opensus
MacPherson strut suspension: hardpoints and kinematics
A MacPherson strut replaces the upper arm of a double wishbone with the strut itself: the damper body is bolted to the upright, and its top sits in a mount on the body. A single lower arm and a tie rod complete the corner. It is compact and cheap, which is why most road cars use it at the front, but it gives less camber gain than a well-designed double wishbone.
The hardpoints
| Point | Name in opensus | What it controls |
|---|---|---|
| Lower arm front and rear inner pivots | lcaFront, lcaRear |
Lower arm pivot axis |
| Lower ball joint | lcaOuter |
With the strut top, the steering axis; scrub radius |
| Strut top mount | strutTop |
Top of the strut and the steering axis; camber plate adjustment |
| Strut base on upright | strutBase |
Where the strut is fixed to the upright |
| Strut axis guide | strutGuide |
A second point on the strut's axis, which sets its direction |
| Tie rod inner and outer | tieInner, tieOuter |
Steering and bump steer |
| Wheel centre | wheelCentre |
Track |
opensus models the strut as a sliding joint: the upright may slide and rotate along the line through the strut base and guide, and that line always passes through the top mount. The steering axis runs from the lower ball joint to the top mount.
How it moves
The front-view instant center lies where a line through the top mount, square to the strut, meets the extended lower arm. Because the strut is long and nearly vertical, that point is usually far from the wheel, so camber gain is low.
The default strut in opensus
Open it with the button on this page (Front → Type → MacPherson). At the design position:
| Result | Value |
|---|---|
| Roll center height | 20.2 mm |
| Roll center movement | -1.97 mm per mm of bump |
| Camber gain | -0.157° per 10 mm of bump |
| Kingpin inclination | 8.53° |
| Caster | 3.43° |
| Scrub radius | 16.8 mm |
| Motion ratio, damper to wheel | 0.979 |
Compare the double wishbone example: about half the camber gain, and a roll center that falls about 1.5 times as fast in bump.
Design notes
- Kingpin inclination is large because the strut top must clear the tire. That raises the wheel as it steers, adding self-centring.
- The roll center drops quickly as the car squats, often below the ground on a lowered car. Lowering a strut car usually needs raised lower arm pivots or roll center adjusters to compensate.
- The strut sees bending loads from cornering, unlike a pure two-force member. The load cases show the side force at the strut slide.
- Motion ratio is near 1: the coilover moves almost as much as the wheel.