trees said:
Think of what "spring rate" means: The weight it takes to depress the spring 1". Then you go from there through these steps: 1. Measure the weight the completed T-Bucket on a front and rear wheel. 2. Set the ride height you want. 3. Install the coil over mounts so the coil over will be near mid range (slightly more up travel than down). 4. Determine how much it must be compressed to fit the mounts. 5. Compute the spring rate you need. For example, your rear wheel weight is 400 pounds (800# total on rear axle) and you need to depress your coil over 2 3/4", then a 150# spring weight will get you very close (funny how NAIRB got 140 rate). Think of it like this: The first 150# compresses the spring 1", the second 150#s compresses another inch, and the remaining 100#s compresses another 2/3", which gets you close enough.
Trees
Trees is absolutely right!!!
It's hard to believe how many people cannot grasp this concept. Most coil over springs are linear in rates, meaning that if it's a 200 lb/in spring, it flat takes 200 lbs of force to compress it 1 inch, 400 lbs to compress 2" and so on.
I've always explained it like this. Choose a shock with a mounted length that approximately matches your approximate mounting points at ride height, mock up the rear axle to achieve the desired rake and frame height, build your mounts with respect to the shocks mounted length and then pick a spring that will carry the vehicle at that height.
The job of the shocks are to dampen the ride.
The job of the coil over spring is to carry the car and keep the shock within it's specified stroke range.
Too many people think they can achieve whatever ride height to whatever they want by simply adjusting the spanner nut, and they fail to realize that you can't be running around and topping or bottoming out the shocks. The spanner nut is there to get the shock into it's correct height. If you have to unwind the spring to the bottom to achieve ride height,the spring is too heavy!! If you have to crank it all the way up, it's too weak!
Sometimes it takes a little finessing and fine tuning to get the ride just how you like it. I've had customers that had to swap springs, but not everybody knows exactly what their car will weigh until they have finished the build.
I had one customer that had a trailing arm car with coils. He was trying to use some sort of junk yard spring, and it rode like a wagon. I estimated his car weight, did a little math and sent him a couple of 5", 170 lb coil springs, and the guy was so happy he couldn't see straight. It's common sense most of the time. A spring is a spring, unless it's a variable rate spring, and they aren't used that often in a rod.
I do believe that larger diameter, taller springs give the best ride, but there isn't always room for such large springs in alot of hot rods.