Rear leaf spring Question 74 Dart
Seems like there's a few terms/properties being interchanged that aren't interchangeable. Spring rate and spring arch are not the same thing.
Leaf springs sag as they get old. The arch changes, and the ride height gets lower. The spring rate, however,
does not significantly change. That's important. The spring rate is a property inherent in the metal used. So your old, worn out saggy springs have a spring rate that's almost identical to what they were originally. The only reason it's different at all is because of the construction of the spring- the clamps loosen, the slide cushions between the leafs break down, etc. For the ride, the biggest changes that you notice with the "floppy ride" are mostly worn out bushings and contacting the bump stops because of the loss of ride height.
The spring rate is what you want to pay attention to. Somewhere I have the breakdown of the spring rates, but this is what they are for A-bodies and most of the available options
stock 5 leaf - 90 lbin (/6 and base 318 cars)
6 leaf "XHD"- 110 lb/in (340 cars and some upgraded 318 cars)
Mopar oval tracks- 120 lb/in
Hotchkis handling- 130 lb/in
Super Stock 002/003- 160 lb/in
The spring arch is what really controls the ride height. Adding a leaf mostly changes the spring rate. If you add a leaf without changing the arch you do end up with a little more ride height, but that's because you have a higher rate so the compressed arch of the spring is higher. The free arch is the same.
Hi guys,
Replacing my rear leaf springs with new ones from General Spring. I saw that there is an option to add and extra leaf if i wanted. I have read that others have added the extra leaf to improve handling but came with a stiffer ride. I am unsure as to the pros cons of doing so. The car is stock 6cyl and just looking to use it as a summer driver until I get into the restoration. So I don't want a ride like a truck but I also don't want that sway. Any help is much appreciated
What you need to find out is the spring rate of the springs you're buying. The construction can be totally different. If they just "add a leaf" to the 5 leaf spring pack you'll end up with a higher spring rate and a little higher ride height. But there's nothing that says that's what they have to do, their "5 leaf springs" and their "6 leaf springs" could have totally different construction- different leaf thicknesses, different arches, etc that could totally change things. More than likely that isn't the case, but if you don't ask you're just assuming.
The factory 5 leaf springs (90 lb/in) are pretty soft, and the factory ride height with them was actually pretty low to start off with. Even the original promotional advertising shows a lot of cars sitting dead level or even a touch nose high. The factory "XHD" springs, which had 6 leafs and a 110 lb/in rate are a nice upgrade if you have the factory 5 leaf springs. The ride is not too stiff, the handling will be improved and the rear end will be slightly higher. Even most of the aftermarket springs intended for good handling are only in the 120-130 lb/in range, which doesn't lead to the car "handling like a truck". They're noticeable firmer, but not crazy as long as you have decent shocks (not KYB's!). The SS springs are like 160 lb/in and while they definitely help launch they absolutely ride like a truck.
Mopar Performance springs as of late have been all over the place for quality. Even the XHD's I put on my Challenger like 8 years ago were made in Mexico and sagged significantly. The spring rate was good, but the car ended up pretty low after 6 months. Fine by me, but not everyone wants their car in the weeds. So I would avoid them unfortunately, because the MP oval tracks make for a great spring if the quality is good.
