One of the biggest problems people run into when ordering coilover shocks is figuring out the correct spring rates. Most guys don’t have corner scales or suspension calculators. And honestly, a lot of builds are still being assembled when the shocks get ordered anyway. That’s exactly why Wide Open Design came up with a simple method to calculate spring rates at home without needing scales or complicated suspension software.
Start With the Correct Shock Length
Before figuring out spring rates, you first need to know:
- shock length
- suspension travel goals
- ride height targets
In the example from the video, the buggy uses:
- 14” shocks
- 7” up travel
- 7” down travel
That gives the suspension a balanced ride height setup right in the middle of the shock travel. Once the shocks are installed, WOD recommends starting with a “test spring” setup to determine the final spring combination.
The Simple Spring Rate Formula
The process starts by installing a single spring on the shock and letting the vehicle settle onto its own weight.
From there:
- Measure the spring free length
- Measure the compressed spring length at ride height
- Calculate how much the spring compressed
Formula:
Spring Length – Compressed Length = X
In the example:
14” spring
compressed to 9.75”
14 – 9.75 = 4.25”
That means the spring compressed:
4.25 inches.
Calculating Supported Weight
Next: take the amount of spring compression and multiply it by the spring rate.
Formula:
X × Spring Rate = Y
Example:
4.25 × 150 lb spring = 637.5 lbs
That means the spring is supporting:
637.5 pounds of vehicle weight.
The beauty of this method is that suspension angles and trailing arms all become irrelevant because you’re measuring the actual installed spring load directly on the vehicle.
Finding the Correct Combined Spring Rate
Now the goal becomes figuring out the correct dual-rate spring setup for the desired ride height.
Formula:
Y ÷ Inches Collapsed at Ride Height
In the example:
637.5 ÷ 7” ride height sag
= roughly 91 lb combined rate
From there, you use a spring stacker chart to determine which upper and lower spring combination produces that combined spring rate.
The final combination ended up being:
- 150 lb upper spring
- 225 lb lower spring
which created the desired combined rate.
Why Dual Rate Springs Matter
Dual-rate coilovers allow the suspension to:
- ride softer initially
- absorb small bumps better
- maintain traction
- still resist bottoming out later in the travel
The lighter upper spring controls initial ride quality while the heavier lower spring takes over deeper into compression. The crossover adjustment nut allows builders to tune when the secondary spring rate engages, bottoming resistance, and suspension feel without needing to completely re-valve the shocks.
No Scales. No Guesswork.
The biggest advantage of this method is simplicity. Builders can calculate the correct spring setup directly on the vehicle at home. It’s fast. It’s repeatable. And it works extremely well for:
- rock crawlers
- Ultra4 cars
- rock bouncers
- trail rigs
- custom tube chassis builds
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