Hot New Data on Driver Cooling

Driver cooling seems to be an issue almost every week. It seems like at least one driver has a cool shirt failure each race. A.J. Allmendinger needed medical attention after COTA due to high ambient temperature and running a road course, where cockpits typically get hotter than other types of tracks.

Some drivers don’t use cool shirts. Some drivers have, shall we say, interesting ideas about how thermodynamics works.

Let’s look at cool shirts, whether McLeod’s argument holds, and a new study that suggests a better alternative to cool shirts.

Rating Hot Drivers

To understand the driver cooling issue, we need to consider two different temperatures:

• Core temperature is the temperature of your internal organs. Your brain, heart and liver produce a lot of heat while doing their jobs. Normal core temperatures are from 96.8 ºF to 99.5 ºF.
• Skin temperature is the temperature of the outermost surface of your body. The skin is the largest organ in the body. It plays a significant role keeping your core temperature within safe levels.
  • If your core temperature drops, your skin can warm the blood traveling through it and that heat goes back to the core.
  • If your core temperature rises, your skin can help cool the core by sweating.
  • Normal skin temperature ranges from 92.3 ºF to 98.4 ºF.

Why Worry about Hot Drivers?

Your body can adapt to higher and lower temperatures, but only to an extent. The hotter your body gets, the less well it works. If your core temperature rises to the level of heat stroke, your organs can fail and you can die.

But temperatures well below heat-stroke-level are still dangerous, particularly if you’re driving at 150 mph. According to David Ferguson, Associate Professor of Kinesiology at Michigan State University, once a driver’s core temperature reaches 102 ºF, he experiences around a 15% reduction in neuromuscular action. That might manifest as muscle cramps, not being able to press the brake pedal as hard, or general fatigue. Performance degradation increases exponentially as core temperature increases.

Overheating also affects reaction time — not just physically how long it takes you to respond to something, but also how quickly you think. That hole between two cars ahead might appear bigger to a driver struggling with heat exhaustion and lead to The Big One at Talladega. An overheated driver might not be fast enough on the brake slowing down on pit road and get a speeding penalty.

While many engineers crow that they would rather have a fast car than a cool, happy driver, Ferguson points out that a cooler driver is a better driver.

Are Modern Drivers Just Wimpier?

Cool shirts are a recent arrival to NASCAR, mostly because car cockpits weren’t as hot in the past. As aerodynamics became more important, engineers sealed up the cockpit to prevent air from getting in and increasing drag. The Gen-7 car has its own additional challenges in how radiator air travels out of the hood louvers and toward the cockpit, plus engine exhausts on both sides of the car.

Back in the early 2010s, for example, a driver might have multiple fans blowing on his hands, feet, crotch, etc. That’s not an option today, in part because NASCAR has instituted rules about how many ducts can be used to bring air into the car, and exactly how they can be routed. It’s hard to believe, I know, but engineers have tried, in the past, to use driver cooling lines for things other than driver cooling.

How Cool Shirts Work

I’ve explained the details of driver cooling elsewhere, but to summarize:

• A cool shirt is a fire-resistant long underwear top with small tubes woven into the fabric.
• Water comes from a refrigerator unit to the shirt. That water is theoretically cooler than the driver’s skin temperature.
• Because heat always moves from high temperature to lower temperature, heat from the driver’s body warms the water.
• The warm water returns to the refrigeration unit, which pulls the heat out of the water, making it cooler.
• The cooler water returns to the shirt.

Cool shirts have a number of downsides:

1. Refrigerators require power, which must come from the car.
2. Refrigeration units are heavy. The more powerful they are, the heavier they are. Weight is a liability in racing.
3. Refrigeration units must be cooled themselves if they are to efficiently exhaust the heat from the warm water. That requires diverting air into the cockpit through a NACA Duct, which negatively affects the aerodynamics.

How Cool Shirts Fail

It is possible for the tubes in a cool shirt to get clogged with algae or other gunk, or for air bubbles in the water to cause pump failure. But neither event is likely if you follow manufacturer instructions for cleaning and using the shirts.

The most common failure mode is due to the same phenomenon that causes engineers to run tires below Goodyear’s recommended tire pressures to try to get just a little more speed from the car. It’s a tradeoff: You can go faster, but at the cost of a possibly blowout. The tradeoff when it comes to cooling systems is that you might have a driver too ill to perform well.

If the refrigerator/pump unit doesn’t get cooled enough, it can’t efficiently pull heat from the water returning from the driver. The next time that water reaches the driver, it’s warmer. Enough cycles like that, and eventually the water is heating the driver instead of cooling him.

Most of the time, cool shirts in NASCAR fail because of choices teams make in implementation and specifications.

A Scientific Study of Driver Cooling

Ferguson and colleagues have just released a new study that compares driver cooling solutions.

