Front Coil Spring Question

This is on my 94 Dakota that has 1,800 lbs on the front end with QA1 coil over conversion shocks/springs.

I was running 400 lb springs, 10 inch and changed to 300 lb springs , 11 inch.

Before making the change, I did basic math (but didn't take into account the spring angle) and the 300 lb spring should have actually been "taller" than the 400 lb spring after being installed.

But, once on the truck, that was not the case. I had to crank the spring all the way to the top of the shock body and it still sits about 1/2" to 3/4" lower than it did with the 400 lb springs. I want to get the front end back up a bit to also help with weight transfer.

Anyone ever use a "coil spring spacer" to install in between the coils on the bottom most coil to get the height? This way, the spring will still have MOST of it's properties. Or, I can use a spacer that sits between the shock adjustment plate and spring to lift the spring up an inch (QA1 9004-110 Spring Spacer).

I don't want to go back to the 400 lb spring.

Thoughts and/or experiences?

Bummer there are no suspension experts on this board anymore... oh the good ole days...

A 300# spring will need to be compressed 1.33 x the 400# springs previously compressed distance. Since the your new 300# spring is 1" longer, that distance will be decreased by 1". Example 400# spring was previously compressed 2". To maintain the same height with the 300# spring. 400# X 2"= 800# that was being supported. Now with the 300# spring 2" X 1.33 = 2.66" -1" =1.66" so in this case the spring platform would actually need to be compressed less to maintain ride height (.66"). Ah but there's more to it. You need to use wheel rates instead of spring rates. These can be calculated from the formula in the diagram below. Then assuming the difference in wheel rate you can then figure the difference in the amount of compression distance of the 2 springs. But by my math up until 4" of compression at ride height the spring perch wouldn't be turned any higher with the 300# spring than the 400# spring. Are you sure that you have the rates of both springs correct?
Doug

I am not an expert but To accurately calculate this you would also have to take into account the leverage of the control arm.
I would not use any kind of spacers between the coils. A spacer under or over the spring would be ok as long as the spring does not coil bind when compressed.

I got treed by DVW. He did a much better explanation of the leverage issue anyway.

I'm no expert, but it reads like the better option may be to install an 11" 350lb spring. Have you consulted with QA1? You may also consider contacting AFCO for technical advise. You already know the front weight of 1800 lbs so you're half way there.
The coil spring spacer seems like a bandaid for the incorrect spring.

Agree. A longer spring with your desired spring rate is the best option as long as coil bind doesn't come into play.

Thanks all! As I mentioned before, I did "basic" math and didn't take into accounts the necessary angles to get exact measurements, since I was only trying to get the reference to each other.

The issue is that I run a QA1 "conversion" coil over and they only offer limited springs so either a 10" 400 lb, 10" 350 lb, or 11" 300 lb. Yes, a 350 lb spring sounds like it would be the correct one, but I can't get one, unless someone else makes one that I am not aware of.

So, I will most likely just add a spring spacer (https://www.amazon.com/gp/product/B01MQL6J71/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1) that shims up the spring about an inch from the bottom of the spring perch.

speedway motors has a chart that tells what percentage stiffer a spring needs to be, depending on the mounting angle of the spring.
i think this chart is for coil over springs, and i'm not sure it pertains to this math, but i thought i would throw this out there just in case it could be useful. the chart is on their website.

Knowing what shocks are on the thing, like a part number, C-C at ride height, etc, would be helpful.

There are no 2.5" ID, 11" springs for a common coil over that I'm aware of. What are the diameters of the springs?

The answer is - typically, you want the lightest spring that will hold the shocks at the desired C-C at ride height without coil binding before the shock is bottomed out, or you run out of threads. If it wants to separate too violently at the hit, control it with the shock. Information on Length At Coil Bind for springs is available from the manufacturer.

I always end up with lighter springs than what the math tells me.

A short spring will have to be heavier than a longer spring to fit in the same space and do the same job. Simply because the long spring will have to be compressed more. Compressing the spring more stores more energy. That helps get the separation at the hit.

This is similar (couldn't find the exact part number off the top of my head) and the spring is 4.1" upper ID, 2.25" lower ID.