Maximum G

Sounds like an energy drink, right?

Listening to Kyle Busch’s press conference Wednesday was alternately fascinating and cringe-worthy. The fact that he remembers so much about the crash is amazing – it will be a great boon to the safety people who probably will use this as a case study in the future. And best wishes to Kyle to get well soon.

Kyle said he left the track at 176 mph, hit at 90 mph and sustained 90 Gs.  My twitter was flooded with people asking “90Gs? No one could survive that kind of a hit.”

That’s actually not true. Trying to quantify a crash via one number is a nice attempt at simplifying things, but totally wrong.

Warning – I wrote and researched this while flying halfway across the country, so we’re likely to need a re-write when I get back home Monday and have a little more time to make this prettier. But let’s start by clarifying terms.

“Gs”

The ‘G’ is quite possibly the most misunderstood unit in racing.  A ‘G’ measures acceleration, not force.   One ‘G’  is equal to the acceleration of any object due to Earth’s gravity. You are experiencing one ‘G’ right now. The product of your mass times the acceleration due to gravity is your weight.

Acceleration is how fast you change speed. If you go from 0 to 62 mph in 2.8 seconds (like the Lykan HyperSport in the Furious 7 Movie), you’ve got an acceleration of 22.4 mph each second. Every second, your speed increases by 22.4 mph. It’s an acceleration of a little more than 1G. (which, by the way does may the Etihad towers jump possible. I did the math, just thought I’d throw that in.)

Let’s set the scale. The Space Shuttle pulled 3G on launch, Apollo 16 pulled 7G on re-entry. A Formula 1 car pulls about 5-6 G laterally during sharp turns and 4-5G during linear acceleration. I’ve got a story in the Physics of NASCAR book about Texas Motor Speedway having to cancel an open-wheel race at the last moment because the drivers were pulling so many Gs that they were having mini blackouts. A good rollercoaster will give you 2-3G.

Electronics spec’ed for the military for use in shells have to survive 15,000 G.

Weight is the force resulting from the acceleration. Remember F-ma? When you experience ’3Gs’ of acceleration, the force you experience is the number of G’s times your weight.

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.

Important: Although Earth’s gravity pulls down (toward the center of the Earth), we use ‘G’ to measure acceleration in any direction:  up or down, back or forth, or sideways.

How Many G’s Can a Person Withstand?

Again, this is by no means meant to minimize Kyle’s experience. He had a really hard crash and broke bones in both legs. So don’t interpret what I’m going to say as trying to say he’s lying or wrong or is trying to exaggerate his injury. It was serious.

But it wasn’t as simple as “90 Gs”

I’m pretty sure the numbers Kyle had were the numbers from the car’s transponder. As far as I know, NASCAR hasn’t instituted in-ear accelerometers like IndyCar.

An accelerometer is exactly what is sounds like: a meter for acceleration. Most iPads and iPhones today have one. Especially given the increasing concern about concussion, IndyCar and F1 have both provided drivers with a tiny accelerometer that fits into the ear and thus gives a much more accurate measurement of the actual acceleration of the head. (Remember that the problem with concussion is that the brain actually hits the inside of the skull.)

NASCAR relies on a transponder located near the frame rails (low) in the car. That means it measures what happens to the car, not the driver. A number of safety measures make the driver slow down less quickly than the car. I’ll come back to that.

There are three primary factors in a crash: The change in speed, the time over which the change in speed happens and the direction of the force.

So it’s not only how fast you’re going when you crash, it’s how fast you stop. When the people who study these things talk about crashes, they talk about the “crash pulse”, which incorporates the first two of these factors. Here’s one I drew for illustration.

Forcevstime2

When someone talks about 90G, they mean that was the peak value of the acceleration vs time curve was 90G. In my plot above, both curves show a crash from the same starting speed. The difference is that the red curve was a case in which the force/acceleration was spread out over a longer time. That’s why the peak value is lower.

How many Gs you experience depends on your starting and ending speeds and how long it takes you to stop. In the case of a crash where you go from 90 mph to stopped over 1 second, you experience about 4 Gs. If it happens in a tenth of a second, you experience 4o Gs.

Now let’s look at a real crash pulse.

BSPEED_CrashPulse

Here, you see the crash and you see the backlash – that’s the negative acceleration on the right side of the graph. The details of these graph give you a much fuller picture of a crash because you learn how the force was distributed in time.

