12:05 Garage- ’70 Duster build

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.

lol! If I had a dollar for every time I went out to my car in my pajamas to measure something I could probably buy a HellCrate for it!

Ok, so with a motion ratio of .807 and a 450 lb spring you should end up with a wheel rate of ~293 lb/in, which is really close to the 300 lb/in wheel rate I run with my 1.12” torsion bars.

Your previous 400 lb/in bars have a wheel rate of ~260 lb/in

I think the 450 lb springs should be a pretty good improvement for you, certainly should cut a little more body roll and give you a good base rate to tune from. There’s still probably more room to go higher, but it will depend on your tire set up and compounds. With the Azeni 615’s at 200 treadwear on my Duster and 275’s up front I’m back to being able to put a little roll into it with spirited mountain road driving. I have a set of Firm Feel 1.18’s sitting in my shop, that’s a 370 lb/in wheel rate. I talked to Firm feel about it, I’m a little skeptical about that big of a jump in WR but they seemed to think it wouldn’t be as crazy as it sounds. Maybe I’ll have to go with some adjustable shocks at some point.

Do either of you have any pictures of the seatbelts as installed? Then I will stop mucking up your thread, Joe. Thanks!

Here you go! Yeah my factory ‘74 retractors were inside the quarter structure behind the door. But the mechanisms were a PITA so I replaced them with the ones from seat belt planet. I just mounted the retractors to the floor where the lap belt anchor was, I don’t recall exactly why I didn’t try to put them in the quarter but they work fine on the floor