Reader Questions: Gen-6 Windshields on Passenger Cars?

 Lexan, Uncategorized  1 Response »
Feb 182013
 
PittCaleb asks:

If we used Lexan in passenger cars, would there be any benefit such as reason repelling our ice buildup? Scratching or other potential downsides? What would the price diff be?

The implementation of superstrength laminated polycarbonate (Lexan) in NASCAR windshields raises the obvious question:   “If it’s so good for NASCAR, why isn’t it in my car?”

A couple considerations:

  • Weight:  The less a car weighs, the less fuel it takes to run, so decreasing weight is not just good for going fast:  it’s good for saving gas as well.  That said, the weight differential replacing a windshield with Lexan is pretty small compared to other places on a car.  the Department of Transportation (DOT) must approve all windshields for use in passenger cars and they require 1/4″ hard-coated Lexan.  Since the density of Lexan is roughly half the density of glass, assuming equal thickness, a Lexan windshield would weigh about half of a laminated glass windshield.   Comparing that to a 3500-lb overall car weight of, you’re not saving much in terms of fuel.
  • Cost:  You can buy a sheet of 1/4″ thick Lexan big enough for a windshield for about a hundred bucks.  BUT:  Lexan (polycarb in general) is extremely susceptible to scratches.  A cheap piece of basic polycarb will be scratched the first time a bit of sand or dirt gets under your windshield wiper.  Manufacturers get around this by putting hard coatings over the polycarb that make it more scratch resistant – but those coatings are expensive and ramp up the cost pretty rapidly.  You’re probably talking 2-3 times the cost of polycarb vs. glass, which is probably a couple hundred dollars for a windshield.  Does that offset the lighter weight?  Depends on the person buying the car.  Polycarb is becoming more common for things like headlight covers, but the only cars using a lot of Lexan are in the class of the Bugatti Veyron – which costs more than most of us make in a decade.
  • Maintenance:  Americans are not very good at taking care of their cars.  Most people don’t check their tire pressure, change the oil regularly, or even pay heed when the engine warning light comes on.  Manufacturers won’t implement  Lexan windshields until they require absolutely no additional TLC relative to glass.
  • Icing:  Interesting point.  The thermal conductivity (how well a material transmits heat) of Lexan is about a quarter that of glass.  That means Lexan is a much better insulator than glass – one attractive point for Lexan from the perspective of environmental aspects is that Lexan would enable you to use less heating and air conditioning.  The thermal conductivity of polycarb can be changed by using glass fillers – but that also changes the strength.  We argued a bit about  this one.  It would take a longer time for the inside of the car to get cold, but since Lexan is a bad thermal conductor, that might not impact the outside of the windshield very much.  If you were using the defroster, it would take a long time for heat on one side of the Lexan to get to the other side, and thus longer to defrost.

The thing might make Lexan a feature of road cars is (in my opinion), if electric vehicles really take off.  Saving ten or twenty pounds on a race car is huge; but it won’t make much of a difference on an internal combustion engine-driven vehicle.  For an electric vehicle, however, every pound counts in terms of extending the car’s range.  Since range limitations are one of the big barriers for EV acceptance, using Lexan to reduce the weight without compromising safety could make a real difference.

 

Secrets of the Gen-6 Car: Superstrength Windshields

 Car Parts, Greg Biffle, Lexan, Robin Pemberton, Tom Gideon, Windshields  5 Responses »
Feb 152013
 

A number of drivers have sounded a common refrain:  the CoT was engineered to be safe … and that’s why it was… well… sort of ugly and not very racy.  The Gen-6 car is a much better looking vehicle and (once the teams get a handle on the engineering) it should also give us a better show.  But don’t let its looks fool you:  NASCAR did not forgot safety in the Gen-6 car.

During a recent visit to the NASCAR R&D Center, Tom Gideon — NASCAR’s Safety Czar — gave me a tour through some of the changes you might not notice… unless something bad happens.  The drivers were briefed on these changes during Daytona testing last month and the reactions have been uniformly positive.

There is a line of Diet Coke cans sitting on the front edge of Tom’s desk.  They aren’t for drinking:  they’re there to become high-speed projectiles.

Why Diet Coke?

Because, Tom explained, if you splat regular Coke onto a car, you’ll spend the rest of the afternoon cleaning up the gooey, nasty mess produced by all that high-sucrose corn syrup.

The soda cans (actually, their less-fortunate cousins) were projected at car windshields.  NASCAR windshield are, of course, not made of glass.  They are made of a plastic called polycarbonate, which is better known by the trade name Lexan.   Lexan is clear, like glass, but it gives more easily and thus can take higher impacts without breaking.  In fact, the shark fins on the Gen-6 cars are made of Lexan, which makes them virtually invisible when the cars are on the track.

Lexan is also more expensive than glass, which is why most passenger cars still use glass in their windshields and windows.  Windshields are made from laminated safety glass.  Laminated means stacking up a bunch of layers of something and compressing them to make a single unit.   Plywood is a laminate of wood, which you can see if you look at the end grain.

Laminated autoglass sandwiches a very thin polymer film (polyvinyl butyral, a.k.a.  PVB) between two layers of glass. The layers are heated under pressure, chemically bonding the glass and the film.  The polymer film (in contrast to glass) is elastic – that’s the layer that allows the glass to absorb energy when something hits it.  This is the same idea as padding your dashboard – the padding extends the time of your collision and thus decreases the overall force.  The polymer film insert in your car’s windshield also absorbs UV rays from the sun, protecting the fabric, leather and plastic inside the car.  Most importantly, if laminated glass breaks, the polymer film holds the pieces together.  The side windows in your car are made from  a different kind of glass that is designed to shatter into a zillion tiny — and rounded — pieces if it breaks.  This allows a person to climb out a broken side window without restriction (or cuts).

