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Rear spring rate

Rusty knuckles

Rusty knuckles

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Any suggestions on a rear leaf spring rate for a 69 charger that has 1780lbs on the rear? Good handling minded is my intent. I'm looking at hyperco composite springs. They appear to start at a 150lb/in rate and go all the way to 225lb/in. Near as I can tell the original springs were in the 100-110lb/in rate.

I need some more rear spring to get the rear to turn a little better. I'm currently on stock torsion bars but plan to bump up to 1.03" bars. I'm also running 1-1/8" front sway bar and a 7/8" rear bar. The balance was pretty good with those sway bars and stock springs. So I don't want to upset that. Thinking like a 175 lb/in rear spring rate.
 
I believe the XHD units are around 130 or so. Kit Car circle track units started at 150 and went up plus had different arches available. I used the 0 arch ones on my Challenger. For my 65 Coronet, Dick from Firm Feel sold me the XHD units. 1.06 bars for the front, 1.25"? front sway bar, Bilstein shocks and on the rear I had to use a 3/4"? one from Hellwig since the rear one I bought from him wouldn't work because of me moving the springs inboard. Newly gained space from the mini tubs and spring movement would have interference between the bar ends and the wheel/tires I'm hoping to run. I don't remember the weight splits between front and rear but the total was 3941 with out me in it. 1/2 tank of fuel, iron headed 440, 727, a/c, p/s.
 
I took some shots out of my Kit Car book for you. #1 & 2. 2nd shows the TB rates they had available. 3rd is from the out of production Mopar Suspension Handling book. I had the 324 springs and the 1.06 bars on the Challenger.

IMG_5036.jpeg


IMG_5037.jpeg


IMG_5035.jpeg


Scan duster 2.jpeg
 
ckessel said:
I don't remember the weight splits between front and rear but the total was 3941 with out me in it. 1/2 tank of fuel, iron headed 440, 727, a/c, p/s.
Click to expand...
Man the earlier cars were a fair bit heavier than the later ones, my '71 Coronet weighs in at 3478 at 1/2 tank, with 1599lbs on the rear. There is a bit of difference in our drivetrains though as mine is an all iron 318/904/8.25/PS/No AC rather than your 440/727/8.75 (I'm guessing there)/PS/AC.

20230828_162914.jpg

To contribute to the actual topic, Hotchkis lists their rear springs at 190lb/in, at least for the lighter 71- B body application.

That also brings up another question, in the Kit Car book referenced above, what length are the torsion bars listed? I ask because the same diameter torsion bar in an A body length would have more rate than the same diameter in the longer B/E body length I'm not that familiar with the Kit Car chassis, so not sure what they were using.
 
My buddy has a 440 1970 Plymouth Cuda that weighs more than my 68 B body.
A few years ago I was at 3750 lbs without me in my car, 451 (400 block), aluminum heads. My car might be a bit lighter now since swapping a few parts out.
 
I'm looking to get a little lighter but not willing to get drastic. I'm at 4100 with a full tank and me in it.

I'm starting to build a 68 charger I'm willing to get a little more aggressive with but I think 3500 is a pipe dream for me without some fiberglass.
 
Rusty knuckles said:
Any suggestions on a rear leaf spring rate for a 69 charger that has 1780lbs on the rear? Good handling minded is my intent. I'm looking at hyperco composite springs. They appear to start at a 150lb/in rate and go all the way to 225lb/in. Near as I can tell the original springs were in the 100-110lb/in rate.

I need some more rear spring to get the rear to turn a little better. I'm currently on stock torsion bars but plan to bump up to 1.03" bars. I'm also running 1-1/8" front sway bar and a 7/8" rear bar. The balance was pretty good with those sway bars and stock springs. So I don't want to upset that. Thinking like a 175 lb/in rear spring rate.
Click to expand...
What is the main use for the car? If it's more track then street, then yeah, go with more spring and bars....but if mostly street, I like factory or just barely above factory springs and a good bit larger sway bars. That gives a better every day straight line ride and yet handles much better in the corners.
 
