May 272014
 

Okay. COSMOS was pre-empted Sunday in favor of the Coca Cola 600 and COSMOS host, astrophysicist Neil deGrasse Tyson, decided to edify us with some NASCAR physics.

NDGTweet1

NDG2

 

I bet 90% of NASCAR fans immediately know there’s something wrong here. In fact, all you had to do was watch the broadcast.  I’ll write a longer post, but here it is in brief.

The calculation for a car rounding a banked track with friction is pretty straightforward. There are a lot of angles in it, but if you work through it, it does make sense.  Check out hyperphysics for the details. The important thing here is that the maximum velocity is determined by the radius of the track, the coefficient of friction between the track and the tires, and the banking of the track.

banked

 

where r is the radius of the track, θ is the banking angle and μs is the coefficient of static friction. I know, it looks complicated, but stay with me.  The next paragraph is just the important stuff.

At Charlotte, the turn radii are 685/625 feet (the two turns are different, depending on whether you’re going into the dogleg or the backstretch) and the banking angle is 24 °.  Hyperphysics even lets you plug in numbers on their site, so I could very quickly determine that 165 mph is what you would get if you assumed a normal coefficient of friction between a regular tire and an asphalt track (around 0.75).  The reality is that NASCAR tires have a much higher coefficient of friction, which is why they easily exceed 165 mph and have no problem staying away from the “embankment” – whatever the heck that is.

Details to follow, but I wanted to get this up because I’m getting flooded with questions.

 

 

May 092014
 

OK, so ‘monozone’ is just a fancy way of saying it’s the old tire.  It’s all in the branding, isn’t it?

Goodyear has been experimenting with multi-zone tires since last year.  Multizone tires attempt to get the best of both worlds by combining a harder compound on the inner 2-3 inches of the tire (for wear resistance) and a softer compound across the rest of the tire (for grip).  I went over the reasons for the need for a tougher inner shoulder to the tire and why the new camber rules in a good amount of detail previously.

Multizone tires were a hit last year in Atlanta, but not as praised much in Kansas.  Likewise, this year the tires made for great racing in Texas, but the Richmond race featured a lot of tire wear and some unhappy drivers.

Goodyear tested a multizone tire at Kansas Speedway on April 14th this year. They weren’t happy with the results and have opted to go with a monozone tire at the race.  They used a combination of information in the historical record regarding previous compounds used at Kansas, along with lab testing to develop a new monozone right-side tire.  The compound is slightly different than the grip-centric compound that had been used on the multizone tires.  The same left-side tire will be used as has been used previously.

Given that Goodyear starts making tires for Daytona back in October, it’s pretty impressive that they were able to generate the tires for the race in a little under a month.

The problem seems to be excessive wear on the inside edges of the tires. I covered the reasons for this previously, but those reasons are exacerbated when you’re at a track with a lot of grip.  This means that the problems are more likely to rear their heads at grippy tracks (i.e. newly paved tracks) and during night races because the cooler temperatures result in more grip.  The other factor is high loads on tires, especially transient (i.e. short-time) loads.

Larry MacReynolds pointed out this morning on SiriusXM NASCAR radio that the teams have the car suspensions set up to be be rather stiff.  In the corners, there isn’t much springy in the car except the tire – the tire becomes the only point with any give. That puts a lot of stress on the tire.  Corner speeds have also been much higher this year – and that’s were much of the load comes about.

The one common thread I’ve heard is that there is a very small window between ‘fine’ and ‘oops’ – the tires are good for a long time and then they just go.  This is a problem because you don’t get much predictability – there’s no way they can run enough testing during practice to know that they can go 54, but not 55 laps before the time starts wearing enough to get into the danger zone.

Related Posts:

 

May 012014
 

We’ve heard a lot, especially this week at Richmond, about tire wear.  A lot of right front tires were wearing excessively.  As seems to be usual at this point, teams would like Goodyear to use a stronger tire and Goodyear would like teams to dial back their setups, especially their camber.

What’s Camber?

Camber is the tilt of the wheels relative to the vertical.  If the top of the wheel is father out (away from the car’s centerline) then the bottom, it’s called positive camber.  If the top of the wheel is closer than the bottom, it’s negative camber.

CamberIllustrated

Turning Left

On oval tracks where all the turns are left, they use negative camber on the right wheel and positive camber on the left.

