The Ultimate Coil Spring Design Guide: Expert Tips
Coil springs are among the most versatile mechanical components, found in everything from automotive suspensions to precision robotics. Getting their design right is critical for safety, performance, and long-term durability. Whether you need compression, extension, or torsion spring design, understanding the fundamentals of coil spring design is the foundation of a reliable solution.
At Lesjöfors, we specialize in designing and manufacturing springs for customers across almost every industry. With more than 170 years of expertise, we combine advanced engineering knowledge with world-class manufacturing to deliver custom coil springs that meet the toughest demands.
In this guide, we’ll walk you through how to design a spring, covering key parameters, calculations, materials, and prototyping. We’ll also explain the unique design differences between compression, extension, and torsion springs, helping you make confident decisions for your application.
What is a coil spring?
Why Suspension Coil Springs Are Key for High Performance
A coil spring is a mechanical device formed by winding metal wire into a helical shape. This design allows the spring to compress, extend, or twist when subjected to external forces, and then return to its original shape once the load is removed. Its ability to store energy and release it in a controlled manner makes coil springs indispensable in managing force, motion, and shock across a wide range of applications.
To explore more about what makes coil springs so versatile, from types and mechanical principles to industrial use cases, check out our detailed coil spring guide.
Types of coil springs
Compression springs
Compression springs resist linear compressive forces. They shorten under load and return to their original length when released. Compression spring design is the most common, widely used in automotive suspensions, electronics, and heavy industrial equipment.
Extension springs
Extension springs absorb and store energy by resisting stretching forces. They elongate under load and contract back when the force is removed. Extension spring design often includes hook or loop ends for attachment, making them common in garage doors, trampolines, and agricultural equipment.
Torsion springs
Torsion springs are designed to resist twisting, storing energy through angular deflection. Their legs apply torque in angular movements. Torsion spring design is critical for applications like clothespins, car trunk lids, and aerospace control mechanisms.
Constant coil springs vs variable coil springs
Constant coil springs
Constant coil springs have a uniform pitch and diameter throughout their length. This design produces a consistent spring rate, ideal for predictable and repeatable performance.
Variable coil springs
Variable coil springs feature changes in pitch or coil diameter to deliver progressive resistance.
Variable pitch
Springs with variable pitch provide a softer initial response that stiffens under load, making them effective in automotive suspension systems where comfort and control are essential. Studies also show that they deliver performance benefits over constant pitch designs, with improved stress distribution and dynamic response, making them a reliable choice for advanced engineering applications.
Variable outside diameters
Springs with tapered diameters help reduce space requirements and allow for compact packaging while still handling high loads.
How to design a coil spring
Designing a spring involves balancing geometry, load requirements, and material selection. Whether you’re working on helical coil spring design or an automotive coil spring design project, following a proven systematic approach, as we do at Lesjöfors, ensures efficiency and performance.
1. Understand key design parameters
Wire diameter
Wire thickness dictates the spring’s strength and flexibility. Small variations can significantly impact performance.
Coil diameter
The outer and inner coil diameters influence load capacity and stress distribution.
Number of coils
More coils generally mean greater flexibility but lower load capacity.
Spring rate
Spring rate – the force required to compress or extend the spring by a unit length – is one of the most critical factors in spring design. It directly links load and deflection, expressed by Hooke’s Law (F = k × x).
Load capacity
The maximum load a spring can carry without permanent deformation must be calculated based on application demands. Designers also consider solid height – the length of the spring when coils are fully compressed – since this limits maximum deflection.
Spring index
The ratio of coil diameter to wire diameter (spring index) affects stress distribution and manufacturability.
End conditions
Squared, ground, or open ends change how the spring interfaces with components and how forces are distributed. Properly designed ends improve seating stability and help reduce stress concentrations at critical points.
2. Set spring specifications
Define working loads, deflection limits, operating environment, and dimensional restrictions. This step ensures alignment with the end-use application.
3. Conduct spring calculations
Use formulas or a coil spring design calculator to determine stresses, deflections, and fatigue life. For compression spring design, Hooke’s Law and stress analysis guide how force and displacement interact, ensuring the spring operates safely within material limits.
For extension spring design specifically, understanding accurate measurement is key. Our guide on how to measure an extension spring provides a seven-step process, covering wire diameter, outside diameter, body length, free length, hook length, hook diameter, and hook opening.
4. Material research & selection
Spring materials must balance strength, fatigue resistance, and environmental compatibility. Common options include carbon steel, stainless steel, and speciality alloys.
Treatments & coatings
Heat treatments, shot peening, and surface coatings improve fatigue resistance and corrosion protection.
5. Manufacture spring prototypes
Prototype production validates theoretical designs, allowing engineers to test performance in real-world conditions.
6. Review, test & refine
Load testing, stress analysis, and fatigue testing help refine the design. Adjustments at this stage prevent costly failures later.
7. Quality control checks
Dimensional accuracy, material consistency, and performance verification are critical steps before full-scale production.
8. Implement in custom spring applications
Once validated, the spring is integrated into its application, ensuring it meets performance, durability, and safety standards.
Design differences: Compression, extension and torsion springs
The table below highlights the key design differences between compression, extension, and torsion springs:
Differences
Compression
Extension
Torsion
Key design parameters
Wire & coil diameter, spring rate
Hook design, preload tension
Leg length, angle of deflection
Main measurement parameter
Free length, solid height
Free length, extended length
Torque per degree
Force direction
Axial compression
Axial extension
Angular rotation
Typical end types
Ground, closed, open
Hooks, loops
Straight, offset, hinged
Key stress points
Inside coil curvature
Hook ends
Coil legs
Example applications
Automotive suspensions, valves
Garage doors, trampolines
Aerospace, industrial hinges
Key Design parameters
Compression - Wire & coil diameter, spring rate
Extension - Hook Design, preload tension
Torsion - Leg length, angle of deflection
Main measurement parameter
Compression - Free length, solid height
Extension - Free length, extended length
Torsion - Torque per degree
Force direction
Compression - Axial compression
Extension - Axial extension
Torsion - Angular rotation
Typical end types
Compression - Ground, closed, open
Extension - Hooks, loops
Torsion - Straight, offset, hinged
Key stress points
Compression - Inside coil curvature
Extension - Hook ends
Torsion - Coil legs
Example applications
Compression - Automotive suspensions, valves
Extension - Garage doors, trampolines
Torsion - Aerospace, industrial hinges
Leading custom spring design & development
At Lesjöfors, we specialize in tailored spring solutions, from helical coil spring design to advanced coil spring suspension design for demanding industries. Our streamlined process covers every stage: concept development, calculations, material selection, prototyping, and testing.
If you’re exploring how to design a spring for your next project, our engineers bring global expertise and precision manufacturing to ensure your solution is efficient, durable, and cost-effective.
The four main types are coil springs, leaf springs, torsion bars, and air springs.
Across industries, the three primary spring types are compression, extension, and torsion.
The general spring rate formula is k = (Gd⁴) / (8D³n), where G is the shear modulus, d is the wire diameter, D is the mean coil diameter, and n is the number of active coils.
Spring design fundamentals include wire diameter, coil diameter, number of coils, material choice, load requirements, and end conditions.
Driving with a broken suspension spring is unsafe and can cause further damage.
