Understanding Coil Springs

Definition of a Coil Spring A mechanical device made from hardened steel and formed in a helix (spiral) used to store and release energy, to absorb shock and to maintain force between contacting surfaces

Purpose of a Coil Spring in a Jeep Suspension

To support the vehicle and maintain ride height
To keep the tires in contact with the road surface
To improve load carrying ability
To maximize wheel travel and ride comfort
To add lift and create better ground clearance
To add clearance for larger tires

Composition/Anatomy of a Coil Spring

Wire Diameter
Pitch
- The distance from the center of one coil to the center of the adjacent coil
- Increasing the pitch will increase the spring rate for a given wire diameter
Coil Angle (Helix)
Outside Coil Diameter
Inside Coil Diameter
Mean Coil Diameter
- An average measurement obtained by adding the inside diameter (ID) and outside diameter (OD) of the coil spring and dividing by 2
Free Length
- Total length of the coil spring when uncompressed.

Considerations for Choosing a Coil Spring

Desired Ride Quality and Vehicle Usage
- On road ride and/or off road ride quality
- Desired articulation
  - Coil springs do not limit downward articulation, but can limit upward articulation if the spring is too long when it is fully compressed
Vehicle Weight that the Spring must Support
- The weight the spring must support at ride height is the weight of the vehicle (measured per corner/wheel) minus the unsprung weight
- Unsprung weight includes; axles, brakes, hubs, wheels, tires and any other components that are not supported by the springs
Ride Height
- The ride height of a vehicle is determined by the coil spring’s free length and spring rate(s) in relationship to the weight the spring must support.
- Measuring the spring’s compression under the weight it will need to support will provide you with the ride height of the vehicle.
Spring Rate
- A value measured in pounds per inch that determines how much load/weight will be required to compression the spring 1 inch
- The lower the spring rate the softer the spring
- There are only three factors that affect the actual spring rate
  - The diameter of the wire
    - As the wire diameter increase so does the spring rate
  - The mean diameter (determined by one of the following)
    - Taking the average measurement of the ID and OD of the coil
    - Measuring the OD of the coil and subtracting the wire diameter
    - Measuring the ID of the coil and adding the wire diameter
      - When the mean diameter increases the spring rate decreases
- The active (free) coils
  - Does not include closed ends or touching coils (coil binding)

Spring Rate Calculation (for us Geeks)

The calculation looks like this
- Load per inch of deflection = modulus of spring steel (11,250,000) x wire diameter4 divided by 8 x number of active (free) coils x mean coil diameter3
Here is an example of the calculation for a spring with the following parameters:
- a wire diameter of .625
- 8 active (free) coils at ride height
- a mean (average) coil diameter of 5”
11,250,000 x .625 x .625 x .625 x .625 = 1,716,614 / 8 x 8 x 5 x 5 x 5 = 8,000

1,716,614 / 8,000 = 214.5 lbs per inch of compression

Types of Coil Springs

There are three basic types of coil springs used in a vehicle’s suspension. They are 1) Linear Rate Coils Springs, 2) Progressive Rate Coils Springs and 3) A any combination of the Linear and Progressive Rate Coils combined into one spring. This last type is what spring manufacturers may call Dual Rate, Triple Rate, Multi-Rate, Variable Rate or any other variety of trademarked names. The bottom line, however, is that, given a consistent wire and coil diameter, the base line spring rate itself will be the same on any type of coil spring. The primary factors that change the spring rate and performance of each spring type are the active (free coils) and the pitch (spacing or lack there-of) between those coils. Let’s take a closer look at each coil type and the primary differences.

1) Linear Rate Coil Springs

Standard OEM equipment
Constant spring rate through compression
A linear 100 lb rated spring will
- Compress 1 inch with 100 lbs of load/weight
- Compress 2 inches with 200 lbs, of load/weight
- Compress 3 inches with 300 lbs, of load/weight
Even when the load/weight on the spring has increased to 300 lbs, the rate of the spring remains at 100 lbs

Pros

Cost effective and easy to manufacture
Provides predictable/consistent ride height and performance
Easier to implement into the suspension system as the spring rate remains the same through all phases of compression
No sudden spring rate changes causing inconsistencies in ride quality, performance and handling
Even under almost full compression the load is distributed evenly among all of the coils

Cons

If the correct spring rate is not matched to the weight it is required to support the ride can be accordingly too soft or too stiff.
Usually shorter in length so not ideal for long travel suspensions
Provides no roll control when cornering as the springs on each side of the vehicle have a the same spring rate

