important roles.

The function most people know is that the foam keeps fuel from sloshing around in the turns.  A full tank of fuel weights about  120 pounds.  When a car corners, everything that isn’t rigidly attached to the wheels feels a force to the right.  The grip you have depends on how much weight is pushing down on each tire.  When weight shifts to the right side of the car, you lose grip on the left side tires.  There’s only so much you can do to keep the body from rolling in the turns – you don’t need the gasoline moving to the right as well.

More importantly, the foam prevents explosions.   If you tossed a match into a container of gasoline (warning: do not attempt this at  or anywhere else), the explosion would happen before the match actually hit the gasoline.  When you have a cup of water, it looks like there is water in the bottom and air on top.  In reality, there are water molecules from the liquid escaping into the air (and water molecules from the air condensing back into the liquid) all the time.  When you microwave that water until it boils, you force more molecules from the liquid phase into the gas phase.  The hotter the water is, the more water molecules in the air immediately above the liquid water.

In order to smell something, the molecules from the thing you’re smelling have to make their way into your nose.  You don’t smell liquids – the molecules from the liquid vaporize and make their way into your nose.

Slide1Gasoline is volatile.  In fact, any liquid that you can smell is volatile, which simply means that the molecules can very easily move from liquid to vapor.  If you look at a traditional fuel tank that is only partially full, it isn’t a layer of gasoline and a layer of air.  It’s a layer of gasoline and a layer of gasoline vapor mixed with air.

When gasoline is purposely combusted in an engine, it has to be sprayed as a very fine mist.  The finer the mist, the more efficiently it combusts.  The top layer in the fuel tank – the gasoline vapor – is highly flammable.  A spark will combust the fuel vapor molecules nearest to it.  That combustion compresses the rest of the fuel vapor, leading to a chain reaction and an eventual explosion.

Foam in a fuel cell prevents a concentration of the fuel vapor plus air mixture, which significantly reduces the probability that the fuel cell will explode in case of a fire. Reducing the amount of foam in a fuel cell, either by not putting it in to the eight-inch mandated height, or by carving out hollows in the interior of the foam, creates a very highly flammable pocket of fuel vapor and a major safety hazard.

While manipulating the foam might give you an advantage in terms of being able to fit more fuel into the tank, it creates a major, major safety hazard by making the fuel cell more likely to explode in case of an accident.  While people talk about messing with the fuel being a major no-no, the big thing here is really the safety aspects.


Apr 202013

Why Turning Fast is Hard

If Isaac Newton had been a racing fan (which I’m sure Sir Isaac would have been if had cars been invented in the 1600′s), he might have stated one of his laws this way:

A race car going straight down the backstretch at 180 mph will keep straight going down the backstretch at 180 mph — unless a net force makes it turn.

Race tracks are rarely circles, but as a first approximation, we can consider each turn to be part of a circle and model the turning of the racecar using uniform circular motion. Uniform circular motion basically means that the object is moving in a circle at constant speed.carturning

If I tie a string to a tennis ball and swing it at constant speed in a circle of radius r over my head, the only reason the ball goes in a circle is because the string is constantly pulling it toward the center of a the circle.  The string forces the ball to turn.

Just like the tennis ball, a turning car needs a force to make it turn. If you want the car to turn left, you have to exert a force to the left.  At each point in the turning circle, the force that makes the car turn is perpendicular to the direction the car is moving, which makes the force always toward the center of the circle.  This center-pointing force is called the centripetal force, and it depends on the mass of the car, the speed of the car and the turn radius of the track.


This equation tells you:

  • The heavier the car, the more turning force it takes
    • Because mass only appears one, if you double the mass of the car, you need twice as much turning force
  • The higher the speed, the more turning force it takes
    • The speed is squared — if you double your speed, you need four times as much turning force.
  • The larger the turn radius, the less turning force it take.
    • The turn radius is in the denominator, so it acts oppositely to the mass and the speed.

