I have been studying coil springs for quite a while now for my custom coilover conversion (almost done).
2 factors are to be taken into consideration
1) Spring rate (measured in LBS/ Inch) which is how many Lbs does it take to compress the spring by 1 inch.
2) Load carrying capacity of the spring
It's a bit tricky to understand but both factors affect each other.
There are many variables that make a spring stiffer or softer...
1) diameter of spring wire
2) number of wounds (coils)
3) outer diameter of each wound
4) free length
here is a site with a formula
to play with....
What you will need greatly depends on your particular application... For starters spring rate is lower (softer) in the front and higher (stiffer) in the rear. Front weight is constant, as opposed to the rear where load can vary... check out Old Man Emu's spring rates chart... if you have a winch, heavy bumper etc... u'll need a higher rated spring....
Angle is critical too.... It has to do with the forces under which the spring has to work.... Given the same vehicle with same weight, the needed spring rate will have to be higher the greater the angle in which it has to work. Check out the stock front coil spring in a suzuki vitara (a relatively light vehicle)... but because they are mounted pretty angled, their rate is double than if they were upright...
To get the rates right you'll need to known the following data"
1) total vehicle weight
2) front vehicle weight
3) rear vehicle weight
4) weight at wheel
5) unsprung weight (like springs, shocks, diffs, wheels and tires)...
It gets a bit more complicated [img]images/graemlins/smirk.gif[/img] if you want progressive or dual rate springs.... you can also try the formulae at SwayAway's website
There's a lot more to know ... be patient... and enjoy the ride.
And that was my 2 cents..... now someone [img]images/graemlins/givemebeer.gif[/img]