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.
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.
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.
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.
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).
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.
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.
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.
Also published on Medium.