CamberIllustrated_TurningLeft

NASCAR opened up the range of allowed cambers on the Gen-6 car.  In 2013, the front wheels could have up to 9 degrees camber and the rear wheels 3.5 degrees.  Clint Bowyer even tweeted about having blown the setup by being over agressive.

Although this makes turning easier, it means that the cars are riding on the edges of their tires when they’re not turning.  See how the inside of the right tire is the lower part?

The camber isn’t the only issue.  When you change direction, brake or accelerate, the car shifts (called load transfer).  When you turn left, the load shifts right.  When you brake, it shifts forward.  So if you brake hard while turning left, or if you have a suspension setup that allows a lot of shifting, you’re going to put a big load on the right-front tire and that also stresses the tire.

Goodyear used their dual-tread tire on the right-side tires at Richmond, where the rightmost ten inches are softer rubber and the innermost two inches are harder – but they were still having problems with cording on those inner edges of the right front tires.

Stu Grant, Goodyear’s General Manager of Global Race Tires, noted that “the operating window between acceptable wear on the right front and unacceptable wear is pretty small.”  Tires seemed to do fine up to a point, and then went catastrophically.

While Goodyear’s decided they’re going to look at the tire wear before this Fall’s Richmond Race (which will have a lot on the line), the teams are also going to have to do some set-up searching and decide how much of a risk they want to take with high levels of camber.

 

 

Mar 272014
 

LasVegas

I know this isn’t a picture of California.  But it’s a picture out my window, which is why this is a sort of short post.

There seems to be a clear division between the people who are upset about the tires Goodyear brought to California and the people who aren’t upset about the tires Goodyear brought to California.

The people who had problems are upset.  The people who didn’t have problems aren’t upset.

Here are the facts:

  • Goodyear brought the same tire they brought in 2013 – the first year of the Gen-6 car, which featured weight reductions, higher speeds, and higher loads.  The same tires (D4522 on the left/D4408 on the right) were used in 2012.  There weren’t a lot of complaints after that race – in fact, those two races have been cited as some of the most exciting races at California ever.
  • NASCAR tweaked the aero package and the suspension setup rules between 2013 and 2014.  The biggest suspension change was the removal of the minimum front ride height. These were relatively small changes compared to the changeover to the Gen-6 car.
  • California continues to change as a track.  The seams between lanes are getting more pronounced.  The bumps down the backstretch are getting bigger.
  • The teams continue to improve their setups as they learn more and more about the Gen-6 car. They are moving closer and closer to the edge of stability in an effort to make their cars faster.

Goodyear looked at the changes and considered how they would affect the current tire. In the end, they opted not to change the tire.  I have no way of knowing how much notice Goodyear had of the suspensions/aero changes for 2014.  They start making Daytona tires in October of the previous year, so they need a minimum of 3-4 months advance notice.

The teams knew about all these changes, and knew that Goodyear was bringing the same tire.  Most teams likely started off with something close to their fastest setup from last year.  A number of the drivers mentioned they had to back off their setups from practice because they weren’t happy with tire wear. That’s part of setting up a car – taking into account changes from the last time you were at the track.

NASCAR is still looking into the issue, but their initial impression was that the teams were too aggressive with their set ups.  They cited very low tire pressures as one reason for the failures, many of which appeared to be sidewall related.  Goodyear recommends 20-22 psi in the left-side tires and apparently some teams were starting off at 11-14 psi.  (Most of the failures were on the left side.)

Teams have always started out with low pressure in the tires because tire pressure increases as the tires heat up.  If you put the pressure you’d like to have in the tires at the start, it would quickly get higher than you want it to.  So you start with a low pressure and let it build up.

The problem with very low pressures is that instead of the air in the tire supporting the car, the tire sidewall is supporting the car.  This puts a lot of stress on the tire and, given how punishing a full fuel run is on tires, the stress can, over time, lead to early tire failure.

This is a very common pattern with race car setups.  NASCAR makes changes, Goodyear tries to anticipate how the tires will behave under the likely setups.  Teams try setups, make changes and Goodyear tries to keep up with the changes.  I suspect the reason we’re seeing the problems this year and not last is that the teams have become more bold with their setups.

It’s a never-ending iterative process, with Goodyear trying to anticipate the effects of new setups, rules changes and track changes, and teams constantly pushing the envelope.

