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How Load Cells are used to Optimize Race Car Performance

Since the earliest days of auto racing, attempts have been made to engineer cars that will go faster and handle better. The difference between winning and losing can often be measured in fractions of a second. Excess weight can result in slower times, but the angle of the car’s nose and the car’s ability to “stick” on curves are also critical to performance. Load cells can help optimize performance, and they are currently used throughout the auto racing industry from NASCAR to the IHRA.

In simple terms, a load cell is a sensor utilizing strain gauge technology. When force is applied, it causes deformation or movement that can be measured. Releasing the force also produces a measurable change. However, load cells themselves are passive, mechanical devices, and they must be linked to a second device, such as a computer or a digital display, to produce meaningful data.

To understand how load cells can prove beneficial for race cars, consider certain basic laws of physics. Newton’s third law of motion states that whenever force is applied, an equal amount of force will be generated in the opposite direction. Perhaps the most obvious real-world example with which most people are familiar is what happens to the occupants of a car during emergency braking, commonly called a panic stop. As the car’s momentum is arrested, the occupants’ bodies continue to move forward at the previous speed. This is in keeping with Newton’s second law — that objects in motion tend to remain in motion unless an external force is encountered. However, once the occupants’ forward momentum has been halted, their bodies will be forced backward with the same force as they moved forward.

The second law of physics involved with load cells for racing is the concept of centripetal, or inward, force. Consider the passengers in a car that makes a right-hand turn at a high rate of speed. During the turn, they will feel that they are being moved to the left. In reality, their bodies are trying to continue moving in a straight line (Newton’s second law), which is no longer possible as the car itself has taken a new direction.

Although other properties of physics apply as well as the math behind factors such as the angle of a banked turn relative to speed, little is to be gained by discussing them at this point. The two examples given are sufficient to understand what happens to a race car at high speeds and why load cells can help engineers improve performance. However, it should perhaps be noted that the car’s center of gravity and rear axle torque play important roles in keeping the car’s nose down, which in turn allows slightly greater speeds.

The laws of physics discussed to provide the clues needed to understand why race cars sometimes spin out or slide sideways on curves. The wheels turn, but the chassis wants to continue in a straight line. Even if the driver retains control of the vehicle, precious milliseconds can be lost. If the driver must slow before entering a turn, even more time can be lost.

A different (but related) problem exists when the car encounters a bump in the pavement. Shocks and springs compress to mitigate the jarring, but they then bounce back. If the shock has been severe, the resulting rebound may force the car’s nose up even higher than it was before the bump. Keeping the car’s nose glued to the track (figuratively speaking) has been a goal since the earliest days of racing as it increases speeds.

Load cells allow engineers to measure the weight supported by each tire and the movement of each wheel during actual operations. The data can reveal how the chassis responds to various speeds, bumps, hard braking, and turns. This allows engineers to make alterations — sometimes extremely small changes — to improve performance.

A case study can provide additional insight. The Cornell Racing FSAE team uses load cells to help optimize performance. The Formula SAE competition is held annually, with 140 schools and 12 countries represented. The Cornell team places in the top 10 virtually every year, and Cornell has won the world championship seven times.

The team chose Transducer Techniques’ MLP-1k load cells to measure the forces exerted on each of the car’s corners and the CSP-3k load cell to measure the forces to which the drive train subsystem was subjected. The data collected allowed the team to refine the suspension system, unsprung parts, and numerous zones within the composite monopod. It also allowed an accurate determination of the drive train components’ fatigue life. As a result, the design team was able to shave weight off all four areas. (A video interview with three of the Cornell team members is available by clicking on this link.)

Although race cars primarily use load cells for suspension systems, engineers are currently using or testing load cells for other areas. Many dragsters are equipped with wheelie bar load cells to measure the launch forces exerted on the wheelie bar. Load cells can measure the force exerted on a gearshift by the driver to help determine how this relates to transmission wear. The brakes are another area that can employ load cells since brakes in a race car respond to the pressure exerted by the driver rather than pedal travel. No doubt, racing engineers will continue to discover new uses for load cells to optimize performance.

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