NASCAR_brokenThe Diet Coke cans on Tom Gideon’s desk were representative of the types of projectiles that might land on a track.  If you think something as seemingly innocent as a soda can  can’t do any real damage, look at this blog I wrote awhile back when a Dallas TV station contacted me to verify the story of a woman who claimed a styrofoam drink cup thrown from a passing car had punched a hole through her windshield.  The hole is shown at right and, yes, a styrofoam drink cup traveling at high speed can break a windshield.

A Lexan windshield wouldn’t have broken, but the windshields in NASCAR racecars have to stand up to much more than soda cans.  They have to survive impacts of things like like wayward car parts.  Last year in Charlotte, a piece of brake rotor hit Greg Biffle’s windshield.  The brake rotor didn’t go through the windshield, but the impact did blow out the interior window braces so that they were hanging over the steering wheel and impeding Biffle’s ability to steer.

Five years ago, NASCAR was in reactive mode when it came to safety.  They weren’t anticipating problems, they were rushing to develop solutions when something happened on the track.  Things have changed:  NASCAR is sufficiently ahead of the game that they aren’t waiting for a catastrophe to happen before they start innovating.

One of the R&D Center’s new toys pieces of scientific equipment is a pneumatic cannon.  It uses compressed gas (just like a potato gun) to propel a projectile toward a windshield at about 50 mph.  In addition to the soda cans, NASCAR used solid metal cylinders to test the Gen-5 windshield using the new pneumatic cannon.  The old windshield wasn’t unsafe, but the R&D Center realized they could do better.

NASCAR_Gen6WindshieldLamination improves the strength and energy-absorbing ability of glass, and it does the same thing for Lexan. The Gen-5 windshield (shown at top left) was a little less than a quarter-inch thick.

The new window uses two pieces of Lexan, each half the thickness of the old window, with a 30 mil (that’s 30 one-thousandths of an inch) polymer film between them.  A really good-quality heavy duty trash bag is 4 mil thick, so the film in the new windshield is about seven or eight trash bags thick.  After heating and pressure treatment, you have a perfectly clear, superstrong windshield that is only thirty thousandths of an inch thicker than the old window.  Robin Pemberton told the media that the new windshield could withstand the equivalent of a connecting rod going 200 mph without breaking.

The pneumatic cannon resulted in some additional developments that I’ll detail in my next post.  Questions and comments are always welcome.

 

Dover: Why Concrete Races Differently than Asphalt

 Asphalt, Bristol Motor Speedway, Dover International Speedway, Mark Martin, Martinsville, Materials, Tracks  6 Responses »
Sep 282012
 

One of the questions you’ll hear drivers and crew chiefs asked a lot this weekend at Dover is how the concrete track affects the racing.  Here’s how:

Asphalt vs. Concrete

Concrete and asphalt are father and son.  They have in common what you and would call it “rocks”, but professionals call it “aggregate”.  Aggregate comes in a huge variety of types, depending on the materials from which the rocks are made, the quality of the material, the size of the rocks and the distribution of sizes of the rocks.

Concrete is an technically any mixture of rocks aggregate stuck together with a binder.  The type of binder determines the properties of the concrete and even the color.

Concrete is the oldest engineered construction material, dating back to the Roman Empire.   The reason only parts of the Roman Colosseum and the Pantheon are missing have more to do with humans than the failure of the materials.  Today’s concrete is more than ten times stronger than the version the Romans developed.

The most common binder in the concrete used in roads, parking lots and sidewalks is Portland cement.  Portland cement (and its close relatives) are mixtures of  limestone and clay, which are crushed to a powder and heated to over 2700 degrees Fahrenheit.  This is the form you buy it in.  To use is, you reconstitute the dry powder with water, and the individual grains form calcium-silicate-hydrate (C-S-H) bonds that make a very strong glue.

Asphalt is a type of concrete, that uses bitumen — tarry black stuff — to hold it all together.   A typical composition for asphalt is 80% aggregate, 15% binder and 5% air voids.  Bitumen comes from the heaviest components of crude oil, and has the consistency of molasses (which is why it has to be heated before being used).   Because bitumen derives from oil, the price of asphalt changes with the price of oil.

 

But Which is Better?

As with most “which is better”, the answer depends on what you what to use it for.  The primary difference between asphalt and concrete is the rigidity of the two materials and how they distribute the load over the base on which they are laid.   The more rigid the pavement, the more the load is distributed over the surface when something like a car move over it.

Asphalt, which is more flexible (relative to concrete), transmits higher, more concentrated loads to the base, as shown below.  I’ve drawn the stress distributions in red.  The concrete spreads out the stress over a larger area, while the asphalt transmits stress to a narrows area.  The narrower area and the same load means that the stress is more concentrated.

Because concrete is stronger, asphalt has to be thicker to get the same rigidity.  Asphalt does have an advantage, however, in that its flexibility allows it to expand and contract with temperature changes with less cracking.  Even so, concrete lasts 10-15 years longer than asphalt.

Asphalt is the traditional material for paved racing surfaces.  Only three Sprint Cup tracks feature concrete:  Dover, Martinsville and Bristol.  They have in common that they are all tracks of one mile or less with significant banking.  (OK – you may not view the 12 degree banking at Martinsville as ‘significant’, but those 12 degrees are the reason the corners are concrete while the rest of the track is asphalt.  The stress on the pavement in the corners necessitated replacing the original asphalt with concrete.)

Dover is one mile with 24-degree banking and Bristol is a little more than a half mile with 24-28 degree banking.  The steep banking and the tight curves make keeping asphalt in good racing condition a challenge.  Having concrete also gives a track a unique character – as well as the opportunity to have a really cool monster statue outside.

