FSU Double Wishbone Design Parameters ... in Progress

Steering Axis Inclination (King pin)

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

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

8° (this is going to change as geometry is defined)

Scrub Radius

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.

• 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.

MS15 used +20 mm scrub radius optimized for force calcs. Should not be fully neutral. Energy efficiency is maximized at slightly negative near neutral.

-10 mm

Caster

Caster is the angle of the steering axis when viewed from the side of the vehicle. It’s the tilt of the axis around which the front wheels pivot when steering. 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. MS15 used 3° and people really liked the way it self centered so we’ll stick with that for now.

3°

Toe

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.

0°

Camber

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

All we care about is energy efficiency so 0° is the best value for that.

0°

UCA and LCA angles

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

_____

Wheel movement

Amount the wheel should move upward 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

80 mm