Good idea Brian, I would really like if it was available on your site. It was posted on RC Tech a few different places, not exactly sure the original poster, or where they got it from. I added a few changes/notes also.
The formula for calculating the spring rate of a compression spring is the following:
K = (G*d^4)/(8*D^3*n) (remember your order of operations guys!
)
where:
K = Constant (spring rate), pounds per inch of deflection, (lb/in)
G = Modulus of rigidity (shear modulus, approx. 11 million psi for steel)
d = wire diameter (in inches)
D = mean coil diameter: outer diameter of spring minus wire diameter (in inches)
n = number of active coils (the very last top and bottom parts where the spring is closed and ground are not considered as active coils)
Here is a calc at Engineer's Edge:
http://www.engineersedge.com/calcula...ring_k_pop.htm
I tested out the formula and calc using a Revo spring of known rate, and it came out just about perfect using 11 million psi for modulus of rigidity. You can convert lb/in to N/mm (common metric unit for spring rate, used by Traxxas for Revo/Jato springs) by dividing by 5.7, and then of course N/mm to lb/in by multiplying by 5.7. Studying the formula you can see how a "progressive rate" springs are made, by either changing the spacing between coils at various parts of the spring, or the mean spring diameter at various points. It could also be accomplished by having a wire diameter that varied in thickness, but that's not very practical compared to the former 2 methods.
Believe me, in the search for the right springs for my 1/8 on-road conversion I learned more about springs than I ever thought I'd want to know!
When I still planned on using the 1/8 buggy fronts (Hyper 17mm Big Bores) I learned I would need to order springs from a spring manufacturer, since none of the available stock springs were firm enough. I found quite a few options, but they ranged anywhere from $10-30 a piece (most places required an order of 100+ to get anywhere near a reasonable price per spring.) That was one of the main deciding factors that made me switch to Revo shocks, which I'm glad of now as they seem perfect for the job.