# Coil spring rate calculator

Discussion in 'Center Of Gravity' started by 88K5Jimmy, Nov 25, 2003.

1. ### 88K5Jimmy1/2 ton status

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This is a coil spring rate calculation that Steve Sharpe (the yellow first gen on rockies that's been pictured recently in this forum) gave me.

Take your axle weight (the weight of the front or rear of your truck as it sits over the axle and divide it by 2 (2200/2=1100)

Take that number and divide it by 2.5 (1100/2.5=440)

and again by 3 (1100/3=366)

So for a soft, flexible, and possible unstable spring you could use a 365# spring. For a more stable but yet flexy spring a 440-450# spring would work.

I have no idea if this based on any scientific prinicples but I have seen his stuff work and been around Steve long enough to know that he knows what he is doing.

Hope this helps you guys as much as its helped me
Ross

2. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Sounds like more of a "Rule of Thumb"....

To paraphrase:

Spring rate that's 40% of vehicle corner weight = Good
Spring rate that's 33% of vehicle corner weight = Bad (Unstable)

I dunno, that's probably a little to "generalized" to be useful in all contexts.... my front springs are only 19% of my corner weight, and I think it's a pretty decent setup.

If he's basing that recommendation on Rockwell axles, that may explain things a bit too. Unsprung weight needs to factor into the decision just like the overall weight does...and Rockwells way a LOT more than a D60/14BFF setup....AFAIK

/forums/images/graemlins/thinking.gif

3. ### az-k51/2 ton status

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The 40 = stable 33 = oops is a good rule of thumb. The most often overlooked factor is location. If the shock body is not parralel (sp) to the movement a lot of the math goes out the window. A spring rated at 365# at a 10° angle, from perpendicular, will act like a spring rated at 306#. The suspension needs to be cycled prior to mounting to find it's natural arc. A well designed 4 link will have minimal arc, but enough to compensate for by placing the shock where it will be perpendicular most often. It all gets even goofier as you run the geometry while the axle is flexed out.
This is just some more math to get in the way of our fun now, but hey I am half way to a mechanical BSE,/forums/images/graemlins/deal.gif and declared a geek by many who know me (I enjoy running goffey numbers).

4. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Interesting!

I was well aware that shocks lose effectiveness as they are angled, but didn't know the same was true for coils.....

I guess the effect is that instead of supporting the weight the spring is deforming and trying to "spit itself out" of the hanger??? I've certainly seen some pics of offroad rigs where the coil is completely deformed like that.

5. ### az-k51/2 ton status

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The effectiveness at an angle is similar on sprigs as shocks or any other part designed to move in a striat line. The angle off of perpendicular allows a leverage force to act on the item. If I could load a cad drawing up on here I would show this in more detail.

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You can also look at it as a geometry problem. Springs are rated in weight/distance. When not mounted in line with travel, then the distance traveled by the axle will exceed the distance the spring is compressed (think right triangles). So the "rate" of the spring is effectively reduced relative to the axle...

7. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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[ QUOTE ]
You can also look at it as a geometry problem. Springs are rated in weight/distance. When not mounted in line with travel, then the distance traveled by the axle will exceed the distance the spring is compressed (think right triangles). So the "rate" of the spring is effectively reduced relative to the axle...

[/ QUOTE ]

Mmmmmm....geometry!

Russ, do you have any mathematical formulas for turkey carving? /forums/images/graemlins/grin.gif

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Yes I do, but that's beyond the scope of this forum. &lt;smack&gt; /forums/images/graemlins/rotfl.gif

9. ### Kyle89K51/2 ton status

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Send me the CAD file and I WILL get it on here /forums/images/graemlins/grin.gif

10. ### Triaged1/2 ton status

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The "installed" spring rate will be

Spring rate*[cosine(angle from verticle)]^2

It is squared because not only are you displacing the coil less you also have more force on it trying to compress it

11. ### 88K5Jimmy1/2 ton status

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angle from vertical

Where do you get that number?

Thanks
Ross

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