opensus
Multilink suspension: five-link layout and virtual kingpin
A multilink suspension locates the upright with separate rigid links instead of A-arms. Five links remove five of the upright's six freedoms, leaving one: wheel travel. Because each link can be placed on its own, a multilink lets you tune camber, toe and anti geometry more independently than a double wishbone, at the cost of more parts and less obvious geometry.
The five-link layout in opensus
| Link | Points | Role |
|---|---|---|
| Upper front | upperFrontIn, upperFrontOut |
With the upper rear link, does the upper arm's job |
| Upper rear | upperRearIn, upperRearOut |
|
| Lower front | lowerFrontIn, lowerFrontOut |
With the lower rear link, does the lower arm's job |
| Lower rear | lowerRearIn, lowerRearOut |
|
| Toe link | tieInner, tieOuter |
Sets toe and toe change |
| Coilover | springUpright, springChassis |
Holds the car up |
Any linkage can be built from points, rigid links, ball joints, hinges and struts: choose blank as the type and add them under Links and joints. The status line says whether the corner has exactly one free motion, and if not, which body is loose or over-constrained.
Virtual kingpin
With split links there is no physical ball joint for the steering axis to pass through. Each pair of upper links, and each pair of lower links, crosses at a virtual ball joint; the steering axis runs between those two virtual points and moves as the wheel travels. opensus computes it from the link lines at every position, so kingpin inclination, caster, scrub radius and trail are reported for multilinks too.
Instant centers from real motion
opensus finds instant centers from how the upright actually moves, not by extending arm lines, which have no meaning for a multilink. The roll center and anti geometry are therefore exact for any linkage that solves.
The default five-link rear
| Result | Value |
|---|---|
| Roll center height | 32.1 mm |
| Roll center movement | -1.41 mm per mm of bump |
| Camber gain | -0.166° per 10 mm of bump |
| Bump steer | -0.121° per 10 mm of bump (toe-out in bump) |
| Anti-squat | -10 % |
| Motion ratio, coilover to wheel | 0.920 |
The default links are a starting shape, not a tuned design: the toe-out in bump and negative anti-squat are what you would work on first. Move tieInner to change the toe curve; see reducing bump steer.