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.


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.


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.


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 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.






A Band Aid for NASCAR’s Tire Bleeding Problem

There are three things you don’t mess with in NASCAR: engines, fuel and tires.

Tuesday, NASCAR handed down a P5 penalty – the penultimate penalty on the books – to Ryan Newman’s 31 team. Crew Chief Luke Lambert was suspended six races, fined $125,000, and Newman and his owner Richard Childress were each docked 75 points. The tire specialist and team engineer were suspended for six races as well. RCR is appealing the penalty, but I wager they’ve got an uphill battle.

NASCAR’s made its stand loud and clear in the last few weeks. Tire bleeding will not be allowed. If you persist in trying, they’ll come down hard on you.


Why Would You Bleed Tires?

The hotter the gas inside a tire gets, the higher the tire pressure gets (says the ideal gas law).


The tire volume changes a little with temperature and pressure, but it’s not a huge change. If you were doing actual calculations to use in a race, you wouldn’t ignore it. For us, it’ll be good enough to approximate that the volume remains constant.  The equation tells us then that the ratio of pressure to temperature has to stay the same. If the temperature goes up, the pressure goes up, and vice-versa.

The video below (from the National Science Foundation) details how and why the tire pressure increases. Steve Letarte is a nice person and a very clear explainer of things. I look forward to seeing how he does when NBC takes over broadcasting NASCAR later this year.

The main problem with changing tire pressures is that grip depends on tire pressure – a lot.  If the tire pressure is too low, you lose energy to rolling resistance. If the tire pressure is too high, the sides of the tread pull away from the track, giving you a smaller contact patch and less grip.

Tire builds can be significant. At some tracks, you might see a 35 psi change in tire pressure. A large build means teams have to start a run with very low tire pressure – 8-10 psi at some tracks. If you look at a car at Martinsville waiting to go out on track, it’ll appear as thought it has flat tires.

Bleeding tires prevents the tire build (increase in pressure) from getting too large by releasing some of the pressure once the tire pressure reaches some value.

Wait… Like a Pop-Off Valve?

This is the same principle teams use in the radiator systems. Put water into a closed metal tube and heat it. We call that “a bomb”. As the liquid gets warm, it turns into gas, the gas pressure increases and eventually the gas inside pushes so hard it breaks the radiator or the tubing in the cooling system.

So we use a little valve called a pop-off valve on the radiator. When you see steam pouring out from near the bottom of the windshield, it means the pop-off valve has popped. The video below explains the pop-off valve in the cooling system.


That’s a great idea, right? They ought to make something like that for tires, so that the tires can’t get overinflated.

TireBleedValvesThey do. It’s called a tire bleed valve. Shown at left, you install it in the valve stem of the tire. Most are adjustable between some range of pressures.

An o-ring sits atop a spring. When the pressure is low enough (left), the spring is relaxed. The o-ring forms a seal on the valve seat,which holds in the air.

When the pressure inside the tire increases past a pre-set value, the spring compresses and unseats the o-ring. Notice how by where it says “no seal” the o-ring doesn’t touch the sides of the valve anymore . This gives air a path to escape. As soon as enough air has escaped so that the pressure returns to the maximum value, the spring relaxes and the valve closes. There’s less air in the tire, which allows the pressure to remain lower.


Seems Like the Perfect Solution. So…?

So bleed valves (or tire pressure relief valves) aren’t legal in NASCAR. However much nitrogen you put into the tire is how much you have and the driver is supposed to deal with the changes in the tire pressure. The harder you drive the tire, the hotter it gets, so having a way to relieve pressure gives the driver the option of pushing the car harder than a driver who is limited by the building tire pressure.

The scuttlebutt around the garage is that the tires on the 31 had small holes poked in the sidewalls. Rubber is stretchy enough that you can get a tiny, tiny puncture and it won’t open up a gaping hole that lets all the air out of your tire. The rubber on the sidewall is thinner than the rubber on the tread, so a pin prick or something similar would do the job.

The disadvantage of this method is that it’s totally random. With a bleeder valve, you can set it to go off at 35 psi and you know it won’t let any air out until 35 psi. With something like poking tiny holes in the tire, you have to guess at the number and placement of holes so that you don’t let out too much or too little. There’s also a safety issue, in that your well-intentioned “tiny” hole might actually do more damage than you intended – or noticed until the right front below out going 180 mph into a turn.

Plus, one of the fundamental tenets of NASCAR is that you do not mess with the tires. It’s bad from a sportsmanship angle and from a safety angle.