The Numbers

Let’s look at some numbers:  The minimum weight of a Gen-6 car is 3300 lbs for a driver of 180-lb, so I’m using a total weight of 3480 lbs (and dividing by 32.2 ft/s2 to get the mass).  Let’s look first at a wide sweeping track like Talladega, with a turn radius of 1100 ft and a speed of 180 mph throughout the turn.  According to the formula, that car needs 6848 lbs of turning force.
Let’s do the same calculation for Richmond, where the turn radius is only 365 ft.  Whoa — you’d need 20,636 lbs to turn at 180 mph.  Why?  The turn radius at Richmond is about 1/3 the turn radius at Talladega, so you need about three times more turning force.  This is why you slow down coming off the exit ramp on a cloverleaf.  70 mph is reasonable on the expressway, but when you’re turning and especially if the turn is tight, then you need to slow down. This is also why cars don’t take the corners at Richmond at 180 mph.
Let’s run the numbers at a more reasonable speed for Richmond, like 100 mph. Then you get about 6,370 mph.  But if you want to go 1oo mph around the corners at Bristol, you need 9,606 lbs of turning force because Bristol has even tighter turns than Richmond.  I put the numbers in a table for easy reference.

Track Turn radius
Turning Force
Talladega 1100 180 6,848 1.97
Richmond 365 180 20,636 5.93
100 6,370 1.83
Bristol 242 100 9,606 2.76


A “G” is a unit.  Just as we call twelve eggs a dozen, we likewise can measure acceleration in units of the earth’s gravitational pull.  A “G” is a unit of acceleration equal to the acceleration of the Earth’s gravity.  One “G” is 32.2 feet per second per second, or 22 mph per second.  An acceleration of  “2Gs” just means twice the acceleration due to gravity. 2G = 64.4 ft/s2 or 44.0 mph/s.

Acceleration is how fast you’re changing speed.  Anything falling with only the force of the Earth’s gravity acting on it will move 32.2 feet per second (or 22mph) faster for every second it is falling.  If you drop a penny, it will be going 22 mph after  one second, 44 mph after two seconds, etc.   Most amusement park rides top out at about 3G; however, some roller coasters go up to 4G (SheiKra Rollercoaster at Tampa) or 4.5G (e.g. the Titan Rollercoaster in Texas).  Accelerations over the 5-6G range cause problems because your heart can’t pump blood well enough to ensure that it makes it everywhere in your body and you’re subject to blacking out.
It seems like a G should always point downward, but just like a dozen eggs could be hen’s eggs or goose eggs, acceleration can be in any direction.  When you’re on a roller coaster coming down a hill, or if you’re falling, the acceleration is downward; however, when you’re taking a corner, the acceleration is sideways.  I talked to one driver who said he can’t handle roller coasters.  He doesn’t mind sideways Gs, but he really hates the up and down Gs.


Please don’t ever use the term “G-force”.  A G is a unit of acceleration, not force.  Force is obtained by multiplying a mass times times an acceleration.

At this moment, you are being pulled toward the center of the Earth.  If the surface of the Earth weren’t there holding you up, you’d be falling and gaining 32.2 feet per second in speed every second you fell.

If you step on a scale, the scale measures the force with which the Earth is pulling down on you.  That force is your mass times one G – which we call your weight.  A drag racer experiencing 5G of acceleration feels a force five times his or her weight.  The number before the “G” is the multiplier for how much force you feel in terms of your weight.

The reasons people use Gs is because you can talk about acceleration independent of mass.  If Danica Patrick and Tony Stewart experience 2 Gs around the corner at Kansas, Patrick (who weighs about 100 lbs) feels a force of 200 lbs.  Stewart (who weighs 180 lbs)  feels a force of 360 lbs.  They both feel the same acceleration, but because they have different masses, they feel different forces.  Everyone throws around numbers like ’50 Gs’, but without understanding that G is really the acceleration due to Earth’s gravity, those numbers have very little meaning.

Apr 062013

Martinsville is my favorite track and it has absolutely nothing to do with the hot dogs.  It’s a short, flat track out in the middle of nowhere.  It doesn’t have the high speeds and pack racing of Daytona and Talladega.  It’s not located near a major metro area like Las Vegas or Chicago where there’s plenty to do outside the track.  But it holds a special place for me.

When I was writing The Physics of NASCAR, I arrived about 6am, before the Sun was up, to make sure I wouldn’t hit traffic and miss the opening of the garage.  I sat in my rental car and read through my notes from the last day… and scanned the weather forecast because there was likely going to be rain.

There weren’t a lot of cars there in the parking lot just off the midway.  Three generations of a family were setting up their breakfast opposite me.  The kids were a little sluggish — apparently not used to getting up quite this early on a Sunday.  Before long there was a grill running and the smell of food that was much better than the McDonald’s I’d picked up along the way.  I enjoy the quiet of the early morning because it’s the perfect time to think – to get yourself oriented for the day.