• No cooling
• A cool shirt, which works by conducting heat away from the skin.
• A helmet blower, which sends air into the driver’s helmet. The air can be directed at the top of the head or directly at the face. Convection and sweat evaporation provide cooling.
• A suit blower is like a helmet blower, but directs air into the driver’s firesuit rather than his helmet.

The cool shirt picture is for illustration only. It is not the brand tested in the paper. Thanks to David Ferguson for the photo of the suit blower used in his study.

The Experiment

Volunteers with body composition and aerobic capacity similar to elite drivers bicycled in full race gear in an environmental chamber at about 90 degrees and 80% relative humidity. Those parameters mimic a heat index of 111.2 ºF.

Each participant cycled for one hour the hour of cycling with each of the cooling solutions, although some stopped some runs early if they felt they were getting too warm.

The scientists measured subjects’ core and skin temperatures, the percentage of body mass lost in sweat, and the person’s subjective opinion about how cool they felt for each trial.

If you’re wondering about the body composition of a typical elite driver, we’re talking body fat below 22% for men and 28% for women. For comparison, the Centers for Disease Control estimates the average U.S. man to have 28.1% body fat and the average U.S. woman to have 39.9% body fat.

Ferguson’s original research proposal planned to test each driver cooling option two ways: using ambient (cockpit) air for cooling, and using refrigerated air. They had to drop the ambient air options because their subjects’ body temperature rose too much.

Ferguson has spent a lot of time at racetracks and with race teams. He estimates cool shirt systems fail about once every five times in their field from his observations. There aren’t any statistics on the topic. The particular cool-shirt system they tested (one used by many NASCAR teams) failed about half the time in the lab conditions of about 90 degrees and 80% relative humidity — and that’s with them being careful to keep the cool shirt cleaned and operated per manufacturer requirements.

Results and Caveats

Before results, let’s look at caveats. Identifying potential limitations is part of any scientific study. Don’t trust someone who doesn’t critique their own experiment.

For example: While they tried to mimic the conditions of an actual race car cockpit, the study wasn’t done on a racetrack in an actual race car. They studied one kind of cool shirt and one blower system; others might perform differently.

Their helmet blower blew directly on the wearer’s face. That can be a problem because of dry eyes and mouth, but also because drivers have to communicate with their teams during the race and the stream of air can be loud.

But I noted one limitation race teams might want to think about. The group discovered that the fans used to blow air into helmets and suits generate their own heat. That’s something you wouldn’t notice in a fan sitting in a window. But when you have a fan feeding directly into tubing, that could be important.

Results

At the midpoint of the trial, participants’ average skin temperature was highest without any cooling, followed by the helmet bower, the cool shirt and the suit blower. Importantly, the group emphasizes, the average skin temperature in the suit blower did not change much after the midpoint, whereas the other cooling modes (helmet blower and cool shirt) did produce a continued rise in skin temperature.

At the end of the exercise trial, the cool shirt produced the highest average core temperatures, followed by the control, the helmet blower and the suit blower.

The suit blower seems to have the advantage of all the different methods.

That’s a rather important finding: the cool shirt actually produced a higher core temperature in this study than no cooling did. That’s after excluding any equipment failure runs.

It sure seems like there needs to be some kind of quick disconnect the drivers can activate when a cool shirt becomes a heating pad.

So Why Aren’t Drivers Using Suit Blowers?

Because no one sells them.

Stand21 made them back in the 2000s, but stopped because they didn’t sell well enough. Remember that drivers had not-so-airtight cockpits and more cooling tools than they have now. Perhaps this study will get someone interested in trying to get a suit/system FIA/SFI approved.

What About McLeod’s Comment?

It is true that it takes longer to cook a big turkey than a little turkey, but that analogy doesn’t work for people because turkeys are dead when you cook them. All of the heat going into raising the turkey’s temperature comes from its surroundings.

In contrast, a driver in a cockpit generates his own heat, in addition to being heated by cockpit conditions. In addition to your internal organs generating heat, muscle also generates heat.

Ferguson explained that a 170-lb man produces about 1800 calories of energy per day. (That’s why a typical caloric recommendation for people is around 2000 calories a day. The food is where your body gets the energy to work)

“If I added 40 pounds of muscle,” Ferguson said, “my heat output would increase to 2600 calories a day.” So a ‘meatier’ driver, as McLeod suggests, would actually produce more heat than a thinner one. That’s why, Ferguson pointed out, women tend to tolerate heat better: They don’t have as much muscle mass and thus produce less heat. (That’s also why the caloric recommendations are lower for women than men.)

Without intending to cast any shade on B.J., I couldn’t resist asking Professor Ferguson how the situation would change if that 40 pounds was fat instead of muscle. Fat doesn’t produce heat.

“Fat is a good insulator,” Ferguson said. That’s important because your body decreases your core temperature via blood circulation. Heat travels from inside your body to your skin via blood. That process is much more efficient in someone with a smaller amount of body fat that someone who is better insulated.

So while B.J. McLeod’s reasoning is perfectly valid for turkeys, it doesn’t quite work for race car drivers.