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Old 01-09-2011, 05:33 PM   #1
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Default How-To: Calculating Spring Rates

If you ever need to calculate spring rates...

The spring rate equation is k=(G*d^4)/(8*n*D^3)
where k is the spring rate, G is the modulus of rigidity, d is the wire diameter, n is the number of active coils, and D is the average diameter (inside diameter plus wire diameter).

Given that G is about 11.4 million psi for steel alloys, that reduces the equation to: k=(1425000*d^4)/(n*D^3),

If that's still too much for you this is about a simple as I can make the equation:
spring rate = (1425000*d*d*d*d)/(n*D*D*D)

OR

Here's a handy online calculator:

http://www.engineersedge.com/spring_comp_calc_k.htm

Hint: As mentioned above, if the springs are pretty much any steel alloy you can use 11,400,000 for G "modulus of rigidity". All other variables can be easily measured at home from your springs.
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Old 01-09-2011, 05:49 PM   #2
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Or use the fatcat spreadsheet
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Old 01-09-2011, 11:23 PM   #3
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Oscar,

The above formula has nothing to do with the FatCat calculator. It calculates the spring rate of a given unknown spring from physical measurements.

Since d is to the fourth power, better bring a good micrometer.
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Old 01-10-2011, 02:35 AM   #4
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Or a ruler and a fat friend.
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Old 01-10-2011, 03:21 AM   #5
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Quote:
Originally Posted by zozo View Post
Oscar,

The above formula has nothing to do with the FatCat calculator. It calculates the spring rate of a given unknown spring from physical measurements.

Since d is to the fourth power, better bring a good micrometer.
I know. I thought it had those formulas too. All you had to do was enter the length and wire thickness etc. of the unknown spring and it would do the calculations for you.
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Old 01-10-2011, 11:16 AM   #6
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Quote:
Originally Posted by Savington View Post
Or a ruler and a fat friend.
or a micrometer and a skinny friend.
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