Strain Gauges Ensure Design of Safe Racecar Components

The Academic Motorsports Club Zurich (AMZ) has been participating for many years now in Formula Student competitions around the world. Formula Student is a series of competitions in which racing cars built by students are raced against each other. In addition to top speeds, Formula Student is mainly about the intelligent and innovative construction of the vehicles.

AMZ frequently reaches pole position, especially in engineering. The team from Zurich won the "Best Engineering Award" in each of the four Formula Student competitions in 2013 - a record.

This engineering performance achieved by students is based on comprehensive planning and testing of the completely electrically driven vehicles. The AMZ team therefore relies on comprehensive testing using HBM strain gauges during modeling and FEM simulation validation.

Validating Modeling and FEM Simulations

The results derived from FEM simulations of many components are directly dependent on the modeling. Since the modeling already includes numerous simplifications and assumptions, errors can accumulate. These errors then flow directly into the simulation, which has a direct influence on the appearance of the finished component. 

The FEM simulation also includes numerous unknowns due to the complexity of the dynamic load behavior and material uncertainties. An inexperienced engineer can rapidly draw false conclusions from FEM analysis and put under-dimensioned components into circulation. If components are too small, in the case of the AMZ, not just the vehicle is endangered, the driver is as well.

Until now, the model used by AMZ was not validated. This was a reason for the experiment. Another reason lies in the validation of the aerodynamic downthrust coefficient. The wings of the vehicle are initially tested during the development phase using CFD simulation on a computer. The problem is the components can only be validated after the racing car has been built.

In order to aerodynamically improve future car generations, it is extremely important to verify the calculations so the quality of the developed construction can be effectively determined. The components are tested in a wind tunnel, and the SG tests can demonstrate the precise properties of the vehicle on the racing track. The wind tunnel is good for direct comparisons of various settings due to the uniform conditions, but it is not optimal for measuring the precise downthrust values in racing operation. These parameters can be directly calculated when the push rod forces acting in operation are known.

All these reasons provided the incentive to carry out proper SG measurements. The model errors and the push rod forces required to determine the aerodynamic downthrust values were obtained using SG and experimental stress analysis.

The Measurement Sequence

The complete measurement process using strain gauges was critical for the success of the AMZ Formula Student project. To ensure that the results actually make sense, it must be assumed that at least the mechanical work has been carefully implemented. This applies most importantly to the application of the strain gauges. Complete calibration of the strain gauges was also implemented before the day of the test.

Test Procedure

Four tests were carried out at the Hockenheim Ring to cover the forces required for the validation:

  • Curve driving
  • Braking
  • Autocross
  • Linear driving

Three differently sized curves were driven during the curve driving test. This allowed different transverse accelerations to be driven. During the braking test, the vehicle was accelerated to maximum speed and then brake at full strength. In the autocross test, a fast autocross track was traversed. The track included all essential elements present in the Formula Student races and required for test operation. These elements included a slalom, a fast curve where braking into the curve was necessary, a straight section where the vehicle could accelerate to maximum speed and other smaller elements. The track length was 422 m. Finally, various tests were implemented on the straight track section at constant speeds.

In order to validate the load cases, the acceleration curve needed to be measured as auxiliary information to the strain gradient. This was measured directly with the G sensors integrated in the vehicle. As these sensors were logged in a separate measurement computer, the two module times needed to be synchronized. This was implemented by pressing four times on the vehicle at the start of each measurement.

This allowed the strain in the SG and a shift in the spring-damper potentiometer to be recorded. Both times could then be synchronized during data analysis.

Significant Differences from Simulation Values

QuantumX data acquisition modules: CX22-W and MX1615


With the help of these measurements, both the load cases in the suspension and the downthrust coefficients could be validated.

Fifteen percent higher loads than calculated were measured in the load cases. The reason for the higher loads are stress peaks due to vibrations of the vehicle caused by unevenness in the road, something not taken into account in the models. The safety factors in the suspension will therefore be adapted on the basis of the load case validation so that these stress peaks can be taken into account and greater security against failure can then be achieved.

During measurement of the downthrust coefficient, values were 10% smaller than those measured in the wind tunnel. This difference is again due to the vibrations in the vehicles and the unevenness of the road - conditions different than the conditions in the wind tunnel.

The SG measurements have led to better understanding of the behavior of the vehicle on the racing track and the data obtained means the set up of the new racing car for the next season will be easier to design.

During their investigations and tests, AMZ not only relied on the HBM strain gauges, but also relied on the QuantumX MX1615 data acquisition system for measuring with strain gauges and the data recorder module CX22-W used for local storage of the measured data.

The QuantumX MX1615 bridge amplifier enables the connection of up to 16 strain gauges per module. Thanks to the carrier frequency technology, the module provides secure measurement results, even under difficult application conditions.

About AMZ

The Academic Motorsports Club Zurich (AMZ) was founded in 2006 by students of ETH Zurich, who - as the only team in Switzerland - develop a prototype each year for various Formula Student competitions in Europe.

Learn more about AMZ

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