If its a helper spring then its super low rate, like 5lbs/in. If its a tender and not fully compressed at static load then its far too stiff. When I was playing with math and tender/assist rear spring for street use I found a low rate (~4k) and high preload was a better choice than a higher rate.

So then you're basically looking at the block height of whatever you put in there as an extension of spring length.

its going to compress for sure. my HS is "20 n/mm" i think, which is like 115 lbs/in (dont quote me, i'm confused with n/mm rates)

ok this has equivalent spring rate to mine
https://performanceshock.com/index.p...oducts_id=3389

yea theyre 3k springs, 2.4" long. i'll prob compress them halfway. car's currently on 8k front 13k rear, may settle it down a little.

it technically has 2 functions (not including the 'spacer')
1) to keep the main spring in place
2) set the position of the plunger/piston of the shock

The assist/tender springs need to be fully compressed when the vehicle is on the ground. Springs in series (stacked on top of each other) result in a rate that is lower than either of those springs.

(3 * 13) / (3 + 13) = 2.43

If your car doesn't fully compress the assist spring at the static ride height your effective spring rate in the rear becomes 2.4k until extra load is applied and the assist spring becomes fully compressed. This can make the car have extra body roll on turn in and can make it feel floaty, something not generally desired.

if my 900lbs spring compresses 6mm or 1/4" that means the rear suspension is supporting 225 lbs each. your 3k springs is only 162lbs per inch but probably long enough to provide more that 225 lbs each. 2inch compression is 162*2

the rear weights of the car were 678/638 (pre acd & batt relo, and a few other things), so it should compress it all the way if it's a 168lb/in spring when it's on the ground. kind of curious about when it leaves and returns to the ground though.

had these sitting in the garage for a while, so with me going to install the front and rear uprights this weekend, i'm interested in how these guys feel and how tweaking it with these will feel

you should subtract the weight of the wheels and brakes. theyre not supported by the spring
*partially control arms, axles, knuckles/hubs, shock also. thats why its easier to just spring rate*spring compression

ah ok. i'll just compress it to leave like half an inch then, see if it behaves weirdly or not. any preloaded amount will raise that end right?

what i call the 'snug' height is just when the shock is fully extended. I'm not sure thats what you mean saying "leave like half an inch". my helper/tender spring cannot compress less than 35mm. pics might help
I dont think its a good idea when the soft spring has gaps in the coil (not fully compressed), especially for a top heavy car. but i have no experience with that
i think "preload" is not a thing anymore when using helper springs

yea not on the car yet but i'll post pics before i make the final set on the suspension. will be for rear.

is it flat coil or round? if its flat then its designed to be fully compressed by the supported weight

wow. those spring rates really make sure they get maximum suspension droop/travel.