opensus
Compliance steer and camber from suspension geometry
Compliance steer is toe change caused by load rather than by travel: cornering force, braking and the tire's aligning torque deflect the joints and links, and the wheel steers. opensus estimates it from geometry alone by giving every joint and link the same reference stiffness, so you can see which layouts multiply loads and deflection, before any bushings or rod ends are chosen.
How it works
The Compliance tab locks the spring and the steering, puts the same stiffness (10 kN/mm by default) on every link, ball joint and the locked spring, and models each hinge as a bearing at each of its two pivot points. It then applies unit loads at the contact patch and solves for how the upright moves.
| Load case | Applied at the outside wheel |
|---|---|
| Cornering, 1 g | Each axle's share of the car's weight as lateral force, plus aligning torque from a pneumatic trail you set (25 mm default) |
| Lateral force | 1 kN toward the car's centre |
| Aligning torque | 100 N m, steering the wheel out of the turn |
| Braking | 1 kN rearward, at the contact patch or the wheel center for inboard brakes |
| Drive | 1 kN forward |
| Vertical | 1 kN upward |
Results scale exactly with the stiffness: halve it and every movement doubles. The loads in each part do not depend on it at all.
Understeer from compliance
A front wheel that steers out of a turn, or a rear wheel that steers into it, adds understeer. opensus combines the outside front and rear wheels' toe change in the 1 g case into compliance understeer in degrees per g: positive adds understeer, negative adds oversteer. On the Formula SAE example at 10 kN/mm it is +0.006°/g: the front's toe-in under lateral force (oversteer) almost exactly cancels the toe-out from aligning torque, and the rear adds a little understeer.
What drives the numbers
- Aligning torque goes into the tie rod. Its load per 100 N m is about 100 000 divided by the steering arm length in mm: the Formula SAE front tie rod carries 1.4 kN per 100 N m, an effective steering arm of about 71 mm. A longer steering arm cuts both the load and the toe change.
- Ball joint spacing. Lateral and braking forces at the ground make a moment about the upright that the two ball joints react as a couple; joints close together need large forces, and large forces mean large deflections.
- Pivot base. A short front-to-rear spacing between an arm's inner pivots raises the pivot loads under braking.
- Toe link position relative to the line through the ball joints decides whether lateral force toes the wheel in or out.
Limits
These are comparisons between layouts, not predictions. Real bushings, rod ends and tube walls differ and are usually softer than the reference. The sway bar is not included, and the left corner of each axle is analysed.