As promised... I did submit a couple fork spring questions to Progressive Suspension and got the following responses from Sean Delshadi, Marketing Manager at Progressive Suspension:
Q1. Is thicker spring steel lower quality than thinner? And for the same spring rate, do thinner gauge coil wires require higher quality in their manufacture?
A1. NOT NECESSARILY – A MORE CORRECT STATEMENT WOULD BE – FOR A GIVEN APPLICATION, A SPRING MADE FROM LARGER DIAMETER WIRE CAN BE MADE OF LOWER QUALITY SPRING STEEL SINCE IT WILL OPERATE WITH LOWER STRESS AND STILL DELIVER APPROPRIATE DURABILITY. LARGE DIAMETER WIRE= LOW STRESS, SMALL DIAMETER WIRE= HIGH STRESS (FOR A GIVEN APPLICATION).
Q2. Is the following statement true? You have to consider the number of active coils - the greater the number of coils, the softer the spring will be.
A2. THIS STATEMENT IS TRUE IF THE SPRINGS BEING COMPARED ARE MADE FROM THE SAME WIRE DIAMETER AND THE SPRINGS HAVE THE SAME OUTER (OR INNER) DIAMETER.
Q3. Also, is the following statement true? With a large number of heavy gauge coils, the spring will lock up much faster, giving you a harsh ride.
A3. NOT A TRUE OR FALSE QUESTION – THAT STATEMENT IS PURELY SUBJECTIVE. IN REALITY, THE PERFORMANCE TARGETS OF THE SPRING GOVERN THE DESIGN DETAILS.
At this point - especially now that springs are installed and forks are back on my bike - I'm going to have to trust that these springs will work well for my bike and my riding style. As I said below, I'll let you know my impressions later this spring.
p.s. I'm a mechanical engineer and haven't really thought about springs / spring design since my coursework covered it 30 years ago. Here are the parameters that effect spring performance:
- Diameter of spring wire, d
- Outer diameter of spring, D
- Free length of spring, L
- Number of active coils, n
- Youngs modulus* of material, E
- Poisson ratio** of material, v
- Density of material, p
*also known as the elastic modulus, is a measure of stiffness of an elastic material. It is used to describe the elastic properties of objects like wires, rods or columns when they are stretched or compressed. It defines the relationship between stress (force per unit area) and strain (proportional deformation) in a material.
**A rod-like specimen subjected to uniaxial tension will exhibit some shrinkage in the lateral direction for most materials. The ratio of lateral strain and axial strain is defined as Poisson's ratio. The Poisson ratio for most metals falls between 0.25 to 0.35. Rubber has a Poisson ratio close to 0.5 and is therefore almost incompressible. Theoretical materials with a Poisson ratio of exactly 0.5 are truly incompressible, since the sum of all their strains leads to a zero volume change. Cork, at the other end, has a Poisson ratio close to zero. This makes cork function well as a bottle stopper, since an axially-loaded cork will not swell laterally to resist bottle insertion.
