FSU Double Wishbone Design Parameters ... in Progress

Steering Axis Inclination (SAI) is the angle between the steering axis (the line around which the wheel pivots when turning) and the vertical axis when viewed from the front of the vehicle. This is used to calculate scrub radius and strongly affects steering. Average values range from 6-14° and large SAI appears to have a negative effect on steering

We want a scrub radius of ~= 20 mm. Using the existing geometry, you calculate the angle required, and that becomes our steering axis inclination.

Update: Due to geometry constraints, the inclination was changed to ~6° to optimize the movement of the shocks.

Scrub radius is the distance between the point where the SAI intersects the ground and the center of the tire’s contact patch.

• Positive scrub radius occurs when the steering axis intersects the ground inside the tire's center, leading to more feedback through the steering wheel, often enhancing responsiveness but making steering heavier. However, it increases the energy required to steer and can add friction, making it less energy-efficient for general driving

• Negative scrub radius happens when the intersection point is outside the tire’s center, reducing steering effort and improving stability, especially under braking, but can reduce road feel.

Optimal Scrub radius is somehwere between 15-10 mm either positive or negative. This value is often heavily impacted by suspension gemetry and was initially intended to be negative.

Update: Scrub radius was made positive and slightly larger so that during turning motions the scrub radius would remain closer to 0. Scrub radius was left undefined in the nodal diagram to allow more freedom in defining the lengths that actually needed to be machined.

Caster is the angle of the steering axis (NOT THE SHOCK) when viewed from the side of the vehicle. It’s the tilt of the axis around which the front wheels pivot when steering. The steering axis is most comonly defined by upright geometry. There are two types:

• Positive caster: The top of the steering axis leans toward the rear of the vehicle. This creates better straight-line stability, helps the wheels return to center after a turn, and enhances steering feedback.

• Negative caster: The top of the steering axis leans toward the front of the vehicle. This one sucks

Positive caster creates a self correcting effect in the vheicle and in speaking with other teams standard values range from 3-5 degrees. Lower caster give less steering correctiong and higher caster increases the force required by the driver. From literature values of caster can be anywhere from 0.5-7° which higher caster often seen in performance cars. MS15 used 3° and people really liked the way it self centered so we’ll stick with that for now.

Toe refers to the angle at which the wheels point relative to the centerline of the vehicle when viewed from above.

• Toe-in: The front of the wheels point slightly inward toward each other. This improves straight-line stability but can increase tire wear.

• Toe-out: The front of the wheels point outward. This can enhance cornering response but may reduce straight-line stability.

All we care about is energy efficiency so 0° is the best value for that. Zero angle minimizes tire scrubbing, allowing for the least rolling resistance and therefore the highest energy efficiency.

Camber is the angle of the wheels when viewed from the front of the vehicle, indicating how much the top of the wheels tilt in or out.

• Positive camber: The top of the wheels tilt outward.

• Negative camber: The top of the wheels tilt inward. This camber can improve cornering but also increases tire wear

• When the wheels are vertically aligned (neutral camber), the entire tread is in even contact with the road surface, reducing the rolling resistance, which is crucial for conserving energy

All we care about is energy efficiency so 0° is the best value for that. Zero angle allows for the most effective ground contact patch in straight line driving which improves energy efficiency. additionally having caster allows the car to create its own highly optimal camber so in non racing vehicles 0 is often used with some amount of caster.

Angles of the upper and lower control arms to the upright relative to the horizontal

Slightly upward angled UCA improves stability and reduces body roll but can cause increased tire wear

Slightly downward LCA improves stability and handling. It also works to reduce body roll.

Having the arms point together alllows the roll center to more easily be defined.

_____ UCA

_____ LCA

Roll Center

Roll Center is an imaginary point around which a vehicle's body rolls during cornering. It is calculated by drawing lines along the plane of the UCA and LCA until they meet

A lower roll center increases body roll, potentially reducing handling precision.

A higher roll center reduces body roll but can make the ride harsher.

We do not want the car to roll under any circumstance so make the roll center as high as is reasonable

_____

Amount the wheel should move throughout its entire suspended motion.

We want 100 mm ride height so with safety factor 80 mm is the most we can get away with in terms of upward movement. If the wheel is off the ground the system can move down around 30 mm to reach the ground more easily and this is defined by the length and stroke of the shock (see shock documentation for more details)

Camber gain refers to the change in the camber angle of a vehicle's wheels as the suspension moves up or down (compresses or rebounds). It is typically expressed in degrees of camber change per inch or millimeter of suspension travel. Camber gain helps maintain optimal tire contact with the road during cornering. When a vehicle leans in a turn, camber gain allows the wheels to tilt inward (negative camber), improving grip and stability by compensating for body roll and ensuring more even tire wear.

Acording to some online diging you want around 0.5° per degree of body roll. This is acording to some online racing forums though so talking to other teams could give us a better idea for this number

~0.45°/°