At first, it was just the low buzz of the people across the way cooking and drinking the first cups of coffee of the day.  And then, off somewhere in the darkness, a low voice started singing a hymn.  Other voices joined in and the sound quietly made its way across the parking lot.  A religious service was being held near the 31 souvenir trailer and the hollers of Martinsville made the perfect acoustics for a church.  A preacher started talking – not a revival-type sermon, but a low, quiet reflection about the importance of thinking about what you do in life and how you treat other people.  About the fact that each of us has the power each day to make someone else’s life better — or worse — and that we need to keep that in mind before we act.  There was more singing, more people joining in as the Sun started to rise and light spread over the parking lots.  I’m not an organized religion person.  The year my grandmother took me to get my throat blessed, I got the mumps.  But sitting there in my car listening to the singing was a truly spiritual moment and a reminder to take a few moments each morning and think about what you can and should do during the rest of the day.

There is a lot I remember about that weekend.  I actually never ate a hot dog.  I was too busy running around trying to keep up with everything.  I do remember the thrill of  standing in the qualifying line on pit road with the car already on the track whizzing by us fifteen feet away.  You know how Kenny Wallace always talks about ‘Kenny Wallace the race car driver’ not being anything like Kenny the television guy?  It’s 100% true.  Wallace started out joking with everyone and as the car crept up to the front of the qualifying line, he got quieter and quieter, focusing only on the qualifying effort.  Walking pit road with Josh, Elliott Sadler’s crew chief at the time, as he gestured with a cup of coffee (the 19 team had the best coffee!) at cracks in the pit stall that might cause a pit crew member to slip during a stop.  Watching Kirk, Elliott’s crew chief, crawl over the little half wall that forms one side of the garage stall and stand a few feet from the track trying to see how the car was transferring weight as it came out of the corner for as long as he could before a NASCAR official told him to move back.  Learning that the best place for me to stand so I’d be out of the way was inevitably wherever the trash can was.   The best National Anthems of any track.  The quiet competence of the head engineer, Chad Johnston — who is now Martin Truex, Jr.’s crew chief — instructing the crew to get a canopy up over the tires when it became clear it would rain.  How nice everyone on that team was to me while I was following them around asking annoying questions.  Watching how hard a disappointing finish affects the driver and the crew.  Seeing how much sheer love of the sport and competitiveness everyone associated with NASCAR has.

Martinsville may not have fancy suites and garages, the best toilets in the series, or the poshest hotels — but if anyone asks me what tracks they ought to make sure they get to, Martinsville is right up there in my top three.

Mar 262013

I got a call out of the blue in the office yesterday.  A biomedical physicist/radiation oncologist from UC-Irvine who had just gone to his first NASCAR race at Auto Club Speedway had a question about my book, The Physics of NASCAR (which, by the way, you don’t have to be a physicist to read.  In fact, it’s probably better if you’re not.)

How did he get interested in NASCAR?

The American Association of Physicists in Medicine 2012 conference was in Charlotte.  The social event (yes, physics conferences do have social events!) was held at the NASCAR Hall of Fame and the exhibits piqued his interest.  Returning home, he found a fan among his coworkers and last weekend, he got to his very first race.  And boy, did he pick a great first race to attend.  NASCAR got a couple new fans last weekend — welcome!

I always tell people who can’t see why racing is interesting that they need to go to a race in person because television and radio can’t capture the in-person sights, sounds and smells.  You’ve got to go to the track to understand why people become fascinated with cars traveling in circle.

One of the questions this physicist had was the noise level.  He had brought earplugs, but he was surprised by how many people didn’t have them.

If you’re ever in the NASCAR garage, take a careful look at the over-50 crowd.  Many of them are wearing hearing aids because they spent a lot of time at the track without earplugs.  A racecar easily breaks 120 decibels 15 feet from the exhaust.  120 dB is the level of a jet engine or a rock concert.  It’s also the threshold for permanent hearing damage.  Despite the great advances we’ve made in science, we are still not able to restore people’s hearing.  Your hearing is already getting worse just because you’re getting older.  Don’t give Mother Nature any help.  Bring a pair of cheap foam earplugs to the track and wear them.  Heck, buy a couple  pair and give your extras to anyone you see who doesn’t have ‘em.

I know… it’s a thrill to hear the cars fly past, but permanent hearing damage is, well…  permanent.

Bonus Tuesday Geek Joke

You’ve all heard this one, right?

Q:  How do you tell the extroverted physicist at the social event?

A:  He’s the one staring down at other people’s shoes.



Mar 062013

As we head for Las Vegas this weekend, I thought I’d repost on of my most popular posts from on 3/5/2008  since the redirects for the old 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.


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.

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