Spring Rates mystery

[cslithergo]

I have seen more than a few threads on springs rates and ran up on an article in Car Craft March issue. The article goes into discussion about how to callculate your spring rate and a few different set up ideas. Its on page 70. Thought I would pass along the info for those looking for ideas.
chris

[Mr.Fixit]

If I remember right, don't you just compress the spring 75% of free length, measure it's force of deflection. compress the spring 20% of free length and measure the force of deflection.

divide the difference in force values by the difference in lenths

(F2-F1) / (L1-L2)

[speed220mph]

First off, compressing a coil spring can be dangerous, so be careful when doing this. It could kick out and hurt someone.

With that said, if it's a suspension spring, compress it a couple of inches and check how much force it takes to do that. Take into account the weight of the spring if you have it sitting on a scale.

You'll have to be accurate with these measurements to get a good number. Now, simply divide the force in pounds by the deflection in inches and you've got spring rate, or K = F/d. Say it took 300 lb to deflect--compress, if you wish--the spring 2 inches, its rate is 300 lb divided by 2 inches, or 150 pounds per inch.

It's never that easy to do, but this will give you an idea about how it's done. What's not as simple is how spring rate translates into wheel rate, or ride rate. That's what counts. Basically, though, it's the lever ratio of wheel travel versus spring travel squared, then divided into spring rate.

This stuff can get complicated, especially when you consider ride frequency and balance, just to mention a few. But that's chassis setup.