How Concrete Changes Racing

 

Grip Level

The grip level can be very different between asphalt and concrete, depending on a lot of factors.   Concrete is inherently more grainy, and its surface can be patterned to create more grip.  Drivers talk about bumps in asphalt as being large and wavy, while bumps in concrete they describe as  more vibrational.  Concrete usually has to be laid down in sections, which means you can have those bumps like you find between slabs on a sidewalk.  The picture at left shows the Google Earth view of Dover’s surface and you can see the individual slabs.

The grip on an asphalt  track depends  on the type of aggregate used, the degree of wear and the character of the bitumen.

For example, Atlanta has a very rough surface because its bitumen wears faster than the aggregate, as I’ve shown at right.   When an asphalt track is first laid down, the surface is very level.  As the bitumen wears away, the tops of the uppermost layer of aggregate are exposed.  The sharp edges of the aggregate are worn down by the tires rubbing against the rocks, but the aggregate sticking out provides a lot of grip.  Eventually, enough bitumen wears away that the aggregate starts coming out, which weakens how well the track holds together and necessitates a re-pave.

Concrete doesn’t wear as fast as asphalt and thus the grip level doesn’t change as much over long periods of time.

Light and Heat

Would you believe that the color of the track makes a big difference in how the track races?

Light comes in a range of wavelengths from smaller than billionths of a meter to larger than billions of meters long.  Our eyes detect a very, very small fraction of that electromagnetic radiation in the nanometer (billionth of a meter) range.  From red to violet, the wavelength ranges from about 800 nanometers to 400 nanometers.  The light from the Sun contains a wide range of wavelengths, including ultraviolet light (UV) (which is smaller wavelength than visibile light), all the colors of the rainbow, and lots of infrared  (IR) radiation.

Our eyes don’t detect the UV or IR light – we see the mixture of all the different colors of light together, which makes white.  Artificial light (like fluorescent) generates a different mixture of wavelengths, which is why it looks different than sunlight.

You see the colors of objects because all materials absorb some wavelengths (colors) of light and reflect others.  When light hits a red object, as I’ve shown at left, all colors except red are absorbed and what comes to your eyes is just the red light.

White surfaces reflect a wide spectrum of wavelengths and absorb very little of the spectrum.  The light that is incident on a white surface is reflected back to our eyes and the broad spectrum of wavelengths we see as ‘white’.  Black is the opposite:  black absorbs a lot of different wavelengths, so very little reflects back to our eyes and we get black.

 

In addition to the visible light, the spectrum from the sun includes the aforementioned ultraviolet  and infrared waves.  Infrared radiation has longer wavelengths than red light.  We don’t see it – we sense it as heat.  You’ll notice that the lamps they use to keep food warm always have a red glow:  they output some visible light, but they mostly output heat .  You will never see food being kept warm by blue light.

How is all this relevant to a racecar?

Put a piece of black paper and a piece of white paper in the Sun and feel their surfaces after a few hours.   The black paper absorbs a lot of the radiation from the Sun and gets very warm.  The white paper doesn’t absorb as much of the Sun’s energy (although it does absorb some), so it stays relatively cooler.  If you measure the temperature of a track over the course of a race, it can change by tens of degrees depending on the weather.

One effect of the changing temperature is how hot the tires get.  If the track is 60 degrees vs. 120 degrees Fahrenheit, that generates a very noticeable level of change in the grip.  But even more importantly, bitumen (the binder in asphalt) is a petroleum product.  As the temperature rises, oils in the bitumen get warmer and make the track more slippery.   Portland cement is crushed-up rocks which (when dry) are not slippery at all.

The end result is that, a concrete track doesn’t change over the course of a race nearly as much as an asphalt track.  Crew chiefs say that the track at Dover is easier to ‘keep up with’ because changes in temperature over the course of the race don’t change the racing surface as much with concrete as they do with asphalt tracks.

The Nature of Friction

There are two types of friction .  The first, called abrasive friction, is the one you learned about in school.  This is the type of friction between sandpaper on a wood block.  The second kind (which I never know about until I wrote The Physics of NASCAR) is adhesive friction, which is the molecular-level stickiness of the track combining with the molecular-level stickiness of the tires.  The heat generated by the tires makes the topmost layer of the track gooey.  The outermost layer of the tire also becomes gooey, resulting in an effect very much like chewing gum stuck on your shoe on a hot sidewalk.  The gooeyness of the track  bonds with the gooeyness of the tires for microseconds and resists forward motion.  That’s grip.

The nature of adhesive friction on asphalt is very different than on concrete because the two materials are so very different.  Concrete has much less adhesive friction.  This doesn’t change the grip level so much (because the abrasive frictions are different) – however, it does make a big difference in what happens when you lose grip. Think about sticking a weight to a piece of wood with gum.  The asphalt surface would be really sticky gum and the concrete surface would be dried up, not-very-sticky gum.  If you turn the wood so that the surface is vertical, the stickier gum is going to hold better.

In terms of a racecar, Mark Martin pointed out:

“… when you lose grip on a concrete surface, you feel like you just got cut loose from a rope. It’s amazing. It’s like losing half of your grip, rather than about 20 or 30 percent that you lose on asphalt.”

All the drivers’ intuitions that are developed on asphalt – which comprise the vast majority of NASCAR tracks – are thus challenged when they drive on concrete.

So there you have it – not necessarily better or worse, just different.

For those of you who have noticed the blog has been quiet the last two weeks, it’s because my older cat, Chaos, was very ill and finally passed away last Sunday. She was my race-watching buddy, although I have to admit that she usually fell asleep somewhere around lap 25 and woke up just in time to see the last 30 laps or so.

I miss her all the time, but I will especially miss her on raceday when she liked to compete with my computer for lap space.

Hey Larry Mac! Cars Do Not Speed Up in the Grass!

 Asphalt, Grass, Larry McReynolds, Tires  2 Responses »
Sep 122012
 

Ever had one of those things that you never noticed before, but when someone brings it to your attention, you notice it and it drives you crazy?  Frank Smith emailed me about an observation made by television commentators that was driving him nuts.  Now that he mentioned it, I keep hearing it and it’s driving me nuts, too.

