Calculator Inputs
Use metric inputs. Motion ratio here means spring travel divided by wheel travel.
Plotly Graph
The graph plots spring force against spring compression and marks preload and static operating points.
Formula Used
- Corner load: Corner Load = (Corner Mass × 9.80665) + Aerodynamic Load
- Required spring force: Spring Force = Corner Load ÷ (Motion Ratio × cos(Spring Angle))
- Preload force: Preload Force = Spring Rate × Preload
- Additional compression from load: Additional Compression = max((Spring Force − Preload Force) ÷ Spring Rate, 0)
- Total spring compression: Total Compression = Preload + Additional Compression
- Installed spring length: Installed Length = Free Length − Total Compression
- Wheel deflection: Wheel Deflection = Additional Compression ÷ Motion Ratio
- Static ride height: Static Ride Height = Reference Ride Height − Wheel Deflection
- Effective wheel rate: Wheel Rate = Spring Rate × Motion Ratio² × cos²(Spring Angle)
- Ride frequency: Frequency = (1 ÷ 2π) × √((Wheel Rate × 1000) ÷ Corner Mass)
- Perch change to target: Perch Change = (Target Ride Height − Static Ride Height) × Motion Ratio
How to Use This Calculator
- Measure the free spring length before installation.
- Enter the spring rate from the spring specification sheet.
- Use corner scales or design data for corner sprung mass.
- Enter motion ratio as spring travel divided by wheel travel.
- Input perch preload in millimeters of initial compression.
- Use the spring angle from vertical, not from horizontal.
- Enter a reference ride height from your chosen chassis datum.
- Add aerodynamic load for race conditions if needed.
- Optionally enter a target ride height for perch guidance.
- Press calculate and review ride height, sag, wheel rate, and frequency.
Positive perch change raises the corner. Negative perch change lowers it.
Example Data Table
| Setup | Free Length (mm) | Rate (N/mm) | Corner Mass (kg) | Motion Ratio | Preload (mm) | Angle (°) | Reference Height (mm) | Calculated Ride Height (mm) | Wheel Rate (N/mm) |
|---|---|---|---|---|---|---|---|---|---|
| Road Coupe | 230 | 68 | 170 | 0.90 | 8 | 4 | 160 | 137.60 | 54.80 |
| Track Sedan | 240 | 75 | 190 | 0.92 | 10 | 5 | 155 | 135.10 | 63.00 |
| Rally Hatch | 260 | 58 | 175 | 0.85 | 12 | 6 | 180 | 152.00 | 41.40 |
Frequently Asked Questions
1) What does this calculator estimate?
It estimates static ride height at one suspension corner. It also shows spring compression, wheel deflection, wheel rate, installed spring length, ride frequency, and an approximate perch change needed for a target height.
2) Why is motion ratio so important?
Motion ratio changes both force and travel transfer between wheel and spring. A lower ratio increases required spring force and reduces effective wheel rate, so ride height can drop more than many basic calculations suggest.
3) Does preload change the spring rate?
No. Preload changes the starting force in the spring, not its linear rate. The spring still gains force by the same amount for each extra millimeter of compression, assuming the spring behaves linearly.
4) Why do you include spring angle?
An inclined spring does not deliver its full force vertically. The cosine correction reduces effective support at the wheel, so angle matters when the spring or damper does not stand close to vertical.
5) What is wheel rate?
Wheel rate is the effective stiffness felt at the wheel after motion ratio and angle losses. It is more useful than raw spring rate when comparing ride response, balance, and approximate ride frequency.
6) What if installed spring length becomes very small?
A very short installed length can signal excessive compression or possible coil bind. Recheck the spring rate, motion ratio, preload, and load inputs. Then compare the result with real manufacturer coil bind data.
7) How accurate is the perch adjustment estimate?
It is a practical first estimate. Real vehicles also react to linkage curves, bushing compliance, tire stiffness, and cross-weight changes. Always verify the final perch change with physical measurements and scales.
8) Can I use this for race cars and road cars?
Yes, for either type, as long as inputs are accurate. The aerodynamic load field is especially useful for race setups. Road cars can usually leave that field at zero during normal static checks.