12:05 Garage- ’70 Duster build

72bluNblu said
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.