What Is G-Force? How to Calculate G-Forces

G-force is a way to measure rapid acceleration or deceleration commonly experienced by astronauts, pilots, race-car drivers and roller coaster riders. We measure g-force with the unit g, where one g equals the normal pull of gravity on Earth's surface above sea level.

Basically, g-force is a way of comparing intense acceleration — in space, in the air or on land — to the regular, everyday acceleration we experience due to gravitational force. Acceleration due to gravity is 32 feet/second² (9.8 meters/second²).

During a space shuttle ascent, astronauts experience an acceleration of up to 3 g — three times the acceleration we're used to on Earth. Aerobatic pilots and fighter pilots may experience 9 to 10 g when they accelerate or make sharp turns in their aircraft.

When subjected to high g-forces for even a few seconds, humans can develop tunnel vision and eventually lose consciousness (g-LOC) as the acceleration causes blood to move from the brain to the lower body.

So, how does a pilot or astronaut stay conscious when subjected to the significant g-forces involved in launching a rocket or making sharp turns on a plane?

Controlled exposure to rapid acceleration, such as g-force training in centrifuge machines, can prepare the human body for rapid acceleration. Because individual tolerance for g-force can vary from day to day, a military pilot may do a g-force warm-up sequence in the air to test their tolerance before attempting more complicated aviation maneuvers.

Pilots and astronauts also wear special suits, called g suits, to prevent blood from pooling in their lower bodies and help them remain conscious. These suits compress the legs by filling with air or water once a certain magnitude of g-force is reached.

On April 29, 2001, CART (Championship Auto Racing Teams) officials canceled a race at the Texas Motor Speedway because the drivers experienced dizziness after as few as 10 laps. The combination of high speeds and tight turns at Texas Motor Speedway can produce forces of almost 5 g in the turns.

At 5 g, a driver experiences a force equal to five times their weight. For instance, during a 5 g turn, there are 60 to 70 pounds (27 to 32 kg) of force pulling the driver's head to the side. Let's see how to calculate how many g a car pulls in a turn and how these Champ cars can stay on the track under so much force.

Calculating the g-forces on the drivers is quite simple. We just need to know the radius of the turns and the speed of the cars. According to Texas Motor Speedway's Track Facts, the turns on the track have a radius of 750 feet (229 meters). During practice, the cars turned laps at around 230 miles per hour (370 kph).

When a car goes around a turn, it accelerates the whole time (this is why, when you make a turn in your own car, you feel a force pulling your body toward the outside of the car). The amount of acceleration is equal to the velocity of the car squared divided by the radius of the turn:

Let's run the numbers:

Banked Turns

How can the car stay on the track under this kind of force? It's because of the banked turns.

The Texas Motor Speedway has 24-degree banking in the turns. The banking doesn't affect how we calculate the g-forces on the driver, but without the banking, the cars could never go around such a tight turn at 230 mph.

If a Champ Car tried to make a flat turn at 230 mph, it would slide right off the track because it doesn't have enough traction. Traction is proportional to the weight on the tires (the more weight, the more traction).

Banking a turn allows some of the g-forces created in the turn to increase the weight on the tires, increasing the traction. To figure out what portion of the g-force gets adds weight to the tires, you multiply the g-forces by the sine of the banking degree.

So, with a 24-degree banking, 1.93 g adds weight to the wheels. In addition, a portion of the 1 g from Earth's gravity also puts some weight on the tires: 1 g x cos24° = 0.91 g. Together, 2.84 g (or 2.84 times the car's weight) push down on the car during the turn, helping it stick to the track.

Downforce

The car's aerodynamics also create significant downforce at 230 mph. On an airplane, the wings provide lift. A Champ Car has spoilers like upside-down wings, providing the opposite of lift: downforce. The downforce keeps the car glued to the track with a downward pressure provided by the front and rear wings, as well as by the body itself.

The amount of downforce is amazing — once the car is traveling at 200 mph (322 kph), there is enough downforce on the car that it could actually adhere itself to the ceiling of a tunnel and drive upside down! In a street-course race, the aerodynamics have enough suction to lift manhole covers — before the race, all of the manhole covers are welded down to prevent this from happening!

Between the downforce and the g-forces, well over four times the weight of the car holds the tires to the track when it goes around one of those 24-degree banked turns at 230 mph.

Final Thoughts

Drivers take an enormous amount of punishment on a track like this. This level of acceleration is higher than most people ever experience. Even the space shuttle only develops 3 g when it takes off. Even more impressive is how long these drivers tolerate this kind of force.

The Texas Motor Speedway is 1.5 miles (2.4 km) long: The front stretch is 2,250 feet (686 m) long, and the backstretch is 1,330 feet (405 m) long.

At 230 mph (337 f/s), the drivers take about 6.5 seconds to go down the front stretch, and then they are slammed by almost 5 g of force for the next 6.5 seconds as they go around the turn. It only takes about 4 seconds to make it down the backstretch before the next turn and another 6.5 seconds of almost 5 g.

If the planned 600-mile (966 km) race had taken place, the drivers would have gone back and forth between 5 and nearly zero g a total of 800 times.