Need help with getting the correct dual rate spring set up in rear

[Jammer75]

Ok. So I have a spring rate of 160.5 now. I am at the upper limit it seems on compression (the fox valving chart). I’m at 35% droop at 1.5” of preload. I want to bring that up to 20% droop and I don’t think that  1/2” more of preload will get me there (I’ll see). 

so can’t I just go up on the spring rate a little (I understand it’s more of a trial and error) as I just need a little ride height as well as a little more for the g-outs. What I have now rides good. Or perhaps the flutter stacks I put in, which felt way better on the chop, but am I suffering loss of compression due to them?

How drastic of a change is it to go up on spring rate I.e. 162,167,171,etc.?

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Is it possible with my spring rates measures at 383 and 288 that originally (car built in ‘08) they were 400 and 300..? (171.4 sr)Possible for that much change in the spring? 

Here’s a pic from previous owner and not sure if id feel comfortable doing this with current spring set up!

 

[Goatpoker]

To use an engineering term. Springs are rated "nominaly' Which means, roughly roughly... lol. They ship in 50lb increments. (maybe able to find 25lb sometimes). So, yeah, it would have been 300/400 and they got soft through the years. 

We also rarely talk about driving style and useable wheel travel.  300/400 with 18" of wheel travel and a flutter will be fine for cruising and whoops at 30mph. Hard turning, big transitions, jumps... you might want more. Then you suffer the harsh penalty. Tune for how you drive. 

I'd say you should move in 10% or greater increments on springs. So, 300/400 -> 350/450 or 300/500 if you don't want to find both tender and main. I don't think you'll notice just changing 50lb on one spring. 

 

[Rockwood]

350/450 would be ideal, but definitely cheaper to go 300/500.

I wouldn’t go less than 33% droop. Less droop means picking up tires more often and falling into larger holes because the wheel can’t fall in the hole without bringing the car with it. Keep in mind that at 20% droop, you’re probably going to have a tight limit strap during hard cornering so you’ll effectively have zero suspension to deal with holes on the inside tire. 

 

[Jammer75]

I want all 4 new. I think the 350/450 combo is gonna be close. Reduce the compression and go up on the rebound.  I’ll post results. 
 

Compression and rebound. I’ll start with @Goatpoker recommendation from the beginning of the post….?