SunRayce N’Energy Car Suspension Analysis
SunRayce is a nation wide competition that allows college teams to design, build and race solar cars. The Northwestern Solar Car Team built a car that competed during the summer of 2001. Currently the team is preparing to build the second-generation car, improving on previous efforts. The team asked our group to assist them by collecting data on the forces of the suspension. With this information, a future design can be optimized for lighter weight.
As the car drives, various forces act on the wheels of the car. These loads are created by bumps or other imperfections in the road and by acceleration of the car. The former primarily produces forces that are normal to the road surface, and the latter produces forces that are primarily tangential to the road surface. Knowing the normal forces on the wheel will tell you the maximum tangential forces on the wheel. Therefore, the magnitude of the normal forces on the wheel is of primary interest to the solar car team.
To the left, a student is shown attaching the strain gages to the shock under analysis.
Here, the strain gages are seen close up after being mounted in a Wheatstone bridge.
SunRayce requested us to find the strain on the shock under various conditions so that they might use the data to better the car's suspension design. With this in mind we ran tests under various conditions. The testing conditions were chosen to closely model a variety of situations that the car might encounter during competition. Since our setup was complete to the point where the only thing we needed to do was run the LabVIEW program (at right) and drive the car, the program was run while the Solar Car was driving in the following scenarios:
1.1. Straight down a smooth road
2.2. Turning left on a smooth road
3.3. Turning right on a smooth road
4.4. Straight down a bumpy road
5.5. Running over a pipe .5'' diameter
In each of these situations, care was taken that car speed, weather conditions and other factors were kept as constant as possible.
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Shown here is the entire experimental setup including a wheat stone bridge of strain gages and a potentiometer with a custom bracket to measure displacement.
In accordance with this, the purpose of this experiment was to determine the magnitude and frequency of forces acting on the front suspension of the solar car. To do our experiment, we utilized each of the tools learned in the ME 224, Experimental Engineering course, from LabVIEW computer programming to circuit setup. Our experiment will improve the SunRayce vehicle and hopefully contribute to a strong Northwestern finish at this year's competition.
Above is a diagram of the LabVIEW coding used to collect data from the shock.
Data analysis revealed that the solar car was of robust design. Several calculations were conducted to determine the fatigue, yield and impact limits. The existing design exceeded all requirements with some safety factors of up to 60 times the expected loads. As might be anticipated, impact loading had the lowest safety factor. Future designs should aim to push these safety factors to much lower levels, as excess weight is the greatest consideration. Other material choices such as aluminum or magnesium alloys could provide the necessary strength without sacrificing weight.
Produced by Alexander Ellis, Ian Harrison, Lars Moravy, and Jonathon Walker – December 2001 – Prof. Espinosa, ME224