That’s the challenge, though.  How much are you willing to risk to get just a little more speed out of your car?

Feb 282014
 

Repaving is the last possible remedy a track wants to use, but when potholes (see: Daytona) show up, there is no choice but to tear up the old asphalt and replace it with new, fresh blacktop.  In the last few years, Daytona, Phoenix, Michigan, Pocono and Kansas have all been repaved.

We hear a lot from race teams about their ‘notebooks’ – the collected wisdom and experience from prior experiences at a track.  When that track is repaved — or reconfigured — the notebook pretty much goes out the window .  They have to start over.

They’re not the only ones.  I had a chance to talk with Greg Stucker, Manager of Race Tire Sales for Goodyear as part of an article I was working on for a British publication called Race Cup Technology about the many challenges inherent to ensuring that the tires provided to the teams are safe and fast.

Goodyear doesn’t categorize tracks the way most of us do, with the superspeedways, mile-and-a-halfs, short tracks and road courses. They have to take into account not only length, but the track surface and the loads the tires experience.

Venue Groupings

Grouping Tracks
Group 1 Daytona, Talladega
Group 2 Charlotte, Chicago, Darlington, Homestead, Kansas, Las Vegas, Michigan, Texas
Group 3 Atlanta, California, Dover, Kentucky
Group 4 Bristol, Indy, Iowa, Phoenix, Pocono
Group 5 New Hampshire, Richmond, Gateway
Group 6 Martinsville
Group 7 Sonoma, Road America, Mid-Ohio, Ontario, Watkins Glen

This results in seven “venue groupings”, with superspeedways and road courses in their  own categories.  Martinsville, that unique concrete corner/asphalt straightaway hybrid also stands in a class by itself.  The remaining tracks are categorized into four groups.

Although Atlanta and Kansas have similar length and shape, they are in different groups. Atlanta’s surface is much older and rougher.  Speeds are higher.  That places different demands on the tire than a smooth, newly paved surface like Kansas.   Goodyear’s categories change almost yearly as tracks age and are repaved.  Kansas used to be in the same group as Atlanta prior to the former’s summer 2012 repave.

You wouldn’t think that Bristol, Iowa, Indy, Phoenix and Pocono had much in common, but as far as tires are concerned, they are all in the same group because of the combination of track surfaces and loads.

There’s another dimension to consider here:  time.  Michigan and Kansas  – two tracks in the same venue grouping – were paved within six months of each other.  Stucker points out that the surfaces aren’t aging the same way.  This is a combination of on-track activity, and the vagaries of weather – how extreme the temperature changes have been, how much moisture the tracks have sustained, etc.

Goodyear is starting a new program to record representative three-dimensional images of track surfaces as a function of time so that they can develop a better understanding of how track surfaces age.

The track-mapping project uses a technique called fringe projection.  Imagine that I create a pattern of alternating light and dark stripes, similar to the one in the upper right hand side of the graphic below.

FringProjection1

I’m going to project that pattern onto a surface and use a camera to capture the reflection.  A perfectly smooth, regular surface will not distort the pattern; however, if there are any irregularities – bumps or voids for example, then the stripe pattern will be distorted.

By varying the pattern and the angle at which the pattern is projected and recorded, you can use some very fancy mathematics to work backward and determine the shape of the surface that’s responsible for the distorted reflection.

The picture below shows you an example of a stripe pattern and the effect of shining the stripe pattern onto a toy fish.

FringeProjection2]

Goodyear’s project will produce a series of 1.5 by 2-inch samples that can display details as small as three micrometers  (about one ten-thousandth of an inch).   This is smaller than the diameter of a red blood cell and about a tenth to a thirtieth the diameter of a human hair.

This surface-mapping project will help clarify both the smaller year-to-year changes of track surfaces due to weather and use, but also will provide information on how major changes like whole-track resurfacing or reconfigurations affects the racing.

Stucker points out that track technology constantly changes.  You can’t compare the most recent repave of Daytona with the repave more than a decade ago.  One big difference is that the binder – the black sticky stuff that sticks the rocks together –  tends to be much denser than it used to be.  The denser binder prevents water penetration and is more durable, which should decrease how frequently the tracks have to be resurfaced; however, that increased density means that it takes a lot longer for the track to start showing signs of wear.

