How to correct stiff leaf springs that sit too high - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-30-2009, 10:31 AM
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How to correct stiff leaf springs that sit too high

I recently had new parrallel leafs made up for my street rod project. Now that the body is back on and most of the dead weight is loaded on as well, I find the rar of the car is sitting about 4" higher than I would like. What is the most preferred method to getting her down?

There are five leafs per spring. I'm using a custom set of short shackles (3" c to c) The rear of the car was weighed, fully assembled, at 1,700 lbs. When calculating the load the springs would need to support, I calculated it as follows:

Weighed Load (at ground) = 1,700 lbs gross weight
Weight of axel, tires and springs = 475 (deducted from gross weight)
Weight of body, interior components supported by rear wheels = 400 (estimated)
Weight of live load (persons, fuel and cargo) = 600 (estimated)

Total weight carried by both springs = 1,700-475-400+600 = 1,425 lbs
Result = 1,425/2 = 712.5 lbs per spring.

Spring length overall is 47.5" and length from front perch to axel pin is 24"
Spring width is 2.5"

I am considering two options at the moment.
Option 1: Remove at least one leaf and re-arch the remaining leafs.

Option 2: Remove two leafs only and do not re-arch the remaining leafs.

Concerns:
1. Which leafs do I remove? The shortest one, or the next one in, or??
2. If I remove leafs will I significantly reduce lateral stiffness?
3. Do re-arching a new spring compromise it's performance?
4. Does the spring rate change as the arch changes?
5. Will removing a leaf or two require me to supplement the spring's ability to resist torque distortion with additional hardware such as drag links?

Any educated advice would be truely appreciated. Thanks

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Old 11-30-2009, 10:52 AM
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What is the rate per inch of the springs you had made??

Just working off of the 1425 lbs load the spring rate per inch required is about 160-180 lbs per inch.

Needing to go down 4" is a lot, you may not be able to get there with the springs you have, even after removine a leaf or two and having the rest re-arched.

Have you tried loosening the bolts through the spring eye and shackle to make sure you aren't bound up there?? Also, what angle are the shacles on relative to the chassis and spring??

How many inches of free arch did the spring have before installation, and how far is the spring compressed now?? i.e how far did the spring settle under the current weight??
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Old 11-30-2009, 12:03 PM
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you maybe able to gain some by useing spacers under the axle. after i get my new tires on my cougar i plan on adjusting my ride height and thats one of my options.
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Old 11-30-2009, 03:07 PM
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Are the springs too stiff, or just too high? One has nothing to do with the other. If they are not too stiff, but just too high, removing leaves is not a good idea.Then the spring will be too weak.

First thing, in my experience new springs settle a lot in the first 100 miles, as much as 2" in some cases. This is after being in the car for some time. So before I did too much, I would drive it a bit. Otherwise you will be asking how to lift it back up.

Once it has settled in, if you still want it lower, blocks will do it,,and give you the option to change it in the future if needed.

Rearching the spring does not change the rate, that is determined by the steel the spring is made of, width, length, and thickness.

Rearching a spring does not hurt them if done properly.

Long leaves as a general rule determine the ride, short leaves as a rule support the load. Removing long leaves give the main leaf less support, so in that case you would need some type of traction device. Remember that a mulileaf spring is designed so each leaf supports the one above it . So removing leaves can compromise the integrity of the spring. Make sure the distance between the ends of the leaves are the same on each one.

Last edited by enjenjo; 11-30-2009 at 03:20 PM.
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Old 12-01-2009, 07:04 AM
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enjenjo and ericnova72, Good Questions. I'll do my best to give you straight answers.

1. I do not recall the spring rate, but we were looking for something around 150-160.

2. The springs seem too stiff.

3. The unsprung arch measured about 6" Thus far, the loaded arch is about 5.25". We loaded the trunk with 750 lbs and the springs deflected about 2.5 - 3 inches.

4. We have checked the spring mounts for binding and found nothing.

5. The angle of the shackle is 14 degrees out of plumb measured along the shackle bolt centers with the bottom shackle bolt back from the top bolt.

From the comments thus far, sounds like I should wait until spring before I make any decisions.

Another Question: Once I get the axle lowered to the desired level, I suspect the drive shaft angle will change. Of course I'll check it, but do you suspect I may have to change the position of the axle seats to get the pinion angle right again?
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Old 12-01-2009, 08:02 AM
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You need a REAL spring shop (few of them around anymore) to make you the correct springs. You can calculate all you want but if you don't have someone do the job right, you are stuck to modifying new springs!

And yes the pinon angle may well need correcting. But wait until you get it down there.


Brian
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