Although the peak force was 90G, that 90G was applied for a short time. Lesser accelerations were experienced during the rest of the crash. A peak force is like a snapshot of a dance. You get one impression, but it’s not the whole picture.

Let’s get back to measuring the car vs. measuring the driver. The driver is belted in by 2 to 3-inch-wide belts over the shoulders, around the lap and around the legs. Those belts are designed to stretch when they’re stressed, which means that the driver doesn’t stop as quickly as the car stops.

Same thing with the HANS device. The tethers on the helmet allow the driver’s head to move forward, but they slow the rate at which the head moves. So even if the car experiences 90G, the driver experiences less. How much less would require a lot of assumptions, but if the various safety devices double the time it takes for a driver to stop, it halves the force.

I mentioned direction is important. That’s because any force on your body also is a force on your blood. Pilots who make sharp accelerations up or down (parallel to the spine) have issues because the heart has to work extra hard to pump the blood. The human body can withstand higher accelerations perpendicular to the spine than parallel to it.

No, Really. How Many G’s Before It’s Really Bad.

StappSledYeah. That’s what you’re really asking, isn’t it? What are the limits of the human body? These are difficult questions to answer because you can’t really do the experiment. People don’t volunteer to be accelerated really fast so scientists can see if they survive.

With one exception.

Col John Stapp (Air Force, shown at left) was active in the late 40s and early 50s. We didn’t know how far or how fast airplanes (and rockets) would allow us to go. And even if we could build the machinery, would a pilot or passengers survive?

The military didn’t want to hand over soldiers for him to run experiments on.

So he experimented on himself.

Today, that would never happen because there’d be so much paperwork that he’d die of old age before he got approval. But back in the 50s, people got away with a lot more.

The picture shows a test in 1954 where Stapp accelerated at 15g for 0.6 seconds and reached a peak acceleration of 22 second. His record was 46 g, and he sustained more than 25 g for 1.1 seconds.

This was no 90 G, but whereas a driver might experience that acceleration for a couple hundredths of a second, Stapp did it for tenths or full seconds.

These experiments had consequences. There is one really big problem with acceleration perpendicular to your spine. Your eyes bug out (or in).

No, seriously. Your eyes are held into your skull by a couple muscles and optic nerves. High accelerations (and decelerations) is like putting your peepers on a bungee cord. What finally stopped Stapp’s experiments was that he sustained major damage to his vision. I highly recommend http://www.ejectionsite.com/stapp.htm if you’d like to learn more.

C’Mon. How Many G’s Has a Human Being Sustained Before…

O.K. A paper (Society of Automotive Engineers. Indy racecar crash analysis. Automotive Engineering International, June 1999, pages 87-90) says that IndyCar drivers have survived 100G+ crashes. I don’t know yet whether those are crashes measured with the in-ear accelerometer, so it’s difficult to make a direct comparison with NASCAR.

But remember that even smaller accelerations – if applied in just the wrong way — can have equally catastrophic results for the driver.

Closing note: You know what they use in doing crash research? Yes, Crash Test Dummies, but the human body is so complex and intricate that a dummy can’t tell you everything.

They use cadavers.

 

 

 

 

 

5 Comments

  1. Kenny Brack survived the highest recorded g-forces since the introduction of crash violence recording systems in IRL, peaking at 214 g (at Texas Motor Speedway)! Just sayin’.

  2. Minor question/quibble: my understanding is that the HANS tethers are designed to keep the head from moving forward beyond a certain point, Aside from the ‘give’ in the strap themselves, there is practically no resistance to a driver’s head moving forward until the straps are fully taut.

    If I have the science right, In a head-on crash, the driver’s head would move forward at the speed of the car prior to impact until the tethers were fully extended and then would continue to move forward at the speed allowed by the flexing of the tethers at which point the head stops moving forward relative to the rest of the body.

  3. @tom: this mechanism you explained is useful (along with the seat which prevents the driver’s head from moving side-to-side and the helmet which protects the skull, but not its contents) when it comes to protecting the cervical spine, but does nothing to prevent concussion or brain damage in a had enough crash.

  4. “Today, that would never happen because there’d be so much paperwork that he’d die of old age before he got approval.” haha so funny and true! I would say that there is considerable variation among individuals (race car drivers) when it comes to g-force tolerance. Moreover that story with 214G always freaked me out! Great explanation here.

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