How is a NASCAR Driver at Bristol like an Astronaut?

The Unique Challenges of Bristol

Many drivers will tell you that, despite not having the highest speeds or a unique shape, or a long race time, Bristol is one of the most exhausting stops on the schedule. Laps take about 15 seconds, with about 60% of each lap being turns. That means you’ve got a total of about six seconds (three on the frontstretch and another three on the backstretch) each lap to go straight. The rest of the time is turning.

Bristol is a tiny track (0.533 miles) with high banking (26–30°) and very tight turns. (242 ft for turns 1/2 and 256 ft for turns 3/4). Turning, as regular readers of the blog know, requires force.

The high banking allows fairly high speed for a short track. Take a look at how the pole speeds have changed over the years. A good lap will average 130 mph.

With the relatively high speeds and tight turns, it takes a lot of force to make a car turn at Bristol. At 110 mph, you need just under six tons of force to make the car and driver turn. Compare that to Daytona, where you only need about 277o lbs.

We’ve talked about the physics of getting the car to turn, but we haven’t talked about the effect of these forces on the drivers.

Let’s fix that, shall we?

G’s and Force

What’s a G?

A G is simply the acceleration due to gravity (32.3 feet per second per second). If you drop something, Earth’s gravity will make it go 32.2 feet per second faster every second it falls. Since most of us don’t think in terms of feet per second, it might be easier to picture this way: gravity makes an object speed up by about 22 mph every second. After one second, the object is going 22 mph. After two, it’s going 44 mph. After three, it’s (rounding off) 66 mph.

When Jeff Gordon steps on the scale and it read 150 lbs, it tells him that the force with which the Earth pulls on him is 150 lbs.

The Earth and Moon are drawn to the correct relative scale. Jeff Gordon is not.

If we sent Jeff to the moon (where gravity is 1/6 of that on Earth), the scale would read something else: It would only read 25 lbs.

Jeff himself hasn’t changed, but the force the planet pulls on him has. (In physics terms, his weight has changed, but his mass has not.)

Side Note: Just out of curiosity, here’s how much Jeff would weigh if you put him on different planets. It’s been a long time since I looked at the gravitational pull of planets, so I was a little surprised at what a wimp Saturn is. But it’s made up mostly of gas, so although it’s a big planet, it’s not very massive and thus doesn’t pull very hard.

Here a G, There a G…

Although the origin of “G” is gravitational pull, we use it as a unit, just like, say, “dozen”. So when you go on a roller coaster, you talk about how many Gs you feel because of the acceleration of the ride. you can feel those Gs in any direction. In racing, we talk about lateral Gs, which are the forces you feel when turning. We can re-do the graph above and express it in terms of ‘Gs’ instead of using a unit of force (the pound)

I found this interesting because you can go 200 mph at Daytona around a turn and never pull more than 2.5G. Conversely, it only takes 90 mph at Bristol to get the same Gs. And Bristol would become dangerous if the corner speed crept up into the 140 mph-150 mph (which won’t happen, so don’t worry about it.)

But the forces drivers experience are very real and do make a difference to their performance. Here’s a simple chart showing how heavy a 150-lb driver feels under different G-forces.

What a 150-lb person feels under that number of ‘G’s.

How Big is a G?

Thank you for asking. Because that’s where the title of the blog came from.

Taking a turn at Bristol at 110 mph creates about 3.3G of lateral force on the driver. That’s comparable to the force astronauts felt on the Space Shuttle as it took off. Of course, the astronauts felt it for a minute or so. NASCAR drivers feel that force nine seconds out of every 15 for 500 laps (266.5 miles).

The Effect of Gs on the Human Body

You might wonder why, if the space shuttle had 37 million horsepower, it accelerated at a measly 3g. The answer is that the engineers decided that this was the safest acceleration for the passengers. Cargo rockets can take off with much higher accelerations. But people are much, much more fragile than equipment.

Sustained vs. Transient Forces

When we talk about forces on drivers, we really should divide them into two categories

Sustained forces are forces that continue over periods of time that are more than a few seconds.

