COIL SPRING RATES
Some thoughts on coil spring.... There has been a theory that spring rates change (pounds per inch of compression). A spring rate is derived from two things. First, the size of the material used, and second, the total number of inches of material used. If none of these things change, your spring rate will remain the same.
Think of a coil spring as a big "slinky" toy. The greater number of coils used, the slinkier it becomes. The same goes for removing coils, the less slinkily it is. The same principal applies to coil springs used in suspension systems.
Since you cannot physically add more coils to a spring, the rate will not get weaker. In effect, the only way to change the rate would be to cut coils or collapse the coils (by heating them) and making them touch each other. These things happening would increase the rate, as you are removing the amount of material in the spring, then comes into play stress of a spring. Our springs are designed to take the stress of racing. Having the right size and amount of material for the application insures it.
A coil spring may take a "set" or loose free length, shortly after installation. Usually this is caused by a poor design or over stressing the spring (using it in the wrong application will over stress a spring). When a spring takes a "set" the rate of the coil has not changed, only the amount of load it will handle does. For example, if you have a 10" free length spring, at 100 pounds per inch rate, it will handle 500 pounds of load. If you reduce the same spring's length by one inch of free length, it will only handle 400 pounds of load before it reaches coil bind (the point at which the coils touch each other).
When asked, "what springs do I need for my race car," a great number of factors must be considered prior to answering. Three major variables, that are never the same between each team, are driver, track and car. Since none are the same, you could climb into the champion's car, take a lap, and wonder how in the world he (or she) managed to control the car. You both have different feels of preference. Other variables include tire stagger, weight load per wheel, body aerodynamics, shock type, and many others.
There is a starting point. You have to judge as to what works or what does not for you and your car.
Every change made to the car should be noted, and the cars reaction to the change should be carefully studied and noted as well. It's a great habit; it will give you data for future reference!
HOW TO RATE COIL SPRINGS WITHOUT A RATER
D=Wire Diameter in Inches
N=Number of Active Coils
D=Mean Coil Diameter in Inches. Mean Diameter is:
I.D. = 1 Wire plus inside Diameter
O.D. = 1 Wire minus outside Diameter
8=A Constant for all Coil Springs
HOW TO DETERMINE ACTIVE COILS OF A COIL SPRING
Count total number of coils, subtract a coil for each coil that touches, these are dead coils. Ground flat ends are a dead coil. Start count with cut-off end facing you directly above would be one and so on. Not all coil springs are even coiled. You can have 1/4, 3/8, 1/2, 5/8, 3/4 or 1/8 of a coil (Example 10 1/8 coils).
- If you cut one coil from a spring, the rate will increase.
- Increasing wire diameter, will cause a great increase in rate.
- Nothing in spring rate calculation indicates that a coil spring ever changes rate. The rate is determined by material and dimension of the spring. Coil springs don't wear out or lose their rate.
- Spring load determines how much load a spring can support at a given height. The rate only tells how much height will change as load is changed. A spring can lose its load height over time if steel is not heat treated properly. When a spring sags, its rate is still the same as when it was new.