2103483 Automotive System Design (2022/2)

Suspension Geometry

Base layout from design spec

wheelbase and trackwidth

Wheelbase : 1560 mm

Trackwidth : 1200 mm

Kingpin offset : 40 mm

Suspension geometry layout from observation

It has been observed that the layout of the suspension geometry of the top teams in the competition is characterized by a sloped Upper A-Arm both front and rear, not parallel to the floor, and the Lower A-Arm both front and back are also slope but less. This suspension geometry was designed to keep the roll center intersection as low as possible.

Suspension geometry design

ROLL CENTER

The roll center positions of the front and rear suspension geometry are the main features affecting the front and rear axles' lateral load transfer rates. Their position and difference in height from front to rear can be used to tune the car's handling behavior (understeer and oversteer).

If a roll center is too high, it will induce a jacking effect on each corner and increase body roll. Therefore, we will design the roll center to be positioned as close to the road surface as possible to reduce them. We also set the position of the rear roll center slightly higher than the front roll center to reduce understeer and retain good controllability.

A-Arm geometry constraint

The width of the A-Arm is limited by FSAE Competition Rule T.1.2 - Internal Cross Section, which states that the cockpit must have a free internal cross-section, and The template, as shown, must pass through the cockpit. The template from SAE is 350 mm in width.

From rule T.1.2 and the kingpin offset value that has been set initially to 40 mm from the wheel's center line. We will get the boundary of the A-Arm position, which is in the red line zone, as shown in the picture. The inner A-Arm mounting point must be at least 400 mm from the center line.

Suspension geometry Optimization

We have used a suspension analyzer to tune and analyze various parameters of the suspension geometry (roll center position and gain, Ackermann toe in error, etc.) to optimize suspension geometry. We also use the suspension template, which includes in the suspension analyzer, then adjusts the coordinate values to match the constraints, as well as change the position of the front roll center to the road surface and the position of the rear roll center is slightly higher than the front, which gives a draft of suspension geometry as pictured.

Front roll center

Rear roll center

After the optimization is complete, we can finalize that our suspension geometry has the position of the front roll center of 0.15 inches (3.81 mm) below the ground, and the rear roll center is 1.51 inches (38.354 mm) above the ground, which is slightly higher than the front, as intended.