Optimized Wishbones for Reduced Lap Time

HBK Measurement Technology Optimises a Vehicle's Wishbone

In the international "Formula Student" competition, teams from colleges and universities compete against each other in a wide variety of disciplines with vehicles they have designed and built themselves.

One of the teams is "High Speed Karlsruhe" from the Karlsruhe University of Applied Sciences, which is constantly searching for new ways to achieve the fastest possible lap time.

Challenge

To achieve the best possible lap time, low weight is crucial for High Speed Karlsruhe's vehicle. This can be achieved by reducing the undamped masses, including the weight of the wishbones.

Solution

The idea is to manufacture the wishbones entirely from carbon fibre-reinforced plastic. Therefore, simulations were carried out to analyse the effects on the wishbone, which now have to be validated with physical tensile and compression tests.

Result

HBK strain gauges are bonded to the surface to determine how the actual forces correspond to the results of the simulation. Moreover, the results provide a basis for further vehicle tests and the design of future wishbones.

The Idea: A Wishbone Made of Carbon Fibre-Reinforced Plastics

To reduce the weight of the vehicle, the undamped masses located between the tires and the spring/damper package must be reduced first and foremost. This also includes the wishbones, which connect the steering knuckle to the chassis. To reduce their weight the wishbones were entirely manufactured from carbon fibre-reinforced plastic. Thanks to the new wishbones, the following benefits can be achieved:

Lower weight
Control through in-house production
Higher stability

In order to exploit the benefits, the action on the wishbones must be analysed in detail, for which simulations are carried out in advance. To verify these simulations, tests must be carried out with bonded strain gauges to determine how the actual forces correspond to the results of the simulation.

Preparation of the Measurement: Simple Installation of Strain Gauges with the HBK Starter Kit

For tensile testing on a test rig, the control arms of the vehicle must be prepared. To optimally record the forces, strain gauges must be installed in the direction of force application along the wishbones. This is done by using the HBK Starter-Kit, which contains all the necessary materials for the installation.

To install the strain gauges correctly, the surface must first be cleaned with a chemically pure cleaning agent (e.g. RMS1), which is included in the starter kit. Cleaning is easy using the enclosed non-woven pads. Subsequently, the surface is roughened in several passes with sandpaper and cleaned again with the enclosed RMS1. The strain gauge is installed on the wishbone using the fast-curing Z70 adhesive and the integrated fluoropolymer release film.

Preparation of the measurement

Installed strain gauge on CFRP wishbone

Arrangement of strain gauges on the wishbone

The Starter Kit is the ideal companion for initial experiments with strain gauges. Furthermore, the enclosed book provides valuable information about strain gauges and measurement circuits and is ideally complemented by video tutorials on the HBM website.

Measurement Setup and Execution of the Measurement

The tensile and compression tests are carried out in the laboratories of the Karlsruhe University of Applied Sciences to test the material connection of the strain gauges to the carbon fibre-reinforced plastic tube and to evaluate whether future tests would work on the moving vehicle.

For this purpose, the control arms are clamped in the testing machine. This is done using a specially designed and manufactured clamping device, which optimally fixes the control arm.

The voltage output of the HBK analogue amplifiers is measured with a multimeter and then compared with the results of the tensile testing machine.

Since only one of the two struts is used for the measurement at a time, the second strain gauge is used for thermal compensation with a quarter-bridge circuit. For this purpose, the settings of the values for the strain gauges, which can be found on the enclosed data sheets, are entered into the measuring amplifier and adjusted with an integrated rotary potentiometer.

The Measurement Results and Their Interpretation

The measurement was carried out according to the principle of linearization to enable different forces to be applied by the testing machine and the stresses to be read off at these points. This makes it possible to determine whether the strain gauges show an identical force curve as, in this case, the tractor.

A tensile test was carried out in which the tension was applied from the relaxed state in 5 kN increments up to 20 kN. This test was chosen because other control arms were damaged at > 25 kN in previous tests.

Measured data of the tensile tests

In the compression test, the load was pulled from a force-free position up to 7.5 kN in 2.5 kN steps. Here, too, a relatively linear progression can be seen.

Measured data of the compression tests

In an additional tensile test, the mounting for the control arm was destroyed. Nevertheless, linear behaviour of the force curve can be clearly seen.

Measured data of the traction unit during a breakage test

In summary, the findings obtained from the tests provide a basis for further vehicle tests and have also been considered for the design of new wishbones.

About High Speed Karlsruhe

For 10 years now, the "High Speed Karlsruhe" project of the Karlsruhe University of Applied Sciences has been participating in the Formula Student competition with specially developed and manufactured vehicles.

The Formula Student offers students of technology and economy the possibility to convert theoretical knowledge into realistic practical experience. The goal is the independent development, construction and production of a racing car by the student teams over one year. Currently, 44 students from a wide variety of courses and alumni of Karlsruhe University of Applied Sciences are working together on the development and production of vehicles.

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