It’s a cool application of a new technology that is being applied to a large number of possible uses.  The “light” used for these measurements can be any type of light, including lasers, visible light and/or infrared or radio waves, depending on the particular data desired.  This technology is used in Microsoft’s Kinect camera, which is used on the XBox to translate a user’s motions onto a computer display.  That system uses infrared light. Other applications include 3D imaging of the mouth (for dental work), of vascular walls, for facial reconstruction, corrosion analysis and failure evaluation.

 

Oct 112013
 

There were 15 cautions last week at Kansas Speedway and at least 15 drivers complaining that driving on the repaved track surface was like driving on ‘razor blades’.  “The worst racetrack I’ve ever driven on.” said race winner Kevin Harvick.

Normally when you have a lot of cautions, the drivers’ first target is Goodyear.  Goodyear even brought a special tire (a multizone tire I analyzed last week) to deal with the specific challenges of repaved tracks.

Surprisingly, the drivers turned their attention this week to the tracks.  Jeff Gordon suggested that the lack of abrasiveness being the origin of the lack of grip.

“To me, it’s really the surface. We’re paving the racetracks with what we pave new highways with, and it’s not a highway.  We had the same issue in Phoenix, at Darlington. We have had the same issue at every repave that we’ve had the last six or seven years.”  (via the Las Vegas Sun)

Friction (the scientific word for ‘grip’) has to do with the rubbing and sticking of two materials against each other.  When you talk about the coefficient of friction for a tire, you are actually talking about the coefficient of friction for a tire on a specific surface.  The coefficient of friction (how ‘sticky’ the tire is) is different on asphalt vs. concrete.  (Also on wet vs. dry surfaces.)

Asphalt is a combination of aggregate (rocks or sometimes metal slag) held together with a binder.  The binder historically has been bitumen, a sticky tarlike substance that comes out of petroleum refining.

The American Chemical Society recently had their annual meeting in Indianapolis and they had some of the folks who developed the Indy race surface speak.  The Heritage Research Group was charged with researching asphalt for the 2004 repave.  They actually worked with Firestone Polymers to develop a superior binder, one that would make a smoother race track without losing any grip.  I also found a really cool presentation on the paving history of Indy.  Nowhere in any of those materials does anyone discuss the 2008 fiasco that was the Brickyard that year.

Gordon’s assertion that there must be roughness may not be true anymore because of all the games we can play with chemicals and making surfaces smooth but still sticky.  There are, however, a ton of variables that come into play.

Kansas was cold.  Very cold, even for Kansas this time of year.  Try this experiment.  Get two sticks of butter.  Put one in the freezer for a couple hours and one in the fridge.  Now get a cheese grater – one of those raspy things would be perfect – and try grating each one of the sticks of the butter.

What you will find is that the frozen butter gives you small powdery stuff and the fridge butter melts and leaves you with a layer of butter stuck on the grater.

That’s exactly what happens with laying down rubber on a race track.  At higher temperatures, you get a nice layer of rubber that comes off the tires.  When it’s cold – like Kansas was – you get powder.  In fact, that’s what drivers and crew chiefs were complaining about – the tires were powdering.

That had little to do with the track or the tires – it had to do with the unexpected temperatures.  Remember that the same tires were used in Atlanta and the drivers were raving about how good they were.

It’s a moving target.  You can’t predict the weather months in advance – the time you need to settle on a particular tire and start making it.  You can’t pave a track such that it will have grip in all possible weather situations.  And you can’t control the weather.

One of the issues, though, is that there are a good number of very smart scientists at Goodyear who know a lot about rubber for race tires.  The folks who research asphalt have their eyes firmly on the millions of miles of roads that you and I travel each day.  The ACS article mentioned that roughness in the asphalt we travel is responsible for 555 million gallons of fuel wasted each year on interstate highways.

If the asphalt isn’t stiff enough, the cars leave dings and dents.  That also takes energy, energy that could be used to move the cars.  They cite a figure of about 200 million gallons of fuel wasted due to destroying the road on interstate highways.

And interstate highways only make up two percent of the road surfaces in the country.

The point is that there aren’t really people who are researching asphalt for racetracks.  All of that research would have to be funded by racetracks.  As roads have gotten better for our cars, the technology that’s been developed might actually be less good for racetracks.

