spring rate v. spring load:
Spring rate -
is how many pounds it takes to compress a spring 1 inch (or for you metric guys, how many N's/kg's per milimeter).
So, a spring that requires 350lbs of force to move 2 inches, is a 175 lbs/inch rate spring.
Spring load -
is the amount of weight it requires to compress a spring to a certain height.
So, lets take that same 175 lb/in spring from before... If your car weighs 2800lbs, the weight on each corner will be, 700lbs...now lets say the suspension and tire make up about 150lbs of UNSPRUNG weight... that means the total SPRUNG weight is about 550lbs. Now if we load that same spring with 550lbs, the spring should compress ~3 inches. That is your total spring load. So, if you are riding on a spring that is only 5" tall... you will probably encounter a very rough ride, as you will only have 2" of suspension travel before riding the bumpstop...(*cough all the RL's aftermarket springs cough*)
which leads to...
Weight of your car and spring rate:
lets take that same car...2800 lbs. same sprung weight of 550 lbs. Lets take any spring, and lets say its 7" tall when the car is up in the air. when we put the car back on the ground, lets say that it compresses the spring to 4" tall. We could agree that the spring rate at the wheel is ~175lbs/in.
Now, when you are traveling down the road, you can easily encounter up to 1G of bump force from the normal freeway. That means that your spring can encounter up to 1100lbs of force acting on it at any given time. that means that your spring, with a wheel rate of 175 in/lb, will compress down to 6 inches.
Most manufacturers consider that soft by OEM standard. Obviously, in racing applications, you will need a higher spring rate, as the car can encounter up to and over 1G of weight transfer in corners.
How to calculate spring rate:
material rigidity or torsional modulus (G) - almost always 11.25 x 10^6, or 11,250,000
wire diameter (d)
number of active coils (n) - only coils that can move
mean coil diameter (D) - (Coil diameter - wire diameter) = mean diameter
so, plug and chug to get k
... then you will have the closest aproximated rate of the spring. Which leads to...
Where to mount springs:
As close to the wheel as possible. This will help reduce the amount of spring rate required to get a desired wheel rate. easy enough? On some old muscle cars, there are springs mounted close to the frame, this requires very high spring rates to keep the suspension from bottoming out, due to the difference in spring rate v. wheel rate.
What is wheel rate?
Unlike Spring rate, which is the amount of force it takes to compress a spring 1-inch, Wheel rate is the spring rate actually measured at the wheel.
In order to calculate wheel rate, you need tohave your car sitting at rest. You will need to have a tape measure, and possibly a protractor/angle ruler. The calculations do not need to be accurate to the .00000001%, a good enough accuracy (for common auto-x'ers) should be +/- 1/8".
Measure the following to calculate wheel rate - the spring rate, the motion ratio and the spring angle, as shown above. Then you can solve the following equation.
Wheel Rate = ( ( MotionRatio^2) * SpringRate ) * sin(Spring Angle)
What is the effect of too much spring rate?
Too much spring rate will cause your tires to lose contact with the road surface due to variations and irregularities in the road surface. Generally, you want to run the least amount of spring rate possible to allow adequate travel of the suspension. The softer spring rates ensure that the tire remains in contact with the road, even under bumpy conditions. more contact with road = more cornering force.
Too much spring rate will also limit suspension travel on bump and rebound directions. Less travel = less ability for the tire to maintain contact with the road surface.
Metric to imperial conversion:
Rates are converted as 1Kg/mm = 56 lb/in. and 1Kg/mm = 9.86 N/mm