Spring rates

Kritter

Krittro Campbell
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k = d^4G/(8D^3N)

where G is shear modulus of elasticity, d is wire diameter, D is mean spring diameter, N is number of active coils which is the number of complete coils plus a fraction that depends on the types of ends the spring has...plain, plain and ground, squared, closed, squared and ground.
 

MNotary

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k is the spring constant....?

When you buy a spring you order by lb..

Typical G for spring steel?

Squared and ground? For a coil over.
 

Kritter

Krittro Campbell
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k is in lbs/in


plain and ground should be a coilover spring..then you need another equation to figure out the fraction of turns to add which I can't give you without a book.

and G you can find on the net somewhere.
 

toddz

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k is the spring constant....?

When you buy a spring you order by lb..

Typical G for spring steel?

Squared and ground? For a coil over.
__________________________________

G(modulus of rigidity) is 30 x 10E6 for all spring steels. Just counting the number of active coils and not worrying too much about the "fractions" at the ends will get you in the ballpark. The "lb." rating on coils is actually their rate, lb./in., rating.

Todd Z.
 

MNotary

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If I was to put the spring in a press with a weight scale and compress it from its unloaded length one inch, the scale would give me the spring rate? Can compare to the calculated rate?
 

Kritter

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yes...but a lot of times spring manufacturesr like to give you a spring rate at mid stroke and back out an average or many other stupid ways of doing it, but if you actually measure it and also calculate it...you should be within +/-10-12% without considering the spring ends and +/-3-4% with allowance for the ends in your calcs.
 

FABRICATOR

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If you're dealing with absolutes...The rate of a coil spring is not exactly linear. On average, it is very close to linear only through about 85 percent of its total travel. The only way to know the exact rate throughout the entire travel is to measure it throughout the entire travel.
 

MNotary

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ON the scale one inch gave me 65lbs on one set of springs and 110lbs on another set of springs. (For a SCORE LITES type car).
 

FABRICATOR

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That's about the spring rate. But the more you compress it, the more accuracy you will get. Or, in other words, the margin of error will be reduced. Try it at 2 or 3 inches, or as much as you can.
 

MNotary

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Using 30E6 for G

d=0.39
D=3
N=19

And I know that is not right....25lb

What is wrong??

I agree on the 2-3 check is probably more accurate but my goofy Wal Mart digital scale doesn't allow it.
 

Kritter

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I come up with 169 lb/in from the numbers you gave, but remember that you are not including the ends and that is going to overshoot your value like I said above.
 

toddz

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Using 30E6 for G

d=0.39
D=3
N=19

And I know that is not right....25lb

What is wrong??
____________________________________

I goofed! 30E6 is some other constant for something my too-long out of engineering school brain can't recall anymore. G is actually 11.25 x 10E6.

Sorry,
Todd Z.
 
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