Dual Spring Rate Calculator

What Is a Dual Spring Rate?

When two springs work together in a suspension system, their stiffness combines into one effective rate.

This combined value is called the dual spring rate.

It depends on how the springs are installed:

1. Parallel (Stacked)
2. Series (End-to-End)

The configuration completely changes how the math works.

Parallel Spring Configuration (Stacked)

In a parallel setup, both springs compress at the same time.

Think of two springs stacked side by side sharing the load.

Formula for Parallel Springs

Combined Rate = Spring A + Spring B

Example

• Spring A = 400 lb/in
• Spring B = 200 lb/in

Combined Rate = 400 + 200 = 600 lb/in

Key Behavior

• The combined rate is always higher than either spring alone
• The stiffer spring carries more load
• Ride becomes firmer

When Parallel Springs Are Used

• Heavy vehicles
• Track cars needing more stiffness
• Applications where high spring rate is required

If one spring is much stiffer than the other, it dominates the setup.

Series Spring Configuration (End-to-End)

In a series setup, springs are installed end-to-end. The load passes through both springs sequentially.

Both springs see the same force.

Formula for Series Springs

Combined Rate = (A × B) / (A + B)

Example

• Spring A = 400 lb/in
• Spring B = 200 lb/in

Combined Rate = (400 × 200) / (400 + 200)
Combined Rate = 80,000 / 600
Combined Rate = 133 lb/in

Key Behavior

• The combined rate is always softer than the softer spring
• The softer spring dominates deflection
• Used for progressive or dual-rate setups

This is common in coilover systems that use a tender spring and a main spring.

Understanding Units

The calculator supports:

• lb/in (pounds per inch)
• N/mm (Newtons per millimeter)
• kg/mm (kilograms per millimeter)

Internally, everything converts to lb/in for calculation.

This keeps the math consistent and accurate.

Motion Ratio Explained

Motion ratio is the relationship between wheel travel and spring travel.

Motion Ratio = Wheel Travel / Spring Travel

If your suspension has a motion ratio of 0.75:

• The wheel moves 1 inch
• The spring moves 0.75 inches

Why It Matters

Spring rate at the wheel is different from spring rate at the shock.

Wheel Rate Formula:

Wheel Rate = Spring Rate × (Motion Ratio)²

Notice the square. Small changes in motion ratio make a big difference.

Example

• Spring Rate = 600 lb/in
• Motion Ratio = 0.75

Wheel Rate = 600 × (0.75)²
Wheel Rate = 600 × 0.5625
Wheel Rate = 338 lb/in

If you ignore motion ratio, your setup calculations will be wrong.

Corner Weight and Static Deflection

Corner weight is the weight supported by one wheel.

If you enter corner weight, the calculator also gives:

• Wheel rate
• Static deflection
• Ride frequency

Static Deflection Formula

Deflection = Corner Weight / Wheel Rate

Example:

• Corner Weight = 1000 lbs
• Wheel Rate = 338 lb/in

Deflection = 1000 / 338 = 2.96 inches

This tells you how much the suspension compresses at rest.

Ride Frequency: What It Means

Ride frequency measures how stiff your suspension feels in motion.

It is measured in Hertz (Hz).

Ride Frequency Formula

Frequency = (1 / 2π) × √[(Wheel Rate × 386.088) / Corner Weight]

You do not need to calculate this manually. The calculator handles it.

What Different Frequencies Mean

If your street car is at 2.8 Hz, it will feel harsh.

If your race car is at 1.2 Hz, it will roll too much.

How to Use the Dual Spring Rate Calculator

1. Enter Spring A rate
2. Select its unit
3. Enter Spring B rate
4. Choose Parallel or Series
5. (Optional) Enter corner weight
6. (Optional) Enter motion ratio
7. Click Calculate

The calculator will show:

• Combined spring rate
• Configuration explanation
• Load contribution comparison
• Wheel rate
• Static deflection
• Ride frequency
• Setup recommendation

It also prevents invalid inputs like zero or negative spring rates.

Parallel vs Series: Quick Comparison

If you want more stiffness, choose parallel.

If you want more compliance or a progressive feel, choose series.

Why Dual Spring Calculations Matter

Many people guess spring rates. That leads to:

• Poor ride quality
• Bottoming out
• Excessive body roll
• Unstable handling

A proper calculation gives:

• Predictable handling
• Balanced ride comfort
• Correct ride frequency
• Better lap times
• Improved safety

Even small math mistakes can change how a car feels on the road.

Real-World Example

Let’s say you are building a street and track car:

• Spring A = 500 lb/in
• Spring B = 300 lb/in
• Configuration = Parallel
• Motion Ratio = 0.8
• Corner Weight = 900 lbs

Step 1: Combined Rate
500 + 300 = 800 lb/in

Step 2: Wheel Rate
800 × (0.8²) = 800 × 0.64 = 512 lb/in

Step 3: Static Deflection
900 / 512 = 1.76 inches

Step 4: Ride Frequency
Around 2.3 Hz

Result:
Firm but usable for aggressive street and track.

This is the kind of clarity the calculator provides instantly.

Common Mistakes to Avoid

• Ignoring motion ratio
• Mixing units without converting
• Using series math for parallel setups
• Forgetting corner weight differences front vs rear
• Choosing springs based only on stiffness

Suspension tuning is a system, not just a number.

Who Should Use a Dual Spring Rate Calculator?

• Performance car owners
• Track day drivers
• Race engineers
• Off-road builders
• Coilover tuners
• DIY suspension builders

If you are modifying suspension, you should calculate, not guess.