What Is a Dual Spring Rate?
Stack two springs on top of each other — the classic coilover setup with a main spring and a helper or tender spring — and the suspension feels them as one single spring with its own rate. That effective number is the dual spring rate, and it matters because it decides how the car actually rides, not what either spring does alone.
Here is the key intuition: in a series arrangement both springs carry the same force, but each one compresses by its own amount. The total squish is the sum of the two squishes, so the pair always moves more than the softer spring would alone — which means the combined rate is always lower than either spring's individual rate.
The Dual Spring Rate Formula
For two springs in series:
[
k_{\text{combined}} = \frac{k_1 \times k_2}{k_1 + k_2}
]
Where:
- $k_1$ is the top spring rate (N/mm, N/m or lb/in)
- $k_2$ is the bottom spring rate, in the same units
- $k_{\text{combined}}$ is the resulting dual spring rate
You may recognise this as the "product over sum" pattern — it is the same formula used for resistors in parallel and capacitors in series. Both input rates must use identical units, because the calculation itself never mixes units.
Don't confuse series with parallel: springs sitting side by side sharing the load simply add ($k_1 + k_2$), producing a stiffer result. Stacked springs produce a softer one.
Worked Example: 7 N/mm Over 14 N/mm
A common coilover pairing: a 7 N/mm main spring with a 14 N/mm helper spring stacked above it.
[
k_{\text{combined}} = \frac{7 \times 14}{7 + 14} = \frac{98}{21} \approx 4.67 \text{ N/mm}
]
Notice the result is well below even the softer 7 N/mm spring — exactly as theory promises. Try those numbers in the calculator above and you'll get the same 4.67 N/mm.
How Soft Can It Get?
| Top Spring Rate | Bottom Spring Rate | Dual Spring Rate |
|---|---|---|
| 7 N/mm | 14 N/mm | 4.67 N/mm |
| 5 N/mm | 20 N/mm | 4.00 N/mm |
| 8 N/mm | 8 N/mm | 6.00 N/mm |
| 10 N/mm | 10 N/mm | 5.00 N/mm |
Two patterns are worth memorising:
- Very unequal pairs land close to the smaller rate: 5 and 20 give 4.00, barely below the 5.
- Equal pairs halve exactly: two 10 N/mm springs behave as a 5 N/mm spring.
Quick Recap
- Series (stacked) springs: multiply the rates and divide by their sum.
- The combined rate is always softer than the softest spring in the pair.
- Equal springs halve; very unequal springs approach the softer one.
- Parallel (side-by-side) springs are the opposite case — their rates add.
- Use the calculator above to check any pairing before you buy springs.
If you want to go further with spring mechanics, the spring force calculator is a natural next step.