Cranky said:
What is the main use for the car? If it's more track then street, then yeah, go with more spring and bars....but if mostly street, I like factory or just barely above factory springs and a good bit larger sway bars. That gives a better every day straight line ride and yet handles much better in the corners.
Click to expand...
It's a bit of both. I don't want it to ride too harsh but looking to step up from where I'm at. It rides very nice now. I'm willing to give up a fair bit there. Rides nicer than any of my modern stuff currently.
 
Rennkafer said:
Man the earlier cars were a fair bit heavier than the later ones, my '71 Coronet weighs in at 3478 at 1/2 tank, with 1599lbs on the rear. There is a bit of difference in our drivetrains though as mine is an all iron 318/904/8.25/PS/No AC rather than your 440/727/8.75 (I'm guessing there)/PS/AC.

View attachment 1679260
To contribute to the actual topic, Hotchkis lists their rear springs at 190lb/in, at least for the lighter 71- B body application.

That also brings up another question, in the Kit Car book referenced above, what length are the torsion bars listed? I ask because the same diameter torsion bar in an A body length would have more rate than the same diameter in the longer B/E body length I'm not that familiar with the Kit Car chassis, so not sure what they were using.
Click to expand...
Years ago when I got my $3k t-shirt from FF, got free parts with it LOL, Dick pretty much knew how much my car would weigh. He said it's heavier than you think. I figured 3600ish. Boy was I surprised. With the aluminum engine I have for it, there is about a 200lb loss right there, but I've added structure pieces to it with most in the middle so it may be a wash. 8pt cage coming too when I can swing it.
On the bar length, the ones I got were B/E body length, same with the springs. Those were Kit Car too. The guy who wrote the handling book also sold parts at the time, late 80's. I ordered t-bars, springs and sway bars from him. Several months went by with no parts but I was charged for them. I tracked down who made them for the guy and was told he had outstanding orders that had not been paid for. Got the money back for the t-bars and springs, he sent me the sway's. I knew a guy who had experience with the KC and he got me in touch from a team who had the needed items hanging on the wall. They were too light for what they wanted but were perfect for me.
 
Rennkafer said:
Man the earlier cars were a fair bit heavier than the later ones, my '71 Coronet weighs in at 3478 at 1/2 tank, with 1599lbs on the rear. There is a bit of difference in our drivetrains though as mine is an all iron 318/904/8.25/PS/No AC rather than your 440/727/8.75 (I'm guessing there)/PS/AC.

View attachment 1679260
To contribute to the actual topic, Hotchkis lists their rear springs at 190lb/in, at least for the lighter 71- B body application.

That also brings up another question, in the Kit Car book referenced above, what length are the torsion bars listed? I ask because the same diameter torsion bar in an A body length would have more rate than the same diameter in the longer B/E body length I'm not that familiar with the Kit Car chassis, so not sure what they were using.
Click to expand...
My guess would be 41", unless that is the a body kit car? The Kit cars were not Nascar "Grand National".
Additionally, I believe the B and A hex TB sockets max out at 1.25" dia, The C body TB max is around 1.40" Dia, I have no idea how they exceeded that unless with custom large sockets or splines. that i have never seen pictured.
I started a project during covid to upgrade to C body hex's in a B body length and use a 1.38" CM hollow TB to get the rate back down a bit and lose 8lbs per TB over 1.25" TB. The heat-treating polishing and cyro are still needing to be done.
I have composite leaf's on three of my cars, and they save a lot of weight,
 
Last edited:
Rusty knuckles said:
I'm looking to get a little lighter but not willing to get drastic. I'm at 4100 with a full tank and me in it.

I'm starting to build a 68 charger I'm willing to get a little more aggressive with but I think 3500 is a pipe dream for me without some fiberglass.
Click to expand...
I have a '68 Charger R/T. 498 w/Indy heads, iron bellhousing, 18 spline, Dana 60 with Caltrac rear springs and leafs.
3550# w/ 10 gallons, no driver, no fiberglass
(no heater or wiper motor either)

20200415_162157.jpg
 
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