ACCELERATION AND G-FORCE - Hold on to your hat for this ride

Drag racing is an acceleration contest. A race in which two cars compete to see which one can achieve the highest average rate of acceleration from a standing start to a finish line 1320 feet away. Sounds simple enough, right? Let’s take a closer look at what goes on during that quarter mile contest with a focus on acceleration.

In regards to race cars the term Acceleration is the rate at which the speed of the vehicle is increased over a given period of time. The greater the increase in a specific amount of time, the higher the rate of acceleration. Although the average rate of acceleration can be calculated mathematically, on a racecar it is measured with a device called an accelerometer. This device is also known as a G-meter. This sensor maps the rate of acceleration force and plots it in relationship to a time line. It is part of the data acquisition system onboard the race car. Crew chiefs use the graph produced by the G-meter to see where their car is accelerating the hardest and where it isn’t. Without dwelling too much on the technical definitions of G-force suffice to say that one G would be equal to an increase in speed of 22 miles per hour in one second of time (three Gs would be a 66 miles per hour increase per second). Furthermore, when standing at sea level there is one G of gravitational force being exerted downward against your body which keeps it from being slung off the face of the earth. If you pointed a G-meter straight up it would register one G. In its horizontal orientation in a race car it registers zero Gs.

When a race car leaves the starting line it does much the same thing as your car does when you drive away from a stop light, just at a greater degree. Depending on how hard you accelerate away from the stop light you feel yourself being pushed back into the seat, your head wants to tilt backwards, and reaching forward becomes more difficult. It is not the speed that is causing this, these sensations are occurring at relatively low speeds. It is the rate of change in speed, or as explained above, the rate of acceleration. This explains how we can have high G-forces at slow speeds, and low G-forces at high speeds. By this we can also see that the higher the rate of acceleration, the quicker the car will cover the quarter mile distance. But wait, there is more.

Racers are fond of quoting the rate of acceleration (G-force) using the highest number recorded. In reference to a front engine top fuel dragster you might hear them speak of numbers between 2.5 and 3 G’s. But that doesn’t tell the whole story. In truth a crew chief is not only looking for the highest rate of acceleration the car can achieve, but more importantly, how long it can sustain that rate. Maintaining the highest G-force rate for the longest period of time, or the highest average G-force over the quarter mile, is the objective and it is sort of like a game of multiplication. A car that can record 2.5 Gs for only two seconds before the rate diminishes will be outrun by a car that records the same Gs for three seconds before the rate decreases (2.5 X 2 = 5 while 2.5 X 3 = 7.5). The higher total number has the advantage. By the same token, even a car that does not accelerate as hard can out run one that records higher momentary Gs if the slower car maintains a higher average rate over a longer period of time. For example, the car above that had the 2.5 G recording for two seconds will fall victim to one that can sustain a lower 2.0 G average for three seconds. Do the math.

So, with that knowledge in hand, you might be wondering what sort of actual G-meter readings are registered by AA/Fuel Dragsters?

Before giving you the numbers you should keep in mind that the rate at which a front engine top fuel dragster can accelerate is limited by a series of rules induced restrictions. The weight of the car versus the size of the engine can be a large factor. AA/FD’s must weight 4.6 pounds for each cubic inch of engine displacement. In line with that are various component restrictions that limit the amount of horsepower the engine can produce. Then you have the fact that these cars are mandated to run a specific rear end gear ratio and they cannot use a transmission. They must utilize a direct drive. This precludes them from using a gear ratio (or gear ratios) that would be to their best advantage. Finally they must use a spec tire which is narrow in comparison to one that would be most advantageous. Without large wings on the car to help plant the tires the crew chiefs are always looking for the optimum point of acceleration just shy of where the tires will lose traction which slows the rate of acceleration.

Okay, on to the numbers.

Most AA/Fuel Dragsters will achieve their maximum G-force reading about 3/10th of a second after it leaves the starting line. At this point the car will have traveled only about five feet, but the rate at which the speed is increasing is changing by the greatest amount and the G-force rockets up to over 3 Gs.

As mentioned earlier this high rate of acceleration is maintained for only a short period of time, usually for about one second, before it starts to taper off, slowly at first and then more rapidly as the car gains speed. Two seconds into the run the rate will decrease to around 2.5 Gs, followed by 2.0 Gs at three seconds, and 1.5 Gs at the four second mark (just past half track). Are you starting to understand why you feel a lot of pressure against your body when taking off in an airplane, but once you near cruising speed the sensations of acceleration are gone? Even though you are flying through the sky at around 500 mile per hour you feel like you are stationary. The plane is no longer increasing its speed in large gobs, so at a constant velocity the rate of acceleration (and horizontal G-force) has dropped to near zero.

As a final note about acceleration the G-forces felt by the drivers of front engine top fuel dragsters is equal to that experienced by the crew of the space shuttle on launch. You will also be interested to know that for each unit of G the rate increases, the feel of the weight of your body also increases. At three Gs a human head, which weighs about 35 pounds, will now feel like it weighs three times as much (105 pounds), which explains why new drivers who aren’t accustomed to rapid increases in G-force will often have their helmet slam back against the roll cage upon launch.

Like we said, hold on to your hat.


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