coil spring interchangeability

I need some advice from GRM, recently picked up a kia sedonna for a beater. The rear springs were starting to sag slightly already, then I added a hitch and it's squatting badly. The problem is, dealership wants $300 PER spring and nobody makes an aftermarket spring for this vehicle. 

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It's an IRS with shocks not in the coils. I'm trying to think of the best solution here.

1. try a set of those urethane spring spacer kits. Have my doubts on this since the springs have 140k on them and are rusty. I imagine I'd get a year or so before a coil snaps. 

2. Find the next closest match from a different vehicle and somehow make it work. Problem is there are no listed specs for the spring online. I could pull the spring off and take some measurements, but I would hate to get said measurements off and do all that work to mistakenly get an even softer rate with more sag.

Any thoughts? 

It's hard to overstate how vital suspension is to your car. It's what keeps your wheels connected to the ground, after all. And if you're rigging your 4x4 to tackle the ruthless terrain of off-road trails, you probably care about that even more than the average driver.

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As you try to figure out the best way to level up the suspension, you'll inevitably find yourself staring at a mountain of springs and trying to pick the right one for your car. Our tips for choosing a spring rate for your car will help you make the best choice.

What Is Spring Rate?

Spring rate refers to how much force or weight needs to be enacted on a spring to make it compress a single inch. If someone tells you a spring has a rate of 150 pounds, they mean it has a rate of 150 pounds per inch. Every 150 pounds makes the spring compress another inch. Springs with lower spring rates are softer, while higher rates are considered stiffer.

Linear Spring Rate Versus Progressive Rate Springs

Standard suspensions have a linear spring rate. This means the rate is consistent throughout the spring, and the same force can be applied to both ends with the same results. However, some springs have what's called a progressive or dual spring rate. This means one side of the spring will have a softer spring rate than the other.

Why Spring Rate Matters

Other than 4x4 upper control arms, your springs are the most important part of your suspension, and your suspension is the most important part of your car's handling. Your suspension's spring rate determines how well it will do that. In general:

• Because stiff springs don't have as much give, cars that use them allow drivers to feel the movement of the vehicle. These cars tend to have less grip when hitting obstacles at high speeds.
• Because soft springs have greater extension and compression, cars that use them grip the road, which keeps the car body still. This will allow for a smoother ride overall; however, since the tires grip the wheels more readily, it may make the car prone to feeling unstable when you hit a turn.

How To Choose a Proper Spring Rate for Your Car

Naturally, how you want your car to handle depends on what you want to use your car for. Those who are trying to trick out their car for racing tend to prefer stiff springs since they'll be driving on smooth, paved roads and need to feel the way their car moves. People driving on normal, rough roads tend to prefer the comfort of soft springs.

What About Off-Roading?

There are mixed messages out there about whether stiff or soft springs are better for off-roading. Soft coils offer the traction that's essential for rolling over obstacles during activities such as rock crawling. However, stiff coils are better for the higher speeds you'll need if you're hitting the dunes. Some opt for the best of both worlds by using progressive-rate coils.

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