Keeping Racecars on the Racetrack

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

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

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

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

Aerodynamic Takeoff

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

BernoulliCar

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

Gen6_RoofandHoodFlaps

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

Mechanical Take-Off

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

CG_Torques

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

Talladega

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

The Fix(es)

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

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

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

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

About Newman’s Comments

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

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

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

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

Here’s an older video about aerodynamics and lift:

Why You Can’t Judge How Dark it is on Television

 Light, Optics, Talladega, Weather  2 Responses »
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.

Iris_Fstops

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.

Why Drying Tracks Takes So Long

 Air Titan, Talladega, Temperature, Uncategorized, Weather  1 Response »
May 042013
 

This is a major revision of a post that originally appeared on the now-defunct stockcarscience.com 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.

Why Turning is Hard

 Bristol Motor Speedway, Centripetal Acceleration, Centripetal Force, Richmond, Talladega  No Responses »
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.

EQ_CentripetalForceWords

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
(ft)		 Speed
(mph)		 Turning Force
(lbs)		 G’s
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

“G”

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.

G-Force

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.

Degrees of Difference: How is Martinsville like Fontana?

 Auto Club Speedway of California, Martinsville, math, Talladega, Texas Motor Speedway, Trigonometry  1 Response »
Oct 252012
 
DLPTXTrackBanking

I love getting questions from readers because I always worry that the geeky stuff I find interesting is only interesting to me.  I love it even more when they not only give me a question, they also supply part of the answer!  This one has to do with the degrees of difference between Martinsville and Fontana.

Michael J. Clark asked a really good question about Martinsville and Fontana:

Why does Fontana (banking in the turns is 14 degrees) seem to have such higher banking than Martinsville (banking in the turns is 12 degrees)?  I would think the 2 degrees more that Fontana has wouldn’t look so dramatically different than Martinsville, but it really does.  I’m guessing it has to do with the fact that Fontana’s turns are about 10 car-widths wide (my estimate) compared to the turns at Martinsville, which seem to be about four car-widths wide.

Great question and another example (like race cars seemingly speeding up when spinning into the grass) of how our perceptions are often subjective.

We always talk about Martinsville being a “flat track”, which is sort of unfair.  It’s flat compared to Talladega and Daytona, but there are still twelve degrees of banking in the turns.  Nothing like a little trigonometry at the racetrack – what does twelve degrees look like?  Let’s start with some definitions so we’re all talking about the same things.

Track width is measured across the track surface and forms the hypotenuse of a right triangle.

Any right triangle can be described by the lengths of any two sides, or the length of one side and one angle.  Remember SOHCAHTOA? You can (finally!!) use your trig to reverse engineer the racetrack.

One degree isn’t really all that large.  A banking angle of one degree means that in order to get a rise of one foot, you need to have a run of about 57 feet.  One degree isn’t very much, as shown in my figure below.

The top picture shows what a banking angle of one degree would look like, with the rise of 1 foot and the run of 57 feet.

The bottom picture is a scale drawing of Martinsville Speedway, which has a track width of about 55 feet (although I think it is a little narrower in the corners).  The banking angle is variously given as 11 degrees or 12 degrees.  I’m using 12 degrees here because that’s what the official NASCAR site says.  Given the hypotenuse (track width) and the banking angle, I can back calculate to show that the rise is about 11.4 feet and the run is about 54 feet.

Now to Michael’s question.  The diagram below shows scale drawings of the banking at Martinsville and California and (just for comparison) Talladega.  I’m using the best numbers I can find on the web.  If someone has more accurate numbers, please let me know.  Kudos, by the way, to Talladega for having one of the best webpages of track data.

Michael has great instincts – the track at Fontana is two-to-three car lengths wider than Martinsville.  This means that the rise is seven feet (one Brad Daugherty) higher than Martinsville at the edge of the track. That increase in rise makes the banking look steeper because you’re looking up a greater distance.

(You always hear that Talladega is five stories tall.  I’m not sure what they’re counting in that calculation because I get 26 feet, which is pretty far short of five stories unless you have very short stories.)

