Tell me if this DIY spring rate tester will work

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09-26-2013, 02:40 PM

#21

sixshooter

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Quote:

Originally Posted by Godless Commie

Having said all this, I still need to come up with data such as number of coils and coil spacing for an 8" spring with an inside diameter of 2.5".
Desired spring rate is 300, 275 and 225 lbs, respectively.
Available wire diameters are 8, 8.5, 9, 9.5, 10 and 10.5 mm. (Al spring steel)

I guess you could use the formula for that, then.

09-26-2013, 02:56 PM

#22

Godless Commie

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Quote:

Originally Posted by Joe Perez

But seriously. I think that the mathematical approach is probably going to yield more accurate answers than something involving a bathroom scale. The basic idea behind the scale contraption is a sound one, but I don't think I'd go that way myself from a practical standpoint.

Joe, please refer to post #19.

09-26-2013, 02:58 PM

#23

Godless Commie

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Quote:

Originally Posted by sixshooter

I guess you could use the formula for that, then.

I'll be crunching numbers as soon as I am done translating regulations and int'l correspondence on horse racing.

09-26-2013, 03:05 PM

#24

Joe Perez

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Quote:

Originally Posted by Godless Commie

Joe, please refer to post #19.

Doh.

Please refer to the timestamps on posts 19 and 20.

09-26-2013, 03:09 PM

#25

Pinky

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Quote:

Originally Posted by sixshooter

I have a better idea. Use mathematics and a set of dial calipers to do it the right way. A formula exists for determining spring rates. I've seen it published a number of times, including in the book by Herb Adams called "Chassis Engineering". I'm sure it is on teh interwebz somewhere.

Suspension Coil Spring Rate Design Equations Formulas Calculator

Another one:Spring Constant Calculator
Another one: Helical Spring Calculators - Spring Index, Spring Rate, Shear Stress, Deflection
Another one: Coil Spring Rate Calculations by Wallace Racing

FORMULA:

spring rate =

modulus of spring steel X wire diameter^4
--------------------------------------------------------------------
8 X number of active coils X mean coil diameter^3

*modulus of spring steel = 11,250,000 pounds/inch^2 = 78,500 newtons/millimeter^2

I have successfully tested this formula on stock Miata springs of a known rate and aftermarket springs of a known rate. I then used it to determine the approximate rates of some additional springs I had around. The modulus of spring steel is a constant.

The Herb Adams book gets a regular workout around here-

Here are the pages that cover it.. If anybody wants hi-res pics of them just pm me your email address.

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09-26-2013, 03:10 PM

#26

Godless Commie

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Quote:

Originally Posted by Joe Perez

Doh.

Please refer to the timestamps on posts 19 and 20.

I'm pretty sure intelligence reports submitted on September 10, 2001 had timestamps on them, too, but that did not prevent the intended recipients from missing them.

09-26-2013, 03:46 PM

#27

mgeoffriau

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09-26-2013, 03:51 PM

#28

JasonC SBB

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Quote:

Originally Posted by sixshooter

Sprign rate is a function of diameter to the power of 4. A 1% error in thickness will give you a 4% error in spring rate. Paint will give you a 1% error.

09-26-2013, 04:23 PM

#29

Savington

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Quote:

Originally Posted by JasonC SBB

Sprign rate is a function of diameter to the power of 4. A 1% error in thickness will give you a 4% error in spring rate. Paint will give you a 1% error.

20ft.lb on a 500ft.lb spring. It will at least put him in the ballpark. If they were decent springs they'd be labeled anyway.

09-26-2013, 04:31 PM

#30

y8s

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Quote:

Originally Posted by Savington

20ft.lb on a 500ft.lb spring. It will at least put him in the ballpark. If they were decent springs they'd be labeled anyway.

And if the labels are missing along with the paint, well accuracy just went up.

09-27-2013, 10:43 AM

#31

Efini~FC3S

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Quote:

Originally Posted by Godless Commie

A formula tells you what the result "should be".
It does not take into account factors such as defective materials, aging, sloppily applied thick paint layer that could affect the caliper reading for wire diameter, treatment failures, etc....

The only material factors that affect the result from the mathematical calculation is the torsional modulus of the steel. All steels have near as makes no difference the same torsional modulus. Spring steel, high-strength low alloy steel, **** chinese steel, steel with a bunch of crap defects in it, etc. etc. etc. That is to say, if you use the number provided, it will be 95%+ correct, no matter what the springs are made of, as long as it's some sort of steel. I'm not sure what you mean by treatment failures (heat treatment? won't affect it).

As others have eluded to, the other factors you have mentioned will only lead to small errors in the calculation.

I would recommend doing both if you are deadset on making a spring compressor testing rig-a-ma-jig. Do the calculation on at least the first few springs you test to validate your test jig.

Have fun