 

 

 

 

Oct 102013
 

Been a busy week – went out to Cincinnati over the weekend, then Vermont for a couple of talks.  More on that and my visit to Thunder Road later!

 

Oct 042013
 

Kansas marks the second appearance of Goodyear’s “Multi-Zone Tread Tire”, which was first used at Atlanta Motor Speedway over Labor Day weekend.

Stop for a moment to appreciate the challenge Goodyear has to face each race.  They must design a fast, durable and safe tire for each track on the circuit – tracks with individual characters that change from year to year due to weathering and re-paving.

The new  Gen-6 car, which  is lighter and faster, adds an additional complication.  NASCAR really hoped that the Gen-6 car would help address the challenge of one-and-a-half mile tracks.  The importance of aerodynamics at these tracks makes it very difficult to pass and has led to a lot of criticism about ‘cookie cutter track’ racing.

Anatomy of a Tire

Race tires are surprisingly similar to  production car tires.  They’re built to be much sturdier because they have to handle much higher forces.  They’re wider to give them better bite.   The most noticeable difference to the eye, however, is the tread.

Most people think ‘tread’ means the grooves.  That’s the tread pattern and it’s there to make tires safe to use in the rain.  NASCAR tires don’t have a tread pattern – we have the good sense to go inside when it rains and wait for it to stop. Tire_Anatomy  (Yes, I’m still bitter about standing outside for eight hours during the Petit Le Mans one year where floods filled the track so deep with water than Porsches could be drowned.)

The tread is simply the outer layer of the tire – the part that comes into contract with the track.  On a NASCAR tire, it’s smooth and only about an eighth of an inch thick.

The tread is made from a tire compound, which is a mysterious (read: proprietary) mixture of rubber, small particles of carbon and other alloys and a laundry list of other chemicals. The particular make-up of a compound is the secret to its durability and grip.

Tire compounding is a complex engineering task because you have to balance two competing behaviors:  soft and grippy vs. hard and durable.

A soft tire compound gives you a lot of grip.  It gets sticky when it heats up, allowing the tire to grab the track and letting you turn a lot faster.  The downside is that softer compounds wear faster.

Underneath the tread are the belts, which are made of woven metal threads, often with a superstrong polymer like Kevlar added for durability.  Belts are made for strength, not generating grip.

When tires wear, they become weak and as soon as one spot becomes weak enough, they blow out.  You can make a really, really grippy tire — but that means you’re going to have to change it very frequently.

The other extreme is a harder compound, which won’t give you as much grip, but also won’t wear as quickly.  Hard tires don’t blow out as easily, but they also don’t let you race as hard.

Goodyear has a constant balancing act:  how to create a tire that’s grippy enough, but also durable enough.

Multi-Zone Tires: A Technology from the Street.

Anyone old enough and from a cold enough climate to remember when we had two sets of tires?  I remember having to walk around a set of ‘snow tires’ as a kid growing up in Wisconsin.  We didn’t mark the change of seasons on a calendar – we knew it was winter when the car slid around the road and Dad had to make an appointment to get the snow tires put on the car.The question of how you balance different qualities in a product is an essential feature of engineering.  Consumers want tires to behave at peak performance in all different conditions, whether they be dry, wet or icy.  The problem is that a tire that performs best in dry conditions rarely also performs best in wet or icy conditions.GoodyearTripleTred

You can change the tread pattern to optimize the tire behavior in different conditions.  The  Goodyear Assurance TripleTred All-Season passenger tire combined three distinct types of tread patterns.  According to Goodyear:

  • The water zone has a sweeping tread pattern that helps move water our from under the tire for better wet traction.  (Those grooves direct  water from the inside of the tire to the outside, getting it out from under the tire and giving the tire more contact with the road.)
  • The “ice zone” has lots of biting edges, which offer enhanced gripping traction on icy and slick roads.
  • The “dry zones” are the large tread blocks along the outer edges, which give you the best behavior when you don’t have to worry about ice or rain.

A tire like this won’t behave perfectly in any one of the three conditions, but it will behave well in all three.  That’s an example of the compromises you make in engineering.

Bringing Multi-Zone Technology to NASCAR

NASCAR tires don’t have tread patterns, but you can apply the same idea to the tire compound.  That’s what Goodyear did in the tires used at Atlanta and at Kansas this week.

