I've decided that I will probably purchase new leaf springs for my 57 Dodge D500 2d ht, since I have a slight case of 'saggy rear' syndrome, and new springs are not as expensive as I thought ($250 plus/minus.)  In the larger scheme of things and all the money I'll be spending, I think it will be money well spent.

SO, does anybody have any suggestions for me in terms of what spring rate I should go for?  Or any other leaf spring data?

According to my research so far, Dodge Dart leaf springs are at least dimensionally correct, although there is no guarantee that they would give the optimal ride height (being designed for a lighter car.)  Also, I am guesstimating that the original spring rates for my car were probably about 100 lbs/inch and a design load of 900lbs.  I would like to go for something that would be the equivalent of the Plymouth Fury.  I am guessing that might be more like 130 lbs/inch.

Any data or recommendations would be much appreciated.

p.s. I also plan on upgrading to the 952/3 torsion bars up front.  They are currently 950/1.

Have you ever considered selling those, or are you saving them for a rainy day ?

I think the answer is...it's all relative.  There is a such thing as too soft, and there is such a thing as too stiff.  Both are bad for handling.

What I've always understood is that it is better to be over-damped than under-damped.  Having a soft spring with a stiff shock would fall into the over-damped category, which is better than the opposite.

BUT the RIGHT stiffness spring with the RIGHT stiffness shock is the ideal.  Then it becomes a matter of what is 'right.', and that can be very subjective.

I personally think these cars are too soft to begin with.  But that is just an opinion.  SO I personally want to up the rates a notch, but I also don't want to go too far.  I'm not building a race car, afterall.

I had an Alfa Romeo once, and they were known to be a little 'soft' in the spring department.  I bought some 12-way adjustable Spax shocks for it, and set them to a rather stiff setting.  I was very happy with it.  I might even say it was one of the best handling cars I've ever owned, despite the fact that they were known for being a little 'soft.'  But that is relative.  Alfa 'soft' is no where near as soft as '50's American car' soft.

I measured my current springs (which I suspect to be worn out) an they are currently about 7.5" arch.  So at 100lbs/inch, they would be at about zero arch, too.  But I don't know how much arch they've lost from new.

I've read that, unless the suspension geometry was specifically designed for it, a leaf spring going into negative arch can have negative implications.  It all depends on the shackle and whether it is moving TOWARDS a 90degree orientation from the frame mount as the spring compresses (good) or away (bad.) However, if that is what our cars are designed for, then that is another story.

It's hard to explain without drawing a picture.  Essentially, as the shackle moves towards vertical, it has a lifting effect on the frame and increase spring rate, which is what you want.  If the shackle is being pulled or pushed AWAY from vertical, this has the opposite effect.  SO if you have a spring that is the same length as the frame mounts, at zero arch the shackle would be vertical.  But as the car leans into a turn and the spring goes negative arch, the shackle will move AWAY from vertical (being pulled in by the compressing spring), thus shortening the distance between the spring frame mount and the spring and thus dropping the frame even farther causing even more lean in that direction.  Also the effective spring rate decreases, creating a sort of 'progressive rate spring'...only progressing to softer rather than stiffer the more the spring is compressed(bad.)  But if the spring is LONGER than the distance between the frame mounts, then the opposite effect will be had.

What you REALLY don't want is for the spring to have a slightly positive arch that then goes negative after being compressed to far, because then you have the chassis getting stiff as it progresses TOWARDS zero arch, and then suddenly going soft as it PASSES zero arch.  I guess this is the case where it is better to be at zero arch to begin with, because then you don't transition from positive to negative (which results in unstable characteristics)...you just go towards negative.  But the spring needs to be longer than the frame mounts at zero arch, so that it pulls the shackle towards vertical as it compresses (shortens.)