How would you tell?  The easiest way to find out if there are tiny holes in the tire is to over pressure the tire (maybe fill it up to 50 psi) and toss it in a bathtub or a swimming pool. If there are holes, you’ll see air bubbles coming out from the holes. (We actually used to use this technique to find big leaks in our vacuum chambers.) If you can’t submerge the tire, you can overpressure the tire and then squirt a little soapy water on the suspicious areas. You’ll see bubbles (from the soap) appearing near the holes.

If you want to be really pedantic about it, you can look at the material under a microscope once you’ve narrowed down where you suspect the holes might be located.

Can You Really Be Sure Someone Cheated?

There are a lot of things that could put a hole in a tire. But not the same size/shape hole multiple times in multiple tires. NASCAR is pretty cautious about not nailing people without solid evidence. I will be majorly surprised if RCR wins their appeal. That’s not to say upholding the penalty means there was a plan by the team to cheat the tires that way. It could have been one person thinking they were helping and the folks who got fined knew nothing about it. Science says nothing about intention or motive.


Fired Up: The Science of Flames

The Scariest Part of Racing?

During the XFINITY series race at Richmond, a malfunctioning fuel can spilled a huge amount of gasoline in the pit stall. A spark ignited the fuel, engulfing gasman Josh Wittman and rear tire changer Anthony O’Brien. A crew member for a team pitting nearby (Clifford Turner, working on Eric McClure’s car) was also injured. Although all the men were conscious and moving around immediately after the incident, all three were taken to the hospital. O’Brien wasn’t released from the hospital until Monday following the Friday incident happened.

If you were to poll racecar drivers about safety, I bet the majority of them would say the scariest situation isn’t a crash. As Elliott Sadler said: Two fears you have as a race car driver: one is being on fire and two is being T-boned in the driver door – everything else you sort of accept.

That quote was from before the Gen-5 car brought additional reinforcement to the drivers side door in the form of additional tubing and IMPAXX energy-absorbing foam. But what can you do to minimize the risk due to fire?


You need three things for fire: BSPEED_FireTriangle Without any one of these three, you don’t get fire. Which is a good thing because we pretty much walk around surrounded by oxygen and fuel all the time. Pretty much any clothing, regardless of whether it’s made of natural or artificial fibers, is fuel. The air is about 21% oxygen, with 78% nitrogen and 1% preservatives and fillers. No, actually the 1% are other gasses, like hydrogen, krypton, neon, etc. and they’re present in such tiny quantities that we don’t care. At all.

Back in The Day…

Way back in the day, drivers and crew wore street clothes and hoped they wouldn’t catch on fire. Then fire-retardant chemicals became available and people would dip their clothing in the chemicals to make it fire-resistant. The problem is that you do tend to want to wash your clothes after driving in a hot car for a couple hours and the chemicals would wash off.

And believe me, after three or four hours in a hot car, you want to wash whatever’s been in there with you.

Polymers = Repeating Molecules

Then we learned how to design polymers. The prefix “poly” means many. Polygon means many sides. Polymer means many units. The unit in this case is a particular arrangement of atoms into a molecule.

For example,  below is a schematic of an ethylene molecule and the polymer polyethylene, which is nothing more than a bunch of ethylene molecules hooked up together.

polymerExamplePONYou can make the polymer long or short by varying how many times you repeat.

Kevlar and Nomex: First Cousin Polymers

Kevlar was discovered by DuPont chemist Stephanie Kwolek, who passed away last June at the age of 90. How Kevlar came to be is an interesting story. A looming expected gasoline shortage led DuPont in the early 1960s to look for strong, yet lightweight fibers for tire manufacturing. One of Kwolek’s attempts at making a liquid that would be spun into a fiber ended up looking rather yuck. It was cloudly and thin, totally unlike what she expected, yet she insisted that it be made into a fiber for testing anyway. 

Her invention was Kevlar, a polymer that is five times stronger than steel by weight. Below is a Kevlar molecule. Grey circles are carbon, Blue are nitrogen, red are oxygen and white are hydrogen. There are actually a bunch more hydrogen atoms in the single molecule that I don’t show because it just makes the picture messy and confusing .



You’ll notice something very interesting about the Kevlar polymer – it’s very straight. That linearity is a big contributor to its strength. Kevlar chains link with each other in a very orderly way and make a fiber that can be used in bulletproof vests, as well as serving as a reinforcement for tires and carbon fiber pieces.