  • The force astronauts feel during lift-off
  • The force drivers feel going around a corner

Transient forces happen very quickly — like in milliseconds (thousandths of a second)

  • The force a driver feels when he or she hits a wall
  • The force you feel when you fall down
  • The force of a slap on the back

When we talk about accidents, we’re concerned primarily with transient force. When there’s a crash, you’ll hear people talk about larger numbers like “75G” — that’s a transient force that might be sustainable for a very, very short time, but that would kill a person if applied for even one second.

But since we’re talking about turns that that 4–5 seconds to traverse, we’re worried about sustained force. In general, sustained forces of 25G and higher are getting into very dangerous territory where you can experience significant injury and even death.

But it turns out that even much lower forces can have a huge effect on drivers.

Direction Matters!

Your body evolved to work superbly in an environment of 1G where you are primarily right side up or lying horizontally. Because we spend most of our time upright, our organs (especially our heart) are designed to work best in that configuration. Animals that spend most of their time upside down (like the sloth) have a different internal organ arrangement that allows them to work best when they’re inverted.

Because we have a preferred direction, the human body is not isotropic (that’s a fancy word for ‘behaves the same in all directions’). If you measure your blood pressure in your legs and in your arms while upright, you’ll find the blood pressure is lower in your arms — and even lower in your head (although harder to measure). When your heart sends blood to your head, the blood has to go ‘uphill’ — it has to fight gravity to get there.

If the mass of the Earth suddenly increased, the force of gravity would increase and now your body would have to work even harder to get blood to your head when you’re standing.

(Of course, if the mass of the Earth suddenly increased, its orbit would change and we’d all die anyway, so blood not getting to your head would not be the biggest problem you need to worry about.)

In real life, when we experience more than the 1G we’re used to, the body has to compensate by working harder to make sure that there’s sufficient blood flow to distribute oxygen throughout the body.

Going Up

Let’s talk about upward acceleration first. The weak link when it come to high acceleration is your eyes. Eyes are extremely sensitive tohypoxia(lack of oxygen, which is due to lack of blood for in this case). An upward acceleration forces blood to the lower portion of your body. Your heart has to work harder to get the blood to important places like your head. As blood flow decreases, oxygen flow to those organs also decreases.

If your eyes don’t get enough oxygen, your peripheral vision starts to disappear. Next, your field of vision constricts (tunnel vision), followed by the loss of ability to see colors (called ‘grey-out’). This is all the brain realizing that you’re not getting enough oxygen to maintain normal vision and gradually shutting things down in an attempt to keep some vision. Military pilots wear pressurized suits that compress the lower extremities, forcing blood back up the body. (They also use external breathing apparatus.)

If the lack of oxygen continues, you lose all vision, then you lose consciousness. This is often referred to as G-LOC (G-induced Loss of Consciousness).

Going Down

If you experience downward acceleration, you have the opposite problem: all the blood goes to you head. This is the classic case of too much of a good thing. Blood pressure in the head increases. The blood vessels in your face are small, but they fill with blood and can burst. The tiny blood vessels in your eyes can become very red (and can burst) which is called red out. This can lead to retinal damage and even hemorrhagic stroke.

Interestingly, you’re able to tolerate upward acceleration much more than downward. Most people can withstand 5G pretty easily in upward acceleration, but only 2–3 G in downward acceleration.

Turning

Here’s the good news: The body is even more resistant when we talk about forces applied perpendicular to the spine. These are called eyeballs in, which means the force originates at your back and pushes you forward and eyeballs out, in which the body is pushed backward. Again, your body doesn’t respond in the same way because the blood vessels in the retina are more sensitive to eyeballs out forces.

But both eyeballs in and eyeballs out forces can be larger. One report I read stated that people could tolerate 17 g eyeballs in for a few moments and still do simple tasks (meaning they hadn’t lost consciousness), but could only tolerate 12 g in the eyeballs out direction.