In addition to the greater width, you also have to remember that there’s a huge difference in overall scale.  Martinsville was the second track I visited while writing The Physics of NASCAR – the first was Atlanta.  Martinsville was the track that made me love short tracks.  You get up close to the action and even though they’re not going 200 mph, when you’re that close to them going 100 mph, it seems really fast.  Short tracks are a great challenge to the crew chief (and the driver) because suspension movement is so much more important than aerodynamics.  And, of course, tempers seem to be proportional to the track length of the track:  at Martinsville, they are both really short.

But you have to realize just how much smaller Martinsville is than the California track.  The straights at Martinsville are 800 feet, while the backstretch at Fontana is 2500 feet.  Martinsville is .524 miles, which is 2777 feet.  If you unrolled the Martinsville track, you could just about fit the entire thing on Fontana’s backstretch.  The picture below is my attempt to make a to-scale drawing of the two tracks.  The banking at Fontana looks huge compared to the banking at Martinsville not only because the track is wider at Fontana, but also because the track is simply bigger.  When you look out into the turns, you simply see a lot more asphalt.

Side note:  The featured picture in the post at the top shows me trying to stand up on the 24-degree banking at Texas Motor Speedway, just to give you an idea of how steep 24 degrees actually is.  This was while we were shooting the Science of Speed video series.

So that’s the difference between the tracks at Martinsville and Fontana.  I’m told there is absolutely no comparison between their hot dogs.

Thanks for the question, Michael!  Questions (and suggestions for the Sirius radio “NASCAR Mythbusters” segment) are always welcome.  Click on the ‘contact’ tab above to send me an email.

I’m heading out to Joliet, IL to give my  Science of Speed talk at Joliet Junior College Friday October 26th at 7:00 p.m.  More information on how to get there can be found on my appearances page.  My talk is aimed at the average NASCAR fan and focuses on why it’s a lot harder to drive fast than most of us think.  Most people leave the talk with even more respect for what professional racecar drivers do.  I promise no pop quizzes, so please come on out and meet me!

 

 

Eliminating Restrictor Plates?

 Daytona, Engines, rear-end gear, Restrictor Plates, Talladega, transmission  7 Responses »
Oct 052012
 

Every return to a restrictor plate track brings suggestions about how we might eliminate the restrictor plate.  Restrictor plates serve the very necessary function of limiting car speeds at Daytona and Talladega so that the cars stay on the ground.  The negative is that they remove throttle response.   One suggestion from some readers that I hadn’t heard of before suggested that NASCAR could just change the rear-end gearing parameters to shift the power curve and reduce horsepower that way. Will that work?

The amount of horsepower an engine make depends on the rotation rate of the engine.  The faster the engine runs, the more combustion events and the more power generated.  This works up to a point, because if you rotate the engine really fast, you start having problems getting enough air into the engine and the power goes down.  The graph below is for a typical unrestricted engine that makes its maximum horsepower around 9300 rpm.

In order to cut the horsepower back to what you’d need to run safely on a plate track, you would need to restrict the engine to run at about 450 hp – which would mean the engine would have to rotate at about 4500 rpm.

Looking at the curve above, it’s evident that the engine is designed to run at its peak horsepower.  What dictates that curve?  Cylinder displacement, engine configuration, head configuration, etc.  But mostly  NASCAR determines the curve because of the rear end gear rules.

NASCAR engines run up to about 10,000 rpm (revolutions per minute).  Rpm is a unit that measures how fast something rotates. It’s like miles per hour, but miles per hour corresponds to a linear motion rather than a rotational motion. The minute hand on your clock, for example, makes one revolution every hour. The seconds hand makes one revolution per minute.

The circumference of a typical tires is around 88 inches.   Every time the tire rotates once, the car moves 88 inches, so 1 tire rpm = 88 inches per minute.  You can convert this into miles per hour.  Since I chose a nice round number like 88  inches for the tire circumference, it works out to a really simple equivalence:  1 tire rpm = 1/12th of a mile per hour.  This means that if you want to go 200 mph, the tires have to rotate at 2400 revolutions per minute.