GoodyearMultiZoneTireThe left-side tires this week are the ‘regular’ single-compound tires and are a little grippier than before.

The right-side tires, however, feature two different compounds, as shown in the diagram at left (from Goodyear).  The inside three inches (meaning the three inches closest to the car body) are made from a harder compound that is more durable and wears more slowly.  The outer nine inches (roughly) are made from a grippier compound that will provide additional traction.

When tire fail, Goodyear studies the specific location of the failure because it teaches them about how to build a better tire.  We’ve talked before about the fact that sometimes you don’t learn things except when an experiment fails.  Well, at 1.5-mile tracks, the inside shoulder of the right-side tire gets a lot of stress.

The old solution would have been to use a harder tire compound throughout the entire tire – but that makes for a less grippy tire.  Less grippy tires tend to lead to more grumpy drivers.  Goodyear came up with a solution that uses a harder compound on the part of the tire that needs to withstand the most stress, but still gives a grippier compound in the area that should be able to handle it.  You get the best of both worlds.

Let me remind you, however, that every step forward Goodyear takes is matched by the advances of the race teams.  It’s the teams’ job to push the limits of the equipment.   The older, single-compound tire limited how aggressive their setups could be.  Too much load on the tire would result in an early blowout, so teams had to back off their setup to preserve tires.

The new tires will take additional stress and I guarantee that at the testing session today, the teams will be experimenting with new setups, trying to eke out that last extra tenth of a second or hundredth of a second.

There is no such thing as “the perfect tire”.  There is always a tradeoff between grip and wear.  This new tire is a great example of the type of compromise engineering is designed to address.

Other tire stories:

Oct 032013
 

Another week?  Already?  Where does the time go?

  • Eric Chemi reviews his great picks for Dover and gives us some data to support the idea that it really is a three-man Chase.
  • @nascarnomics is looking into the NASCAR attendance issue.  The great thing about this blog is that he explains his analysis, instead of just saying “attendance is down”.  This is a great opportunity to learn something about the power of numbers and (for the teachers) using graphs and charts to communicate data.  And face it, “censored regression modeling” sounds like something that ought to be labeled NSFW.
  • @PitRho has their reflection on Dover as well as their predictions for Kansas
  • I chimed in with a little piece about how banking doesn’t actually appear in the equation that dictates turning force, but why it’s important in the , anyway.
  • Not cars, but my friend @asymptotia appears in FailLab Ep 3 where he explains how it’s possible to grab and electric fence.  Great fun.
  • Racecar Engineering is offering a free supplement issue on engines
  • Did you know NASCAR has a whole website for their Green Initiative?  In response to a facebook question, I learned this week that Goodyear recycles all the tires they take back from the track – lots of other info on the site.
  • NASCAR and the Department of Energy signed a Memorandum of Understanding that identifies several “transformative energy technologies that will benefit NASCAR and its fans”, including electric vehicle charging stations, solid-oxide fuel cells, advanced biofuels and emerging natural gas technologies.    Remember that NASCAR is a huge enterprise.  It isn’t just about the cars on track.  There are race shops, offices, haulers, individuals working in NASCAR… energy efficiency in those areas  has more of an impact than if they change the cars on the track.  Little things like getting hauler drivers to shut off their generators makes a huge difference in energy consumption and greenhouse gas generation.
  • If you’re in Vermont, I’ll be giving two talks (October 8 and 9) there and appearing on the Mark Johnson show on WDEV on October 8th with Lt. Gov. Phil Scott – a racecar driver!  More details here.
Sep 272013
 

Dover is a fascinating track – twenty-four degrees of banking, but only a mile in length.carturning  A student approached me with a question:  Higher-banked tracks generate higher centripetal forces – so why doesn’t the track banking appear in the equation for centripetal force?

I’ve talked about centripetal forces in detail before, but let’s have a quick reminder.   I tie a string to a tennis ball and swing it at constant speed in a horizontal circle around my head.  Assuming I am coordinated enough not to hit myself in the head with the tennis ball (don’t laugh – it happens), the reason the ball goes in a circle is because of the string.

The string exerts a force on the ball.   At every moment, the ball tries go straight and the string forces it to turn.  The direction of the force the string exerts on the ball is along the string – which means it is always toward the center of the circle.  Centripetal literally means “center seeking”.