Not to denigrate Larry Mac and the other television commentators.  I’ve learned a lot from Mr. McReynolds.   There’s a perfectly good physics explanation for why he (and others) keep telling us that cars speed up when they get into the grass on a racetrack.

Assuming that the driver has the presence of mind to take his foot off the gas, this is impossible.

Why Cars Slow Down

Let’s say you’ve got a car doing 100 mph coming onto the frontstretch.  The force of the engine pushes the car forward and the forces of friction push in the opposite direction as the car is moving.  Frictional forces include friction between the various parts in the motor and drivetrain, air resistance (which is  friction between the car surfaces and the air molecules) and friction between the tires and the track.  The force of the engine must be greater than or equal to the force of friction in order for the car to move to the right (as shown in my drawing).  If the force of the engine is just equal to the force of friction, the car will move at constant speed (no acceleration).  If the force of the engine is greater than the force of friction, the car will accelerate to the right.

Now let’s say the driver takes his foot off the brake and lets the car coast.  We’ve removed the force of the engine, so all we’re left with is the frictional force.  The frictional force causes the car to slow down (decelerate) until it comes to a stop.  This is just an example of Newton’s Second Law:  F=ma, or (in words)

The larger the net force, the larger the acceleration.  It’s a vector equation, so the direction of the acceleration is in the direction of the net force.

Bring on the Grass

Now let’s look at a case in which the driver lets up on the gas, the car travels on the frontstretch for a little while, then goes into the grass.  The frictional forces in the engine/drivetrain and the air resistance remain the same, regardless of what surface the car is traveling on.

It is impossible for the car to speed up unless there is a force pushing it to the right.

Our NASCAR  television commentators are not stupid.  They’re reporting what they see and it does look like the car starts to speed up when it transitions from asphalt to grass.

This is an example of relative motion.  Have you ever sat at a train track and focused only on the train going past in front of your car?  If you only look at the moving train, you can make it feel as though the train were standing still and you were moving in the opposite direction as the train.   That’s relative motion.  To you, the train is moving to the left at 50 mph.  To someone on the train, you’re moving to the right at 50 mph.

We’ve had this issue arise before during Carl Edwards’ restart penalty at the first Richmond race:  If the car that is supposed to start the race spins his tires, his lack of acceleration can make the car beside him look like it’s accelerating, even though it’s actually moving at constant speed.  I’ve got some animations at the link above to show you how it’s very easy for your eyes to be fooled.

We judge speed relative to what’s around it.  If you’re going 65 mph on the expressway and another car is going 60 mph, you’re going 5 mph faster than the other car.  If we just took your cars, without anything else surrounding you, it would look exactly the same to you as if you were going 200 mph and the other car was going 195 mph.  What gives you the ability to distinguish 200 mph from 65 mph are the stationary objects you pass.  In the Richmond case, you can’t just look at the 99 and the 2 cars and make a valid observation about their speeds – you have to look at their cars relative to something standing still, like the lines on the wall that indicate the restart lane.

The same thing is happening in our car-in-the-grass scenario.  The big difference between the two surfaces is the coefficient of friction between asphalt and grass.  The coefficient of friction between tires and asphalt is about 0.7-0.8.  The coefficient of friction between tires and grass is 0.35 – roughly half what it is when the car is on the asphalt.

The frictional force due to the tires is proportional to the coefficient of friction.  The grass has a lower coefficient of friction, so the frictional force decreases when the car travels into the grass.  The car doesn’t speed up – it slows down at a slower rate.  It looks like it speeds up because your brain is watching the car on the asphalt and expecting the car to keep decelerating the same way.  When the car moves onto the grass and the frictional force changes, the deceleration changes and it looks to us like the car is speeding up, even though it isn’t.

This all assumes, again, that the driver isn’t on the gas.  But if  you’re spinning out into the grass and your foot is pushing down on the accelerator, you’ve got much bigger problems than not understanding basic physics.

The Trouble with Pit Road Walls

 Collisions, Foam, Michigan International Speedway, Safety, Uncategorized  4 Responses »
Aug 192012
 

We saw a very scary incident during the Cup race Sunday when Mark Martin was T-boned by the edge of the pit road wall.  Luckily, the car hit the wall behind the driver’s seat — otherwise, that could have been very serious.  (The link has the video).

The ends of walls are probably the biggest safety problem NASCAR has right now.  The SAFER barriers have radically improved the ability of drivers to walk away from standard crashes, but there are still some vulnerable areas.

When a car comes to a stop by hitting a wall, it experiences some force.  The SAFER barriers spread that force out over a longer time, so the peak force is less.  The end of a wall poses a different types of problem.  Pressure is the force the car experiences divided by the area over which the force is applied. When a car hits a wall broadside, the force is spread out over the entire area of the car’s side.  The end of the wall presents a very small area.  Given the same force, the narrow end of the wall creates a very large pressure – which is why we saw the wall intrude a good foot into the car.

The doors on both sides of the car are heavily reinforced with horizontal bars.  The topmost bar sticks out further beyond the next bar down and so on, with the idea being that when the car hits the wall, the bars will successively give.  Again, the idea is to spread out the force over time.  There is also a sheet of Tegris (the material used in the splitter) in the door to protect against cockpit intrusions by something narrow enough to fit between the bars.  The material also gives you an additional layer of reinforcement.

There are a lot of edges on the racetrack.  The end of Pit Road is probably the most likely to create a hazard, but anywhere there is an opening in the wall, there is an edge.

