opensus
How to set roll center height on a double wishbone
To raise the roll center on a double wishbone, raise the lower arm's inner pivots or lower the upper arm's inner pivots: both tilt the arms so their front-view lines meet higher and closer to the wheel. Every move that shifts the roll center also changes camber gain, so set the two together.
Measured effects
Starting from the docs example double wishbone (roll center 89.9 mm, camber gain -0.317° per 10 mm), moving one pair of inner pivots by 10 mm:
| Change | Roll center | Camber gain, per 10 mm | Roll center movement, per mm of bump |
|---|---|---|---|
| None | 89.9 mm | -0.317° | -1.32 mm |
| Lower arm inner pivots up 10 mm | 114.9 mm | -0.387° | -1.32 mm |
| Lower arm inner pivots down 10 mm | 64.5 mm | -0.247° | -1.32 mm |
| Upper arm inner pivots up 10 mm | 78.0 mm | -0.219° | -1.33 mm |
| Upper arm inner pivots down 10 mm | 101.5 mm | -0.413° | -1.31 mm |
(The tie rod inner pivot moved with the lower arm pivots, to keep bump steer at zero.)
The lower pivots move the roll center about 2.5 mm per mm; the upper pivots about 1.2 mm per mm. Both raise camber gain as they raise the roll center, because both shorten the front-view swing arm.
Step by step
- Decide the target. Race cars commonly sit between ground level and about 100 mm at ride height, front lower than rear so the roll axis tilts nose-down. The roll center's height sets how much lateral load transfer goes through the links; see roll center.
- Change the lower inner pivots for big moves (Points → Lower arm, front inner and rear inner, Z). They move it furthest per millimetre.
- Use the upper inner pivots to trim camber gain once the roll center is close: they move camber gain more, relative to the roll center, than the lower pivots do.
- Check migration. Watch Roll centre height in Kinematics → Heave and Roll. A roll center that crosses the ground in bump, or swings far sideways in roll (Roll centre lateral), makes the car's balance change with load.
- Re-check bump steer. Moving arm pivots changes the arm arcs; move the tie rod to match (see reducing bump steer).
- Look at the whole car. Cornering and braking shows the resulting roll angle and front share of load transfer at a given lateral g, and the 3D view's Roll axis button draws the axis through both roll centers.