DESIGN OF WHEEL ASSEMBLY OF A FORMULA STUDENT CAR
BACKGROUND INFORMATION
The wheel assembly is an integral part of the vehicle which comprises of several components that are Upright, Spindle, Rotor and the wheel. The main function of this assembly is to transfer the forces generated at the tyre to suspension and the chassis. Every part needs to be engineered to take into consideration variables that will impact the overall assembly and its performance. These consist of brake fade, un sprung mass, camber, and the Ackermann steering geometry.
ACC 20C Design Goals
The major design goal for 2020 vehicle were as follows (Priorities written in descending order):
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Reliability: The most important goal was to build a car which would clear all scrutiny tests and successfully complete all dynamic events.
Drivability: Focus was given on good driver ergonomics and an interface that would allow the driver to respond quickly to the feedback received from the car; empowering him to extract the most out of the car and improve lap times.
Simplicity: With emphasis on simple design and part count reduction, the team aimed at maximizing time for testing various systems and for driver training.
Manufacturability: Designing components to aid accurate, simple and fast manufacturing to maximize testing time and ensure easily replaceable parts.
Weight: A target weight of 175kg was set, a 15kg reduction from the previous iteration.
CHALLENGES
Improve Reliability
Unlike in previous vehicle, no Fatigue failure for upright considering 500 kms of testing prior to competition.
Reduce Manufacturing Cost and Time
Design components in such a way that they could be manufactured in house. This would drastically reduce manufacturing time and cost.
Packaging Wheel Assembly in R10 Rims.
We decided to switch from R13 to R10 rims for our 2020 vehicle. This significantly reduced the space available for packaging the wheel assembly than previous iterations.
SOLUTION IMPLEMENTED
Finalize suspension pickup points and fix the geometry of Upright.
Using the upright and wheel position fix spindle dimensions.
Fix Caliper and Rotor position to achieve maximum possible effective radius without interference with rim.
Basic Geometry for Wheel Assembly fixed.
MATERIAL SELECTION FOR UPRIGHT
The next step after finalizing the Upright geometry was Material Selection. The two main materials considered for manufacturing of uprights were Aluminum and Mild Steel. Both had their own advantages and disadvantages. For Aluminum upright the design would have to be machined from Aluminum T6 - 6061 whereas for Mild Steel I could manufacture it by welding Mild Steel Sheets.
Aluminum
Cons
Pros
Mild Steel
Pros
Cons
The weight of the Upright could be significantly reduced if manufactured using Aluminum.
Using Aluminum for Upright would significantly increase the manufacturing cost and time as we didn't have in house Aluminum machining facility. Also the cost of material is more than Mild Steel.
The fatigue life of Aluminum is not long enough. Due to this Aluminum cannot be used for large number of cycles. This would reduce the reliability of the whole system.
MS Sheet Metal welded Uprights could be manufactured in house. This reduced our manufacturing time and cost.
The weight for Mild Steel Upright would be significantly higher than Aluminum Uprights.
The fatigue life of Mild Steel is higher than Aluminum. This allowed us to use same set of Uprights for testing and competition.
As Reliability and Manufacturability were on a higher priority for 2020 vehicle's design, I decided to implement Sheet Metal Welded uprights instead of Aluminum machined uprights.
ITERATIONS
In my first iteration I designed a Upright with Mild Steel casing inside it. But during testing I found out that the bearings were getting damaged as the tolerances inside the casing could not be maintained due to welding contraction.
To counter this I came up with another iteration of upright where a C45 axle was welded to Mild Steel sheets. Bearings were put on this axle which were then press fitted into the Spindle.
For Spindle I used Aluminum T6 2014. This provided me optimum weight to stiffness ratio. Brake Rotor was assembled with the Spindle using Brake Buttons and E Clips.
Individual and assembly analysis of the components was done using Ansys R 18.2.
MANUFACTURING
While manufacturing I had to make sure that the geometry of uprights didn't change due to welding contraction. To ensure this I designed the fixtures for manufacturing of Upright. Mild Steel Laser cuts were welded together to form the final design.
FINAL ASSEMBLY