Summary

The cost effectiveness, ease of manufacturing and consistency in rate, ride height and performance make this the go-to spring for manufacturers and after-market spring suppliers

2) Progressive Rate Coil Springs (Variable Pitch)

Increasing spring rate through compression.
- This is achieved by varying the pitch (space between) the coils throughout the spring.
- Each coil, regardless of pitch, will travel the same distance under load
- Coils with less pitch require less force to deflect/compress, effectively decreasing the effected spring rate of the coil
- Coils with a greater pitch (more space between the coils) require more force to deflect/compress, effectively increasing the effected rate of the coil
- As more load/weight is added, the coils with a smaller pitch will begin to compress to the point of coil bind.
- Once coil binding occurs, the coils lose their active energy absorbing ability which allows the remaining free coils, with the greater pitch, to create a stiffer overall active spring rate.
Softer rate when extended, stiffer rate when compressed
Progressive rate coils are designed to offer optimal ride characteristics and performance based on operating conditions
Also common in situations where load variance is typical/frequent
- Added cargo or equipment

Pros

Provides smoother suspension operation under a variety of loads/weights
Softer more comfortable ride under normal operating conditions
As load increases, such as higher speeds or cornering, the spring compresses and stiffens improving performance and handling.
Provides roll control as the spring that is under the greater load during cornering will be compressed further than it’s opposing spring creating a stiffer spring rate and more force where needed

Cons

Determining the correct spring for a desired ride height is more challenging as ride height changes more unpredictably as weight is added. This is due to the transitioning of the various spring rates
Difficult to tune with shocks as potentially varying ride heights based on weight will affect shock travel
Do not like to be fully compressed as this puts a greater strain on the last few coils. This could cause coil sag over time
Potential for undesired coil bind and increased spring rate at ride height if over weighted

Summary

If your load/mass on the springs is fairly consistent, the progressive springs offer the best of both worlds. Great ride quality and superior handling as well as load carrying capability under more compression. If, however, your load varies based on gear, passenger, etc., progressive springs may not be the way to go for you.

3) Dual Rate, Triple Rate, Multi-Rate, Variable Rate Coil Springs

A spring that typically utilizes one or more combinations of
- Coil binding
  - Providing solid spacing (zero pitch and not a part of the active suspension) at ride height and providing resistance under full flex
- Linear rate coils
  - Having a consistent spring rate (coil pitch)
- Progressive rate coils (varied pitch)
  - Having a one or more different coil pitches
    - Coils with less pitch require less force to deflect/compress (decreases spring)
    - Coils with a greater pitch require more force to deflect/compress (increases spring rate)

Pros

Provides smoother suspension operation under a variety of loads/weights
Softer more comfortable ride under normal operating conditions
As load increases, such as higher speeds or cornering, the spring compresses, increasing spring rate and improving performance/handling
Provides roll control as the spring that is under the greater load during cornering will be compressed further than it’s opposing spring creating a stiffer spring rate and more force where needed
Excellent for long travel suspensions
- Tightly wound coils (coil binding) can be used to help keep the spring seated when the spring is fully extended.

Cons

Determining correct spring for a desired ride height is more challenging as identifying the transition points is difficult and changes based on weight/load
- Too much preload (weight) at ride height will dramatically change the spring rate
Difficult to tune with shocks as potentially varying ride heights based on weight will affect shock travel
Transitions between rates can be sudden causing unwanted ride characteristics
Potential for undesired coil bind at ride height if over weighted
The constant pressure can unload and cause unwanted upward force on vehicle when climbing a steep obstacle

Summary

The most hybrid of all coil springs by attempting to combine the best characteristics of linear and progressive springs. It is also the most difficult to implement into your suspension due to the many variables in spring rates, rate transitions and load variances. May not the best option for an ease of use standpoint.

How to Determine the Ride Height based on your Selected Coil Spring

Determine the weight for each supported corner of the vehicle and subtract the corresponding unsprung weight.
- This weight will be used to determine how much the spring will be compressed at ride height
- Unsprung weight includes; axles, brakes, hubs, wheels, tires and any other components that are not supported by the springs
Determine the desired ride height that you would like
- Do you want the vehicle to remain at stock ride height?
- Are you lifting the vehicle 1 inch, 2 inches, etc?
Choose your coil spring type based on desired performance and usage
- Considerations
  - On road manners
  - Off road performance
Contact the prospective spring manufacturers and provide them with these three desired values
- Have them recommend the spring that they believe will work in your situation