Helical Spring Design & Equations


Helical spring design equations are the key to finding out if a spring will function the way you expect them to in your device or mechanism. These helical spring design equations will help you calculate your helical spring’s physical dimensions as well as the force and travel your spring will eject once it is in function.

There are many basic helical spring design equations to calculate your spring’s physical dimensions. You will use this to make sure that your spring will fit properly in your mechanism. These helical spring design equations are the ones provided below.

Outer Diameter Equation:

If your spring is going inside a hole, you want to make sure that the outer diameter of your helical spring isn’t going to be too tight around the inside walls of the hole or even too big to fit in the hole. The equation to calculate this helical spring dimension is to multiply the wire diameter by two and add the product to the inner diameter as shown below.

2(Wire Diameter) + Inner Diameter = Outer Diameter

2WD + ID = OD

If your spring is going inside a hole please use this "Spring Hole Clearance Guide" as a reference.
  • Spring Holes .100 to .250 allow for .010 to .015 clearance
  • Spring Holes .251 to .500 allow for .020 to .025 clearance
  • Spring Holes .501 to .750 allow for .025 to .030 clearance
  • Spring Holes .751 to 1.000 allow for .031 to .040 clearance
  • Spring Holes 1.001 to 1.500 allow for .040 .050 clearance
  • Spring Holes 1.501 to 2.000 allow for .050 to .075 clearance

Inner Diameter Equation

If your spring is going over a shaft or mandrel, you want to make sure that the inner diameter of your helical spring isn’t going to be too tight around the outer walls of the mandrel or shaft or even too small to fit over the shaft. The equation to calculate this helical spring dimension is to multiply the wire diameter by two and subtract it from the outer diameter as shown below.
Outer Diameter – 2(Wire Diameter) = Inner Diameter

OD – 2(WD) = ID

If your spring is going over a shaft please use this "Spring Shaft Clearance Guide" as a reference.
  • Spring Shafts .100 to .250 allow for .010 to .015 clearance
  • Spring Shafts .251 to .500 allow for .020 to .025 clearance
  • Spring Shafts .501 to .750 allow for .025 to .030 clearance
  • Spring Shafts .751 to 1.000 allow for .031 to .040 clearance
  • Spring Shafts 1.001 to 1.500 allow for .040 .050 clearance
  • Spring Shafts 1.501 to 2.000 allow for .050 to .075 clearance



Suspension Design Moving Through Uneven Road Surfaces

Mean diameter of a coil D = 48 mm

Diameter of wire d = 8 mm

Number of turns (n) = 16

Free length (Lf) = 256 mm

Pitch (p) = 16 mm

Spring index (c = D/d) = 6

Outer diameter of spring coil (Do = D + d) = 56 mm

Total weight of (bike + 1 person +2 person) = 300 kg



Figure: Helical coil suspension


As per road conditions the range of dimensions are prescribed which are summarized below:

1. The pitch value should range from 14.4mm to 17.6mm.

2. The diameter of suspension wire should range from 7.2mm to 8.8mm

3. The radius of suspension ranges from 21.6mm to 26.4mm.