Racing

When driving on a banked track, the drivers experience a complicated combination of forces in multiple directions. While the 3.3G felt at Bristol isn’t in the dangerous level of grey-out, the drivers definitely feel effects. Their bodies have to work harder to move (your 10-lb head feels like it’s 33 lbs under 3.3G). Their hearts are working harder to pump blood around the body.

There have been issues in the past with excessive G’s at race tracks. I tell the story in my book, The Physics of NASCAR, of how the first CART race at Texas Motor Speedway had to be cancelled because the drivers were pulling 5–6 Gs around the turns and were reporting moments of grey- or black-out. The race was cancelled hours before it was to start. I still am mystified that no one realized earlier that this was going to be a problem before they actually got to the race.

SideBar: The Human Guinea Pig of Gs

A lot of what we know is due to the work of Air Force Col John Stapp, who used himself as a guinea pig to understand how the human body responded to various magnitude accelerations. There are now many laws and forbidding paperwork to prevent anyone from doing this, but he operated back in the 1950s when we seatbelts were optional equipment in cars and people were encouraged to smoke because it was cool and maybe even healthy.

Stapp strapped himself to a rocket sled and accelerated, then stopped the sled (and himself). He recorded his runs and, of course, had the first-hand experience to add to his data. During the course of his experiments, he lost dental fillings, cracked ribs and broke bones. All in the name of science. (I can hardly look at the pictures. I want to yell at him to put on a HANS device (which wasn’t invented yet, I know.)

His last run (spoiler alert: he did survive it) was a rocket sled that accelerated him to 632 mph in 5 seconds — and then came to a stop in 1.1 seconds. He experienced a peak “eyeballs-out” force of 46.2 G and also survived 25 G for a little over a second. Although his experiments don’t seem to have had any effect on his longevity since he lived another 45 years, dying in 1999 at age 89, that last experiment permanently damaged his vision.

When he was experiencing those 46.2 Gs, his body (168-lbs) felt like 7,700 lbs.

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How Tracks Take and Lose Rubber

The two words we heard most frequently last week in Pocono (after “still raining”) were “green racetrack”.

Rain doesn’t just delay racing. When the race is finally run after a rain delay, it’s run on a different racetrack. To get the details of how a racetrack changes from “rubbered up” to “green”, I talked to Greg Stucker (Goodyear’s Director of Racing).

The Process

We often talk about a racetrack rubbering up as though it’s a one-way deal and the only time rubber is removed is when there’s a rain or the track is cleaned. Greg Stucker explained that the racetrack is constantly changing because rubber is being laid down AND taken up throughout practice and the race. Here’s the overview, as a picture, then I’ll go through the steps.

Rubbering Up

The right-hand side of the graphic (the red circles) shows the process by which the track gains rubber.

The faster cars go, the hotter their tires get. While rubber is a solid at normal temperatures, when tires reach 200 F and higher, the surfaces of the tires are so hot that the bonds between rubber molecules weaken to the point where the abrasion between the asphalt and the tire can pull rubber molecules off the tire and stick them to the road.

Once that first layer of rubber has been laid down (which can take awhile with, for example, a brand new racetrack), subsequent layers go down on top. The more hot tires in contact with the track, the more rubber you lay down.

Rubber adheres to aggregate and to the asphalt matrix that holds the aggregate together. The adhesion of the second layer of rubber to the first layer of rubber is weaker than the adhesion of the first layer of rubber to the pavement — and this becomes important.

More Is Not Always Better

If you’ve ever painted something that’s been painted many times (and this could be anything from drywall to your fingernails), you know that there is a point at which your coating becomes too thick. It starts to crack and peel and come off in nasty little pieces.

Despite behaving differently, rubber and paint are both polymers and they share a lot of similar properties. The same type of peeling and cracking happens with rubber as you find in paint. As you build up more and more rubber, the layers become stressed. One layer can crack, which pulls on the layers it’s attached to.

