12:05 Garage- ’70 Duster build

Yeah with the motion ratio you’d be able to calculate the wheel rate, so you’d be able to compare it to similar Mopars with torsion bars. This is a decent explanation

Spring Rates Part 1: The Motion Ratio

And RideTech has a full on calculator for calculating spring rates based on corner weights, etc. But one part is that is a decent schematic showing how to measure motion ratio

Spring Rate Calculator - Ridetech

For @DionR, my seat belts are in fact from from seat belt planet. They’re a custom set because they didn’t have an application for a Duster at the time, they’re 140" length, 12" sleeve (bucket seats), 8" extension on the shoulder loop. I feel like they fit pretty well with placement of the belt for me anyway.

3 Point Retractable Seat Belt with All Metal Push Button Buckle
I got impatient and went out and took some measurements this morning and used the ride tech calculator.

Note: I estimated corner weight based on a total car weight of 3500lbs with me in it. I haven't weighed it with the HDK setup (scales at Moparty were backed up). It was previously 3535 with me and t-bar suspension. I also lost a couple lbs since last year...lol. I also guess at the unsprung weight.

I used a few different corner weights, based on front/rear balance percentage to see how the results would change.

Motion Ratio = .807
55/45 = 400lb spring rate
56/44 = 425lb spring rate
57/43 = 425lb spring rate
58/42 = 425lb spring rate
59/41 = 450lb spring rate
60/40 = 450lb spring rate

Changing the unsprung weight just bounced these results between 400 and 450.

It looks like I did a good job guessing on the initial spring rate and ordering the next size up, 450lb. (sometimes its better to be lucky than good) QA1 doesn't have a 12" 425lb spring. I'm sure I can find another brand, but I doubt it will make much difference, especially since my weights are guestimates. If it's too much and I get some oversteer, I can always loosed the rear bar up. It's full tight right now.