Just like the tennis ball, a car needs a force to make it turn.  The amount of force depends on the mass (weight) of the car, the speed of the car and the turn radius of the track.

EQ_CentripetalForceWords

As my student pointed out, the degree of banking doesn’t even show up in this equation.  All this equation tells you is that you need more force:

  • to take a tighter turn
  • to turn a heavier car
  • to turn faster

The numbers are interesting, in part because they are so big!  Every track in the NASCAR circuit has different parameters, so we have to do a different calculation for every track.  Here’s the numbers for Dover

BSPEED_TN_Dover_EnglishUnits

Teachers:  You can download metric and English unit version of this figure, along with some fun science facts about Miles the Monster.

The Numbers

Using a typical weight for a Gen-6 car (3300 lbs of car and 180 lbs of driver), we can figure out how much force it takes to make a car turn.  (Disclaimer: Parts of this table are from my previous blog.  But I did double check them and added this week’s track – Dover.)

Track Turn radius
(ft)
Speed
(mph)
Turning Force
(lbs)
G’s
Talladega 1000 180 6,848 1.97
200 8,456 2.43
Richmond 365 180 20,636 5.93
100 6,370 1.83
Bristol 242 100 9,606 2.76
Dover 500 130 7,858 2.26

We can see from this:

  • Even though Talladega is a higher speed track, the g-forces are comparable to smaller tracks – because Talladega has wide, sweeping turns.
  • Speed in the turns is limited at a small track like Richmond – the turns are very tight and you’d need a) an unrealistic amount of turning force and b) the g’s would be so high that the drivers would be likely to pass out.
  • Look at how much a 2o mph increase in speed increases the g’s experienced at the track.

“Gs”

The ‘G’ is quite possibly the most misunderstood unit in racing.  A ‘G’ measures acceleration, not force.   We use ‘G’ because the unit is equal to the acceleration of any object due to Earth’s gravity.

You are standing (or sitting) and the ground (or your chair) is exerting a force upward equal to your weight.  As a result, you do not accelerate up or down.  If the chair were to spontaneously disappear, you would accelerate toward the ground.

We use the unit ‘G’ just like a unit like ‘dozen’.  I can express anything in terms of dozens:  a dozen eggs, a dozen jellybeans or a dozen beers.  Likewise, we can use the unit ‘G’ to express the acceleration of anything.  I can measure the acceleration when you step on the gas after stopping at a red light in ‘G’s.   I can measure the acceleration you feel on a rollercoaster in Gs.

Just for reference, 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).

Although Earth’s gravity pulls down (toward the center of the Earth), I can use the unit ‘G’ to measure acceleration in any direction:  up or down, back or forth, or sideways.  Drag racers experience accelerations of about 5G backward at take off.  When you’re turning at constant speed, the acceleration is sideways (which engineers call ‘lateral’).

G-Force

Please don’t use the term “G-force”.   It’s wrong because a ‘G’ is a unit of acceleration, not force.  When you experience ‘3Gs’ of acceleration, the force you experience is the number of G’s times your weight.

Compare Danica Patrick (who weighs about 100 lbs) with Ryan Newman  who (according to Yahoo! Sports) weighs 207 lbs.  They both experience 3 ‘G’ in a turn.  That means Danica experiences a force of 300 lbs, while Ryan experiences a force of 621 lbs.  (Their cars, which weigh the same, experience the same force.)

If they experience such different forces, why do we use ‘G’s?  Because the ‘G’-value doesn’t depends on who or what is accelerating. It’s the same number.  You may remember that force = mass x acceleration.  If you take the mass out of the equation above, you have the formula for centripetal acceleration.

But What About the Banking?

As I noted above, the equation for centripetal force doesn’t address banking.  Banking doesn’t directly enter into the equation;  it’s just mass, speed and turn radius.

BUT:  A banked track lets you go faster around the turns.  If Dover were flat, you would still experience 2.26 ‘G’ at 130 mph – but you wouldn’t be able to go 130 mph in the first place.  The greater the banking, the higher speeds around the corners and thus greater G-values; however, a high-banked track with very wide corners could conceivably have lower ‘G’-values than a relatively flat track with really, really tight turns.  It – like so much of racing – is a tradeoff.

Related Posts:

Why Concrete Races Differently than Asphalt

Why Turning is Hard