While this is interesting, it really is a little puzzling since a lot of the aftermarket springs seem to have softer rear spring rates:
Suspension & brake aftermarket parts index
As per KYB Ph when they've released the KYB rear springs for the hatchback in 2016, the stock rear spring (probably models earlier than 2016) has a rate of 1.9kg/mm, while their new springs are rated at 2.3kg/mm.
2016 springs were significantly squishy than the KYB, which gives me the impression that it's rated far below 2.3kg/mm (0A468041991A, not sure if this was the part #). Furthermore, despite most claims, kyb increased right height by 3/4 of an inch but solved the bottoming issue as expected with stiffer rates.
Recently, I had a chance to peek under the rear of the newer models assembled here in the Philippines, surprised me with a newer series of springs with the serial/part number inscribed 0A468045820AL. These models also had higher rear height than 2016 models from thailand, so I'd assume they've put more stiffer springs on these new models. I'm yet to ride a friend's 2017 model soon to figure out.
Misleading info here:
Let's say the free length of the spring is 12" and the installed length is 9". Probably not too far off from reality, but it doesn't matter... just an example.Amazon (AD) rear spring (-5lbs for weight of spring and wood)
1" - 50lbs
2" - 99lbs
3" - 159lbs
If you're measuring a progressive spring rate, your effective spring rate at normal ride height is the rate after 3" of compression. Most progressive springs have a set of "tight" coils that will be completely compressed under the static weight of the car.
That's so that when you go flying over a railroad crossing at Mach 2 and the suspension unloads completely, when you come back DOWN, you don't get a sudden jolt. You get a softer spring rate for the first few inches as the suspension recovers.
The calculation for spring rates includes the thickness of the wire and the diameter of the spring. But, a major factor is the NUMBER OF COILS in the spring. More coils = softer rate. So, the typical progressive spring has coils that are closer together and fully compress quickly. Once they are "bound", then they're out of the equation, you have less active coils and more spring rate.
What I'm saying is that the test method above, if you're talking about a progressive spring, is probably flawed. Would be more accurate to measure the spring and crunch the numbers. "What's the spring rate with all coils active?" "What's the spring rate with the progressive coils compressed?"
I like to use a calculator like this one because I'm lazy:
Physics doesn't lie!
Simplify and add lightness.