If the edges of walls are known to be dangerous, why haven’t they been fixed?  A couple reasons:

1.  The chances of having a serious accident involving the end of a wall are small.  NASCAR has to balance the cost of developing, testing and installing new safety devices with the likelihood that they will be called upon during a race.  They’ve tried to anticipate the locations on the track where accidents happen the most and protect those first.  Drivers hit inside walls far less frequently than they hit outside walls, so outside walls were the first to be addressed.  Over the years, we’ve seen drivers hitting the inside walls at particular tracks and they’ve installed SAFER barriers there.  Even Mark Martin said (in an article by Bob Pockrass) that this was a ‘freak accident’ and he wasn’t sure whether it was possible to protect against these rare occurrences.

2.  This is a tough problem to solve.  Think about the constraints:  The safety device has to absorb a hit and not scatter material all over the track.  It has to be able to be ready for a second hit almost immediately, so any solution that requires repair of the wall when there is a routine hit is going to be nixed.  It can’t interfere with drivers getting where they need to go, or with emergency vehicles having access to the track.  It has to be easy to retrofit and can’t cost too much.

A couple people suggested rounding the wall, so that it’s a semicircular profile instead of flat. Unfortunately, that decreases the area of contact.  Making it a right angle so that there isn’t a wall end is a possibility, as long as you keep the corners rounded; however, a large round surface will either decrease the opening size or decrease the adjacent pit box.

One suggestion a lot of people had was to make movable gates that would cover the openings until they were needed.  The big problem becomes:  what if the car manages to hit the hinge (or other opening mechanism) and jams the gate so that you can’t open it?  If we’re talking about one of the openings where you see emergency vehicles and people waiting so that they can respond quickly, you are delaying the time it takes for a potentially injured driver to receive aid.  I know that research is ongoing as to how to adapt SAFER barriers to these openings.  If they had a feasible solution, we’d be hearing about it already.

Another interesting solution NASCAR is considering was discussed in a Popular Science article from 2006:  It’s a very special foam that can deform to 1/7th its volume during an impact, and then return to its original configuration within minutes.  Called FlexAll, it was developed by Battelle and is currently being adapted for applications in the military and on the highways you and I drive — where the ends of safety barriers represent a major safety hazard as well.  The problem with FlexAll is the cost:  I’m seeing numbers on the order of $30,000 per wall end.  That makes it suitable for the military, but difficult to justify for highways… or perhaps NASCAR.

 

 

 

 

 

How Many “Cookie Cutter Tracks” are There?

 Asphalt, Atlanta Motor Speedway, Kansas, Las Vegas Motor Speedway, Materials, Weather  19 Responses »
Apr 132012
 
MSM_TrackTaxonomy

One of those phrases you tend to pick up as a NASCAR fan without thinking is “cookie cutter track”.  That’s the accusation commonly directed at the one-and-a-half mile tracks (like Texas Motor Speedway, which we’re visiting this week).  The complaint is that these tracks are so identical that it’s almost not worth bothering to watch.  But are they really identical?

My disclosure, before we start:  I am a fan of short tracks.  I like being able to see the whole race and I like watching drivers try to pass each other.  So I started out with a bit of a bias toward the intermediate tracks — Atlanta, Charlotte, Chicagoland, Homestead, Kansas, Kentucky, Las Vegas and Texas — myself.  But there is nothing a scientist likes more than sitting down with a pile of data and trying to make sense of it, so (armed with Google Maps, a load of data from racing-reference.info, and the Excel file in which I’ve collected track parameters), I dug in to see for myself how similar these tracks are.  After a couple false starts, I decided to try to develop a taxonomy.  Taxonomy is a Greek word meaning a ‘method of arrangement’.  My 1.5-mile track taxonomy is shown below.

The Differences

 

We start at the top with 1.5-mile tracks and a missing hyphen.  The first distinction is the track shape because not all ovals are equal.  Homestead actually is oval shaped. The other tracks have one of two shapes:  the D-shaped oval and the quad-oval.  Atlanta, Charlotte and Texas fall in the latter category, with Chicagoland, Kansas, Kentucky and Las Vegas in the D-shaped oval Camp.  The difference is evident in the figure below.  The photo in the upper left is Kansas (our representative of the D-shaped oval) and in the lower right is Atlanta.  The difference is subtle:  the D-shaped oval is more of a triangle while the quad-oval has a double dogleg.  I’ve driven at Texas and you can see two distinct angles in the wall as you approach them.  The D-shaped oval looks like someone grabbed an oval in the middle of one of its long sides and pulled on it.  The D-shaped oval is not specific to intermediate tracks.  California, Michigan and Richmond are D-shaped ovals, too.

Within the D-Shaped oval category, no two tracks have the same corner banking.  Kentucky is 14°, Kansas is 15°, and Chicagoland is 18°.  Las Vegas is in a class of its own, as it has progressive banking that runs up to 20°.  (Homestead also has progressive banking.)  The quad-ovals all have the same corner banking (24°), so we can’t differentiate that class any further in this level.  These three tracks really are very similar.  To make any distinction, we have to look at things like the backstretch length.  Charlotte and Texas have approximately the same backstretch length (~1350 ft), while Atlanta has an 1800 ft. backstretch.  Although Charlotte and Texas have similar frontstretch lengths, they do differ by 300 feet, so if you were really looking for an excuse to put them in separate categories, that’s about the most obvious division.

The tracks definitely race differently.  The pole speeds on these track vary from an average of 174.8 mph to 193.0 mph (values given are averages over the last four races run).  Out of curiosity, I plotted the pole speeds for the tracks as a function of different variables and finally found the following relationship.  Note that there is no data for Kentucky since the first Cup race was run just last year and starting order was determined by owner points.  The pole speed definitely depends on corner banking, which makes perfect sense.  Banking helps the cars turn by providing some of the required centripetal force.  More banking means more speed given that the track length is constant.

There is still, however, a 4.2 mph difference on the three quad-oval tracks, which suggests that there are other factors to be considered beyond shape.

