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Dr. Diandra: Firesuits for race cars

NASCAR Cup cars fire

Kevin Harvick exits his car after it caught fire during the Southern 500 on Sept. 4, 2022. (Photo: NBC Sports)

Endurance racer Stephen Cox relates a bone-chilling first-person account (in two parts) of his car catching fire at 125 mph. He was amazed at how the smoke made it almost impossible for him to see his belts or the window net release.

Fire is not unique to any one racing series. When Ryan Blaney’s car caught on fire in the 2018 Coca Cola 600, his crew chief asked if he had activated the fire suppression system.

“Dude, I was trying to get out of it. I didn’t look for the pin. I couldn’t see it regardless,” Blaney told him.

The Next Gen car has had more than its share of fires in its inaugural season. Tyler Reddick experienced the first Next Gen fire at a March test at Kansas. During the season, fire has impacted Chris Buescher, Joey Logano, Cole Custer, Chase Briscoe, J.J. Yeley and Kevin Harvick — all Ford drivers — and Alex Bowman.

After the Indianapolis race, NASCAR mandated an insulating shroud around the right-side exhaust pipes. Officials theorized that contact moved the pipes, allowing them to set fire to the foam inside the door panel.

But fires persisted.

So did driver frustration.

After a fire knocked championship contender Harvick out of the Darlington playoff race, NASCAR issued more technical changes. These modifications are based on a new theory: that small pieces of rubber inside a car start fires.

What causes fire?

Knowing how fire starts is the key to stopping it.

Fire is a chemical reaction between fuel and oxygen in the presence of heat. Engines use spark plugs to combust gasoline. Your body oxidizes glucose and fats in a similar reaction to keep your metabolism going.

Sustaining a fire requires all three components — fuel, oxygen and heat — in quantities sufficient to sustain the chain reaction that keeps the blaze going. Fires stop when one or more of the three components is reduced or eliminated.

Air is about 21% oxygen. Rolling someone on fire on the ground or in a blanket separates the fire from oxygen. That’s the same reason putting out a grease fire by clapping a lid on the pan works. No oxygen, no fire.

Water cools, thus eliminating the heat component. But water won’t extinguish a gasoline fire for the same reason it doesn’t work on grease fires. Liquids like gasoline, paint thinner, etc. don’t mix with water.

NASCAR uses chemical fire suppressants in the car and in the containers safety personnel carry. These chemicals gobble oxygen. A thermally activated extinguisher is required in the fuel cell area, but the suppression system in the driver’s compartment is manually operated.

How firesuits work

The zeroth rule of motorsports safety is: Hope for the best and plan for the worst. That means not just trying to avoid fires, but protecting people if a fire does start.

No material is fireproof. Drivers’ firesuits protect them by providing thermal insulation, being lousy fuel sources and blocking oxygen.

Quilting traps air between a firesuit’s layers. Air is a great thermal insulator, which decreases the amount of heat that can penetrate the firesuit.

Most firesuits are made from Nomex or Nomex blends, although there are increasingly more material options. Nomex may not have the super strength of its sibling molecule Kevlar, but Nomex doesn’t burn: It chars. Charring forms a layer of carbon around the fiber, as shown in the drawings and photos below.

A drawing with photos showing how Nomex creates a char layer around the fiber instead of burning

Carbon is a superb thermal insulator that provides additional protection from the heat. Because the char is not flammable, it doesn’t provide fuel for the fire.

Finally, the expanding Nomex fibers close the holes in the weave of the fabric, as I’ve drawn below. That prevents oxygen and flame from getting to the driver’s skin.

A drawing showing how the expanding Nomex fibers close the holes in a firesuit's weave to prevent oxygen from getting through

All driver gear must be fire-resistant, from headsocks to shoes and underwear. Even patches must conform to standards established by the SFI Foundation. That gear gives the driver about 10 additional seconds before he or she will suffer second-degree burns.

If you want to learn more about firesuits, check out this video I made with the National Science Foundation.

A firesuit for the car

Any material will combust or melt given enough heat and oxygen. Carbon fiber is stable to very high temperatures, but the resins and glues holding carbon-fiber composite together are flammable at lower temperatures.

NBC Sports analyst Steve Letarte broke down the most-recent NASCAR-mandated changes in the video below. The first fix seals places where bits of rubber can get into the car. The second fix replaces part of the polymer right-side door panel — the area closest to the exhaust pipes — with stainless steel.

A video from NBC Sport's Steve Letarte explaining the fixes NASCAR mandated to stop car fires

The only thing I’ll add to the video is that smaller pieces of fuel catch fire more easily. You can put a propane torch to a tire and it won’t burn. But marbles — small pieces of rubber and track grit — have a lot of surface area. That means a lot of places to initiate combustion. That may also be why the problem didn’t make itself known until actual races at tracks that collect a lot of rubber.

NASCAR suggested one more optional fix: intumescent paint. This paint is both a great piece of materials science and fun to say.

Intumescent (in-too-MESS-scent) means expanding, often in the presence of heat. Research on intumescent paint took off after 9/11 and the surprising way the steel in the World Trade Center melted. Intumescent paints are now required in many commercial buildings. They’re also common in drag racing.

Intumescent paint works much like Nomex. Heat swells the paint, but to a much greater degree. The video below shows an example of an intumescent paint used in construction.

A video showing intumescent paint expanding

Also like Nomex, the char cannot become fuel. The example below uses a regular piece of cardboard with and without protective paint.

A video showing how intumescent paint prevents cardboard from burning

NASCAR limits painted areas to where the exhaust passes close to the body panels and the right-side foam. Teams also have the option of painting the door foam.

There’s a small weight penalty, but it’s nowhere as big as the consequences of being knocked out of a playoff race because of a fire.