Because the bonds between layers aren’t equal strength, eventually you get cracks that go through many layers . While that can lead to the rubber delaminating from the track, that’s not the main process by which rubber is removed.

Below, the ‘BEFORE’ picture shows the successive deposition of many layers of rubber. I’ve idealized it a bit. They aren’t necessarily continuous and you can’t really tell the difference between the layers — but otherwise it would’ve been one giant black blob and what would you learn from that, huh?

‘AFTER’ shows what happens when the layer is thick enough that the layers begin to crack and peel. It looks very much like the paint above, but we’re talking smaller cracks and much thinner layers, so you don’t necessarily see this happening, especially at early stages.

Caution: Rubber Coming Up

Here’s something I hadn’t thought of. The deposition of rubber depends on high speeds because those produce hot tires.

So what happens when we slow down? Let’s say we’ve had a nice green-flag run and built up a nice layer of tire rubber on the track.

The yellow flag comes out and the cars slow down — which means the tires cool down.

Now, instead of depositing rubber, the cooler tires actually start picking it up from the track. It either sticks to the tires or gets flung out onto the track. Greg pointed out that this process is much more noticeable in F1. (This isn’t a knock against Pirelli, who supply tires for F1. Any tire company would have to provide the same types of tires to satisfy the needs of a F1 car. But the type of tires F1 requires just produce a whole lot more deposition and take-up of rubber.)

The pieces that stay on the track are good-sized chunks of rubber called “marbles” because drivers say driving over them is like driving over marbles. On the left, you can see the huge amount of marbles on the racetrack that accumulate during the course of the race. They clean the racetrack before the race starts. Everything you see is created during the race. The cars push the marbles out of the racing line, but heaven forbid you get a little out of sorts and get off the clean track. There’s an example of where rubber on the racetrack does NOT improve grip.

On the right is a tire that just came in from the track after a caution. The pattern you see is not due to tire wear: The snake-like surface texture is all rubber picked up from the track. Greg tells me you can see the difference between tires that come in from a green-flag stop and those that come in after the car’s run two or three laps of caution. Something to look for the next time you’re on pit road.

You’ve probably seen the tire specialist using a propane torch to clean off tires that have just come in. That’s to remove the picked-up rubber that is covering up the tread indicators. If you tried to measure how much tread you used without cleaning the tires, you’d get garbage results.

When drivers don’t get new tires, they have an additional challenge because they’ve got all the rubber crap on their tires. Yes, swerving does heat your tires up, but more importantly, the back and forth motion scrubs the chunks of rubber sticking to their tires off. It’s like scraping your boot on a step to get the mud off.

A chunk of rubber picked up on a tire is like a piece of chewing gum on your shoe. It sure ain’t gonna make you go faster and it’ll probably slow you down.

Just for comparison, here’s a NASCAR tire from Martinsville. It’s not nearly as bad as the F1 tire, but I wanted to show you how big the chunks of rubber are. Soft rubber is like chewing gum: you just keep gathering it up. Your take-away?

A cold tire running around the track during a caution is like a Swiffer for rubber

Why Some Tracks Don’t Rubber Up

How well a track rubbers up depends on the track as well as the temperatures. I know you’ve seen this graphic a couple times, but it’s important to remember how tracks wear. The asphalt between the aggregate (i.e. the rocks) wears away. The rocks themselves wear a little, but smaller rocks may actually be pulled out of the track as they have less and less matrix surrounding them.

 

When a track is brand now, the aggregate is nice and level and the asphalt comes up to the top of the aggregate. It’s smooth and grippy. But look what happens after the track wears: the aggregate pokes through a little more. The track is even grippier. Everyone loves a track like this. When the track wears down, you start losing aggregate and the aggregate that is there rounds off.

NASCAR laser maps their tracks. The pictures below show the maps for Kansas and Atlanta. Zero (think of it as sea level) is in that muddy place right between the red and the green. As you move from red to orange to yellow, the track is getting higher — you’re looking at hills. As you move down to blue and violet, the track is getting lower and you’re looking at valleys. The topmost yellow corresponds to 2 millimeter (2 mm) and the bottommost violet corresponds to a negative 2 millimeters. The area we’re looking at in each one is about 20 mm x 20 mm.