The track surface makes a huge difference in speed.  Asphalt is a composite of aggregate (stones) and binder (bitumen).  A host of variables such as the size distribution of the aggregate, the chemical makeup of the asphalt and the conditions under which the asphalt is laid down have a huge impact on the track’s grip and how it wears.  The track is changed constantly by the weathering and no two tracks experience the same combination of factors.  The diagram at left shows how the aggregate (grey) and asphalt (black) wear over time.  More of the aggregate is exposed with time and sharp edges get worn down.  The track also changes in response to temperature and, again, different tracks will change in different ways.  Atlanta, for example, is known as a tire-eating track because its rough surface is very hard on rubber.

The same issue arises over the course of a single race.  When you hear a driver or crew chief talk about “chasing the race track”, it means that the setup they had that worked so well at the start of the race didn’t work as well during the race.  A track changes significantly over the course of a race:  it heats up due to friction between tires and the track, plus it may heat or cool due to the way the Sun hits the track (or portions of the track) or even just because a race goes into evening and the overall temperature changes.  Different weather means different racing.

In addition to the small-scale roughness discussed above, some tracks have unique, larger perturbations in the track surface.  Texas has a major bump between Turns 1 and 2 that was caused by the track settling over the entrance to the infield.  In 2007, they drilled a bunch of holes in the area and injected a structural urethane to try to fix the giant distraction.  They made it better, but you have still heard drivers all week talking about “the bump”.  This isn’t unique to Texas:  Charlotte has a big bump entering Turn 1.   Those bumps pose major challenges for setting up the suspension.  The ideal position for the splitter is as close to the track as possible – but if there’s a big bump, you have to make sure that the splitter doesn’t hit the bump.  There are also issues like seams and patches, where the texture or type of asphalt changes, that challenge drivers.

The Similarities

This is not to say that these tracks don’t share some similarities.  They are all fairly wide (50-60 feet) compared to the smaller tracks.  The most important similarity is less a function of the track and more a function of the car.  The current version of the NASCAR stockcar is highly aerodependent on one-and-a-half-mile tracks.  Aerodynamic forces go like the speed squared, so these high-speed tracks have three-to-four times more emphasis on aero than short tracks.

A car depends on air rushing over it to push its tires into the track.  Turbulent air – like you find in the wake of a high-speed car – doesn’t provide as much downforce as laminar (straight-flowing) air.  This is why drivers value “clean air”.   If you’re the first car in line, you don’t have turbulent air from the car in front of you because there is no car in front of you.  Another feature of 1.5-mile tracks is that, because it is larger, you don’t run up on lap traffic as much as you do at a short track, and there’s plenty of room for a lapped car to get out of the way.  At these tracks, being out front gives you have a huge advantage. That leads to a car that can easily put quite a distance between itself and the rest of the field.

The ‘aeropush’ effect happens when you get too close to the car in front of you.  The air coming off its rear end is turbulent and doesn’t give you as much downforce as laminar flow would provide.  It’s like running over ice:  the only thing you can do is slow down.  The aero-push makes it really hard to pass because you have to get close to the car in front of you in order to pass it.  If the cars weren’t so dependent on aerodynamic downforce, then losing a fraction of that downforce wouldn’t affect them a significantly.

 The Conclusion

I’d say there are actually only three ‘cookie cutter’ tracks:  Texas and Charlotte are identical twins that get their hair cut differently and refuse to wear identical clothing.  Atlanta is a fraternal twin to Texas and Charlotte.  Lumping the D-Shaped Ovals in with these tracks, however, is unfair.  The issues that many race fans have with racing at these tracks requires changing the car rather than changing the track.

 

 

 

 

 

Atlanta is known as a really rough track that eats tires.

Potholes in Atlanta?

 Asphalt, Atlanta Motor Speedway, Materials, Tracks, Uncategorized  2 Responses »
Sep 042010
 

There was an interesting comment during practice this morning from Jeff Hammond (channeling Darryl Waltrip) about dark spots on the track, which indicate (he said) the cars were “knocking off” asphalt.  I received a number of questions about this and whether it might indicate that Atlanta could have the same problems we saw in the season opener at Daytona?

Atlanta was the first race at which I followed around the 19 team for my book.  It’s been about 13 years since the track was last paved.  Josh Browne took me out to the track and showed me how rough the track surface is.  Atlanta is one of the fastest and roughest tracks on the circuit and part of the reason for that is the composition of the track.

Most NASCAR tracks are made from asphalt – only a few are concrete.  Asphalt is a combination of aggregate (small rocks) and bitumen, the tarry black stuff that holds it all together.  Asphalt is type of composite – a material made of two things, but having properties superior to either.    Bitumen comes from the heaviest components of crude oil, with a consistency of molasses (which is why it has to be heated before being applied to anything).  We often refer to the viscous black stuff as ‘asphalt’, which irritates the heck out of the people who deal with roads for a living.

About 95 percent of the paved roads in the US have asphalt surfaces.  Most airports use asphalt for their runways because it stands up to heavy loads well.  Aggregate makes up 80-95% percent of the volume, with the remainder being the binder and air voids.  The picture at right shows voids, which were either there when the surface was originally laid, or represent places where there used to be a piece (or pieces) of aggregate.  I took the pictures, incidentally, at the Smithsonian Museum of Natural History, where they take the difference between asphalt and concrete very seriously.

The size of the aggregate used varies, depending on the requirements for the surface use and when the surface was laid.  As we’ve learned more and more about the long-term behavior of composite materials like asphalt, recipes have evolved.  Binders nowadays add some polymeric molecules that increase the adhesion of the rocks to each other, and the resiliency of the road.

As the track weathers — which means gets hotter and colder, wetter and dryer, loaded by racecars and sitting idle — it changes.