The power curve above shows that the engine makes the most horsepower at 9300 rpm.  This produces a problem:  the engine is driving the car at 9300 rpm and the wheels are rotating at 2400 rpm.  That’s why you have a transmission and a rear-end gear, as illustrated at right.  The diagram shows the gear ratios for a Borg Warner MM6 manual transmission and a GU6 3.42 rear-end gear, as might be found in a Corvette.  Note that NASCAR cars are not allowed to use any gear that increase the rotation rate between the engine and the wheels.  No 5th or 6th gear, either.  1:1 is the best you are allowed — which means that the rotation rate coming out of the transmission is the same as the engine rotation rate.

At maximum speed, the transmission is using a 1:1 gear, so the only reduction occurs at the rear end gear.  A 4:1 gear means that one gear makes four rotations for every one rotation the other gear makes.  If the engine is rotating at 10,000 rpm, and it passes through a 4:1 rear-end gear, you have 2500 rpm at the tires (which is 208 mph).

The whole point of this discussion is to keep the cars at lower speeds so they stay on the ground.  Let’s say you want to limit the cars to 190 mph – that requires the wheel rotate at 2280 rpm.  We don’t want more than 450 hp, so the rear-end gear has to take the rotation rate from 4500 rpm to 2280 rpm, which means 4500/2280 = 1.97, so you need essentially a 2:1 rear-end gear.  (Just for comparison, a typical rear-end gear is 3.3-3.9, depending on the track.)

So it is possible to gear the car down so that it simply doesn’t produce as much horsepower.  It is a better solution than what we currently do?

Restrictor plates work by reducing the air coming into the engine, which means you can give the engine less gas and thus you produce less horsepower.  Gearing down would reducing the horsepower by making the engine run in a much-less efficient range.

One consequence of a lower rpm is that you would have really back problems with knocking.  Knocking happens when the air-fuel mixture auto-ignites (ignites before the spark plug fires).  Knocking is much more likely at low engine speeds because the the combustion happens so much more slowly than in a fast-running engine.

Another consequence is that my engine design friends tell me that they can probably tweak an engine, within the rules, to produce more horsepower at 4500 rpm such that NASCAR would have to further change the gear and run the engines at 3500-4000 rpm, which exacerbates the knocking problem.

I wondered whether taking the engine speed down might increase throttle response, but none of the experts I spoke with thought that it would.  The problem, they say, is that as long as cars are running at full out power, aerodynamics will dominate plate tracks.  You’d have to decrease downforce and increase drag to really make a difference.

Finally, there’s an aesthetics issue.  The sound of an engine changes with its frequency.  If you went to Indy and we blindfolded you and asked you tell us whether the car on the track was a NASCAR racecar or an Indy car, it would be easy:  Indy (and F1) cars sound like mosquitoes.  They run at about twice the speed of a NASCAR engine.  If you forced a NASCAR engine to race at 3500-4500 rpm, the car would sound like it was in pain – you’d get a low moan instead of the engine sound associated with 9000 rpm that we’ve all come to know and love.

If you were paying attention, you ought to be wondering why you couldn’t just run at very HIGH rpm – the power curve goes down on each side of the peak, so you could have the engine run at 13,000-14,000 rpm and output 450 horsepower.  The problem on that side is that NASCAR initiated the gear rule so that teams wouldn’t have to deal with the incredible stress on engine parts that have to run at those very high speeds.  High-speed engines would significantly increase the cost to teams – it would be cheaper in the end to just let them build a dedicated open (not restricted) plate track engine.

In conclusion, yes – gearing down would work in theory, but it would introduce its own unique problems that would offset the advantage.

Plate Racing Rules: Getting Ready for Talladega

 Cooling, Engines, radiators, Talladega  No Responses »
May 032012
 

Most of the issues we were talking about at the start of the year regarding the measures NASCAR has taken to eliminate or reduce the two-car draft are still in play, so I thought I’d put the most important in one place as you start getting ready for Talladega this weekend.

One of the major changes is the radiator: The water capacity was decreased, which means that it can’t cool as effectively as it could with a larger volume of water. That limits how long cars can draft together in close formation, where the trailing car’s radiator is blocked and doesn’t get as much air circulating.

A related issue is the small, but extremely important limiter on the radiator called a pop-off valve This is one of the easiest last-minute changes NASCAR can make to adapt to changing temperatures — and new innovations teams have made to get around the current rules.

Finally, it seems as though bump drafting has gotten harder to do correctly. It’s all a matter of preventing cars from getting torqued. Literally.