But Kevlar isn’t a miracle material. It has its limitations. When you heat it up to 900 degrees F, Kevlar literally falls apart. The atoms start letting go of each other.

But check this out.



Compare the molecules left to right. Exact same atoms, just arranged differently. Kevlar is this nice straight molecule, but Nomex is… well… Nomex is a little kinky.

That difference in conformation – straight vs. kinked – makes all the difference. Nomex is nowhere near as strong as Kevlar; however, when you heat Nomex, it doesn’t melt and it doesn’t burn.

It chars. While that might seem like a bad thing, it’s actually good.



When the Nomex fiber chars, it forms a layer of carbon on the outside. That makes the fiber thicker, which does two things: First, the thicker fabric gives you a little more protection from heat transfer, but second, the thickening of the fibers  closes the air gaps and prevents oxygen from getting through to the skin and feeding the fire.

The video below is one of DuPont’s promoting Nomex. Reminder. Don’t try this at home.


Nomex used to have a monopoly on the market, but recently there’s been a new material making waves. CarbonX is a blend of oxidized polyacrylonitrile and other strengthening fibers and is inherently non-flammable. Polyacrylonitrile is the precursor for 90% of carbon fiber production. One issue is that oxidized PAN is pretty much available in your choice of black or black, so the fibers have to be blended with other fire-resistant fibers to get colors.

One of the things CarbonX has is a very high LOI (Limiting Oxygen Index). That’s the percentage of oxygen that has to be present before the material will combust. CarbonX won’t combust unless 55% of the air is oxygen. Remember oxygen makes up about 21% of normal air, so to some extent, that’s a moot point because anything with a LOI over 21% is going to work about the same as far as motorsports goes.  You can hold it at 2600 F for two minutes and it won’t ignite or burn.

Prices have come way down on CarbonX since I first investigated them. You can get a CarbonX sport bra for about $80 and a CarbonX balaclava for about $65 now. A good Nomex balaclava will cost you almost the same.

The choice of CarbonX vs Nomex comes down to comfort, since they both will protect you in a fire. Drivers worry about the weight of the suit, mobility and breathability. The people I know who have tried both feel like CarbonX suits are heavier, but more breathable and less scratchy.

Can We Fireproof Racing?

FuelingApronPeople are very careful with their terminology when talking about fire safety. Nomex is not fireproof. Nomex firesuits are fire-resistant. Firesuits are made in layers, with the air between the layers also providing insulation against heat. That works the same way the air gaps between double-pane windows works.

SFI, a non-profit foundation that writes specifications and tests motorsports safety equipment, rates firesuits in terms of how long you can be exposed to fire before you’d get a second degree burn. For example, a 3.2A/1 rated firesuit gives you three seconds of protection, while a 3.2A/5 rated suit gives you 10 seconds of protection. (For the curious 3.2A is the SFI specification that deals with fire resistant uniforms).

NASCAR mandates that drivers wear a 3.2A/5 rated firesuit, as well as cover the remaining parts of the body with accessories that meet SFI specifications, including shoes and gloves. Crew members who go over the wall are required to have 3.2A/1-rated suits, although the NASCAR rulebook recommends going to the 3.2A/5.  The exception is that anyone handling gas must have the the 3.2A/5 suit and must wear a fire-resistant apron.

Fire resistant underwear and socks aren’t mandated, but they are recommended. The danger here is that if you close enough to a fire, synthetic fibers like nylon and rayon melt. Then they stick to the skin and are very difficult (and painful) to remove. So for the weekend racers, if you can’t afford a full set of Nomex undies, at least make sure everything else you’re wearing is 100% cotton. And ladies – no metal hooks, clasps or underwires. Metal heats up faster than fabrics and you’ll get burned in particularly bad places.

As you might expect, the higher the level of protection, the more expensive the suits are – although you’d like to think that a couple hundred dollars per suit difference is worth a few days in the hospital – or worse. Used to be the pit crews didn’t wear firesuits or helmets. If a fire similar to the one in Richmond happened then, it would likely be fatal.

Let’s also note that the fire wouldn’t have happened if there hadn’t been a malfunction in the fuel can that allowed a couple gallons of gasoline to flood the pit lane and probably get on the gas man as well. I also want to note that the stuff they use in fire extinguishers is a pretty nasty brew of chemicals that aren’t exactly good for people to breathe – but they’re a lot better than burning to death.

Safety is about protecting people on all fronts. Even though the gas can failed, the safety equipment stepped up to the worst-case scenario. Thank heavens everyone is safe.