Some liquids have a high vapor pressure, which means they evaporate easily.  Acetone (like in nail polish remover) or toluene (paint thinner) disappear if you leave them out of their container because the molecules of the solvent gradually diffuse into the air.  Water evaporates much more slowly than volatile solvents like the ones I mentioned above.  Believe it or not, some of the oils in the bitumen also evaporate over time.  This happens on a time scale of months to years and it happens faster when the track gets warm.  The surface oils go first and the oils deeper in the surface start making their way up to the surface.  The force of the cars running along the track also wears the asphalt.  Binder, as well as small bits of aggregate, come loose during a race  and form ‘marbles’

The diagram at left illustrates how a track might change over time.  The top picture shows the track as it was laid down, with the second and third pictures showing later times.  The asphalt is gradually worn down.  You can imagine a gentle wear, like on a surface street, where speeds are rarely high and the surface doesn’t change temperature much.   You also can imagine that race cars might be a little tougher on the road.  The wear depends entirely on the type of bitumen (how sticky is it, how does it respond to heat) and the mix of rocks used for the aggregate.  The middle picture shows that some of the binder has worn down, exposing more of the rocks and making a bumpier surface.  The last picture shows the situation after a lot of the bitumen has worn off.  A lot of the rocks have come loose or are ready to do so.  The longer it’s been since a track has been resurfaced, the faster it is likely to wear.  The teams aren’t making it any easier on the track, either.  Aerodynamically, you want the splitter as low to the track as possible.  Every time a splitter bangs on the track, it knocks around the surface.  Cars on low tire pressure are lower to the ground and they can bottom out, scraping metal against the asphalt and essentially shaving the track.  Even the jackposts can bang on the track and knock the asphalt surface.

Another major contributor to wear is freezing and thawing of water.  Water is one of the few liquids that expands when it gets colder.  Since asphalt is porous, water can get down into the voids between the rocks, freeze, and push outward, creating internal stresses.  Even when the water liquefies again, there is some residual damage.

You can see seams on the Atlanta track, almost line line markers.  When the asphalt is laid, it is laid by a machine with a finite width.  The lanes being laid down right now at Daytona are 21 feet wide.  The asphalt isn’t a continuous layer all the way across the track.  One of the reasons asphalt is so strong is that each piece of rock interlocks with the rocks surrounding it.  Not only is the binder keeping the rock together, the rock is interlocked like the old rock walls that dot the Northeastern farms, which makes it much stronger.  The tamping down of the rock (pushing very hard on it while vibrating) is designed to rotate the rocks so that they pack as closely as possible.   That’s not the case at the seams, which is why those areas are the ones where you often see cracks first.  As I’ve shown at right, even though the bitumen is essentially continuous (I left a thin line for emphasis), there is a big difference in how the rocks interlock on either side of the seam.

The Daytona pothole was a big deal because a big ol’ chunk of asphalt came off all at once, leaving a big hole.  It is much easier to patch a small area than a large one. Putting sealer over the seams is a precaution, as those areas are inherently weaker than the others.  There’s a big difference between gradual wearing and catastrophic failure.

The problem, of course, is that asphalt is entirely opaque.  Even though the track sends people out to inspect the surface before the race, after practices, etc., all they can see is the track surface.  They can’t see whether there’s a crack just below the surface waiting for a car to come around, bottom out and take a piece of the road surface with it.  Asphalt surfaces can sustain some amount of cracks, but there’s a point at which the structural integrity is compromised.  Reaching that point is like walking off a cliff you didn’t know was there:  there’s no warning and everyone will ask you afterward why you didn’t anticipate it.

This is one reason drivers are always so tentative about a track when a repaving is announced.  The chances that the track will change are about 100 percent.

The Federal Highway Administration statistics tell us that there are 2,734,102 miles of paved public roads in the U.S.  There are scientists and engineers who study how different types of roads stand up to traffic and weather.  Racing doesn’t have that advantage.  Add up the total miles of NASCAR racetracks in the country and even if you count the local tracks, there are only a few hundred miles of pavement.  Tracks are used irregularly, by very different types of vehicles.  We don’t have a comprehensive database of how different types of asphalt age.  I can’t imagine we even have two tracks in the country with identical banking, identical types of asphalt and identical weather.  The folks who work at racetracks have to be ready for virtually anything.

Aside

Come see me and a bunch of my friends on October 23rd and 24th in Washington DC.  We’re going to be part of the very first USA Science and Engineering Festival.  The National Mall (and surrounding areas) will be invaded by hundreds of scientists, engineers and educators.  You can hear talks from the rock-guitar-playing Director of the National Institutes of Health, astronauts, inventors and more.  Our booth (on the science of motorsports, of course!) will be located at 13th and Pennsylvania Avenue.  The Office Depot show car will be there, along with opportunities for you to learn firsthand how tires can handle tons of force, how a 3,600-lb racecar can put six and a half tons of force on a set of tires, and why physicists don’t believe in centrifugal force.  In addition, you can learn about green racing and how what happens on the track might eventually end up in your own passenger car.  More information can be found at http://www.usasciencefestival.org/.  Stop by and say hello!

Daytona Potholes: Avoidable or Not?

 Asphalt, Daytona, Materials, Tracks  1 Response »
Feb 182010
 

I wake up in the morning listening to our local NPR station. A couple weeks ago, they said that the George Bush Turnpike was closed due to “a buckle in the road”. My husband commented that he knew Texans had big belt buckles, but he didn’t think they were big enough to shut a whole side of the tollway.

Well, the buckle they were talking about was actually three feet high and spanned two lanes. Apparently, the heavy rains we had received created a lot of pressure in the adjoining retaining wall and that pressure pushed the pavement until it buckled and formed our own little miniature mountain range right there in Carrollton.

The problems at Daytona last Sunday weren’t quite of that magnitude (the pothole was about 9″ x 15″ and only 2″ high, but that tiny pothole impacted a lot more people. Including me, who had assured my husband that the race certainly would be over by five as he planned Valentine’s dinner. What happened and how could it have been prevented?