Kansas, which was recently paved, is pretty darn smooth. It’s mostly green and red — there’s no yellow and only a very little blue.

Atlanta, however, has big peaks and low valleys. What you’re seeing here is the real-life equivalents to “Too New” and the “Just Right” in the graphics above.

Imagine we multiply the scale so now 2 millimeters is two miles and I ask you to walk from one side to another. If you walk Kansas, you’ll go a distance that would be equivalent on our scale to about 20 miles; however, if you do Atlanta at its peakiest part, you’ll travel further than 20 miles because you’ve got to go up the hills and down it.

ASIDE: Did you know that physicists have scientifically proven that Kansas is indeed flatter than a pancake?

Back to work. In case you don’t see it, Compare the two shapes below. One is a flat surface and the other is the same sized flat surface with a mountain in the middle.

I traced the surface and then extended the lines out. See how much further you have to go with the bump in the middle?

Now let’s apply that to our track.

 

I do not have the patience or skill to do the same thing with the complicated lines in that picture that I did above, but hopefully you can see that there is more red on the “Just Right” track surface than either of the others. When a track is ‘just right’, it gives the rubber more places to stick to. Those tracks rubber up better than the other types of tracks.

This is the same reason why you need a lot more pain to paint a textured wall than a plain flat wall. Ever tried painting brick? You need a ton of paint because of all the little dips and ridges and holes. Surface area is bad for painting, but really good for racetracks.

Concrete

Greg Stucker pointed out that it is much harder for rubber to stick to concrete than on asphalt. Goodyear developed compounds specifically designed to lay down rubber on the concrete tracks so those tracks do get grippier. One more thing (in addition to speed and safety) Goodyear has to consider when designing a tire.

Rain, Rain, Wash Away…

One of the questions I hear a lot is some variant on this:

If rubber on a racetrack can stand up to a 3500-lb racecar going 200 mph, how come the same rubber comes off with just water?

Some people think that rain dissolves the bonds between the rubber and the track and the rubber is slowly eroded away. It’s actually a physical interaction, the same as if you went out on the track and scrubbed it.

A raindrop in a drizzle might be going 5 mph when it hits the ground, but in a good storm, a raindrop can splat with a speed of 15–20 mph.

It has a force of about one Newton (which is about the same as the weight of an apple). But remember that the force is spread out over a very tiny area, since the average raindrop is on the order of 2.5 millimeter (which is about a tenth of an inch). Pressure is force divided by area. A woman in a high-heeled shoe can exert as much pressure as a small elephant because all the force is focused into a point.

A good strong rain (like the one they had in Pocono last weekend) is similar to sandblasting or pressure washing. If there was already a good buildup of rubber, that rubber layer is already stressed and just waiting to crack and come up, so the rain gives it the extra impetus and physically washes the track clean.

Air Titan Plays a Role, Too

The old way of drying the track used heat from jet engines to evaporate the water. In addition to being slow, the sudden temperature changes and intensity of the air removed rubber from the track as well.

The Air Titan works by first pushing water off the track using air. Greg told me that the Air Titan purposely uses as low an air pressure as they can to remove water while preserving as much rubber on the track as they can.

So… Is Rubber on the Track Always Good?

Rubber is always being laid down and taken up during the course of a race, with the amounts depending on the track conditions and the temperature.

Relatively thin layers of rubber on the track are the best. They give the tires more to stick to and improve grip, but…

When rubber layers get too thick, they start to crack and peel like old paint, which means they can get picked up the the cars’ tires and either reduce the tires’ grip or end up on the track as marbles, which are also reduce grip.

So, like anything else, rubber on the track is good for increasing grip — as long as it’s the right size and in the right place!

*** Many thanks to Mike Siberini of Goodyear for coordinating the interview — especially on short notice.

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