Project Description

The old trailing arm was heavy and lacked a simulated safety factor. The new project had to meet safety factor requirements and should be lighter than the old design.

Performance Review

Ground Clearance: After assembly, the minimum ground clearance of the trailing arm was measured to be 133mm, with the lowest point of the assembly being the motor mount bolts (excluding the wheel).

Weight: The final weight of the trailing arm, motor mount, and fasteners was measured to be 12.6 lbs using a kitchen scale.

Toe Adjustability: The toe adjustability was tested using trigonometry proofs. By turning the rod end, the length of one side of the drawn triangle is increased which changes the interior angles of the triangle. Using this and some trig function the length that the rod end has to be can be calculated.

Trailing Arm Safety Factor: The trailing arm was designed to withstand a worst case, simultaneous 2G bump, 1.5G turn, and 1G brake force applied at the contact patch of the tire on the ground. To ensure this design requirement was met, the team used Altair Inspire’s FEA to predict the minimum safety factor to be 1.3.

Motor Mount Safety Factor: Similarly to the trailing arm, the motor mount was designed to withstand a 2G bump, 1.5G turn, and 1G brake force applied at the contact patch of the tire on the ground. Using FEA from Altair Inspire, the minimum safety factor of the motor mount is predicted to be 1.9.

Rod end Safety Factor: To find the minimum safety factor of the ¾ rod end, a static analysis of the trailing arm was performed to calculate the worst-case load applied to the rod end. The safety factor of the rod end was then calculated to be 1.46.

Design Description

Design Description: The rear suspension system consists of a brake caliper mount (1), motor mount (2), rod end (3), trailing arm (4), and angled shock (5).

1. Brake caliper mount: The brake caliper mount is a metal bracket fastened to the rear of the trailing arm. This placement allows the brake cable to be routed under the car and eliminate interference with the shock.
2. Motor mount: The motor mount is made from aluminum 6061-T6 to reduce weight. It mounts to the back plate of the trailing arm to reduce the length of the rear suspension.
3. Adjustable Rod End: An adjustable rod end connects the trailing arm to the chassis of the car. It can be extended or retracted to quickly adjust the toe angle of the rear wheel.
4. Trailing Arm: The trailing arm is made of 4130 plate steel. This is a higher strength steel than the previous design. This reduces the amount of material required and weight. It is optimized using FEA software to remove unnecesary material and further reduce weight.
5. Angled Shock: The shock is mounted at an angle. The mounting point on the trailing arm is further back than on the previous design. The shock and mounting hardware are reused from the previous design to reduce cost.

Conclusion

Our design met all major design requirements. The actual deign varies from the modeled design, most notably in the ground clearance (170mm, 133mm) This descrepancy is due to the way that the trailing arm was welded to the chassis. Rather than welding the trailing arm level to the car it was welded so that when weighted the sag brought it to level. The toe adjustability also was slightly better than we modeled due to the movement in the spherical bearing and rod end which were not modeled with all degrees of freedom in SolidWorks

Lessons Learned

The team gained valuable skills in multiple fields, including hardware selection, manufacturing, presentation skills, software skills, and communication skills to list a few. Within designing the brake mount, the importance of seeing the whole design became clear. The brake mount appeared to align in SolidWorks and was cut out to match the file created. Once cut out the part required some welding, at this point it was noted that the heads of the bolts holding the brake mount to the trailing arm would interfere with even a small welding bead. Because of this the team had to outsource welding to a shop that knew how to tig weld since it has a smaller bead that didn’t interfere with the bolt heads. With that overcome another issue arose. The placement of the bolts one the drivers left side are very difficult to reach when the wheel is attached due to where the brake rotor is. While not a major flaw this issue could be fixed if more thought was put into the base design.

Future Works

In the future, additional testing for the service interval and longevity of the trailing arm should done. There are design requirements for the trailing arm to last 200 hours between service intervals. At the time of making this poster 200 hours of testing has not been reached so it is not clear if that requirement is met.