(photo Bill Friel)

Let’s start with thermal expansion. If you’ve ever had a lid stuck on a jar, or a ring stuck on your finger, you may have tried running the jar or the ring under hot water. The metal jar lid would expand faster than the glass jar, thus loosening the seal and allowing you to remove the stubborn lid. That’s because different materials expand at different rates. Metals expand faster than glass and fingers. (The water also provides some lubrication and in the case of jars, may dissolve anything sticky that might be inbetween the threads.)

Most things expand when heated and contract when cooled. Not water. This is good and bad. On the good side, ice is less dense than water, which means that ice can float on top of a pond while warmer, denser water goes to the bottom. The fish and anything else that wants to survive also goes to the bottom. On the bad side — as you know if you’ve ever left a bottle of soda or juice in your car overnight when it got really cold — water expanding at the wrong time can be a mess.

Water freezing and thawing can wreak havoc in other places. Putting in lawn edging in the North is an exercise in futility because the freeze/thaw cycles push the edging up so that, by April, it’s lying on the ground.

The word ‘cycles’ here is important. Most materials are designed to handle constant loads. A car rolling along a flat surface exerts about the same force everywhere along the surface. When you subject a material to repeated cycles of pulling and pushing on it, eventually, it breaks. You can bend a paper clip back and forth a couple of times, but it gets harder and harder to do, and then finally breaks. Each time you bend the paper clip, you make a little change in its microstructure. It’s like a game of pick-up sticks (or Kerplunk). Everything is fine up to a point, but when you push just a little too far, the whole thing comes down.

Normal temperature changes outside make most things expand and contact. There are joints in concrete sidewalks, for example, to allow for this expansion. Otherwise, two slabs of concrete would start pushing against each other and you’d have your own miniature version of plate tectonics.

Asphalt is made up of two components: aggregate (small pieces of rocks) and binder. Go get a bunch of rocks roughly 1/2 inch in diameter and put them in a jar. Try to pack them as closely as possible. It’s not easy to do, and if you don’t believe me, fill the jar up with water, then measure how much water you got in there.

The rocks are mixed with a liquid binder to hold it together, but in the end, asphalt looks like a sponge: rocks held together by binder, with a little bit of air space inbetween. A typical composition for asphalt might be 80% rock, 15% binder and 5% air voids. Here’s a picture from “The Idiot’s Guide to Highway Paving” showing some asphalt close up.

porous asphalt

You want some porosity in the asphalt. Porosity helps asphalt absorb water. A completely smooth, impervious surface would take a very long time to dry and would be more prone to hydroplaning than a rough surface.

The pores, however, cause problems, too. When water gets between stones and freezes, it exerts stress on the asphalt. Not a lot of stress, but enough cycles of stress will eventually produce weak spots and finally cracks. Once a crack is started, it’s very hard to stop (just like runs in nylons) and everytime a car goes over it, the crack gets wet. The weather in Florida was abnormally wet and cold the last few months. Don’t forget that Daytona was literally underwater last summer.

“Well, why didn’t they take that possibility into account?”, some of you are asking. If there is one thing we ought to be teaching in school science, it is that science never has absolute solutions. You can only increase downforce if you’re willing to pay a price in terms of drag or engine heating.

Likewise, if you engineered a track that was totally impervious to freezing and thawing, it wouldn’t drain well and would take a long time to dry when wet. Florida is much more likely to have rain and a need for lots of track drying than it is to have freezing. No track design is perfect. Although asphalt has been in use for many years (the Sumerians used it way back in 3000 B.C. as an adhesive on statues), we don’t have a lot of data on how highly banked asphalt racetracks that see speeds of 200 mph behave. There are really only two superspeedways, both constructed 1959-1960 and you can tell from the racing that they have very different characteristics, despite their apparent similarities.

Asphalt is not an easy material to work with, either. You start with crude oil, remove everything that seems useful (gasoline, diesel, oil, paraffin, etc.) and the sticky, goopy mess left over is used to make binder. You’ve probably seen (and/or smelled) asphalt machines puffing smoke near highway construction sites. The binder softens when it is warm and hardens when cool. Asphalt is usually laid down around 275-300 degrees Fahrenheit and gradually cools to a solid.

Liquid asphalt patches often consists of asphalt binder in a solvent — the same way pigment molecules are suspended in a solvent to make paint. You apply the liquid and wait for the solvent to evaporate, leaving behind a solid. The problem is that evaporation usually takes a long time. A re-surfaced asphalt driveway usually needs a day or two before it’s ready to be used. Heating will quicken the process, which is why the track workers were using a blowtorch on the patched area. Of course, the area that had the problem was the one part of the track that wasn’t in the Sun and thus was colder than everywhere else!

Eventually, they literally turned to Bondo. (My first car was a ’69 Buick LeSabre, so I know all about Bondo!) Bondo is a two-part putty that cures via a chemical reaction that is significantly less sensitive to temperature than asphalt patches. Of course, Bondo won’t stick as well to asphalt as asphalt sticks to asphalt, so Bondo is not the ideal solution. There’s that tradeoff again: you can make a fast repair that doesn’t last very long, or a slow repair that lasts longer. With a race in progress and FOX rapidly reaching the point where they were ready to interview drivers’ dogs because everyone else had already been interviewed, any repair that would get us to the end of the race was the right one.

Repaving is estimated at about $20 million dollars, and there’s no guarantee that (if it had been done between February and July ’09), the torrential rains of summer ’09 and the cool weather wouldn’t have caused problems. The next repave is tentatively scheduled for February 2012. Repaving can totally change the character of a track and not always for the better. They have plenty of time to patch the track between now and July (although there are other events scheduled for the track). An in-depth evaluation by an engineering company is in process. Whether patching will be sufficient or a total re-paving is necessary will be determined by the results of that evaluation. And while the folks doing the evaluation are some of the best in the business, the nature of the world is that there are no guarantees. The only Law of Nature that is certain is Murphy’s Law.