Pirate 4x4 banner

1/4 Elliptic spring rates

10K views 16 replies 5 participants last post by  4Bangler 
#1 · (Edited)
Okay, I'm mulling over my winter project for my YJ, 1/4 elliptical rear suspension. I've been reading over all the suspension link threads an think I've got a good handle on what I want to do, but none of them 1/4 ellip topics I could find discuss spring rates. I have no problem with building custom springs, I have a good spring shop nearby that will build anything I ask for, but the specialize in heavy trucks and scratch their heads at anything under 500lbs/in, so I'll be designing the springs myself.

My question is, how does mounting a leaf spring in a 1/4 elliptiacl fashion effect the spring rate?

I'm running whipped out 140lb/in springs now and they seem to work okay.

I'll be mounting my 1/4 ellip's with a spring eye at the frame, then an adjustable solid frame mount about six inches down the spring at the spring pin, then a swiveling shackle at the axle end. The adjustable solid mount can be adjusted to even out static ride height and can be disconnected if I ever what to wow the mall crawlers at an RTI comp. I don't like the "my axle is falling off" feeling of a 1/4 ellip setup with no solid mount that only provides spring rate upon compression, so my main leaf will have support leaves above it to support the vehicle weight, and a couple leaves below it to control down travel. I can calulate the rate for a conventional semi-elliptical spring setup, but will the added leverage of the 1/4 ellip mounting cause a need for increased spring rate? I need to get my sprung and un-sprung weights nailed down, as well as my COG before I start, but I've got some gear racks and a new cage to build first. If I design the spring for about 170lbs/in up and maybe 80lbs/in down, do I use the length from tip to my solid mount as my working length, since that is the only length of the spring that is working? Do the formulas work the same for a conventional spring that is compressign both ends to support weight applied to the middle vs. my 1/4 ellip that is compressing one end to support the weight six inches from the other end? All the leaves will extend to the spring eye on the mounted end, so there shouldn't be any deflection between the solid mount and the frame spring eye.
 
See less See more
#2 ·
Here's an equally feeble sketch to accompany my feeble descriptive skills
 

Attachments

#3 ·
like this....
 

Attachments

#4 ·
Only with controlable extension, like this...
 

Attachments

#6 ·
Nice write up, sounds like he had many of the same concerns I have, and I'm glad I went with a 9" rear, none of that diff trussing hoop-in-a-joob to deal with, fab the link brackets and weld away. I'm also concerned about the rear doing all the work and the front suspension staying at static ride height, but I think by controling the rear suspension's extension, and pulling some tricks out of my hat for my new SOA 3/4 ellip front, it should stay pretty balanced.

He used off the shelf spring packs, so my question is still unanswered.
 
#7 ·
Any of you suspension geeks have anything to add to this? Or has everyone gone the coil-boinger way?
 
#9 ·
I think I got the jist of your intentions.
I don't think that worrying over the spring rate needs to be down to the gnat's ass. you can always adjust your leverage on the spring by moving your frame mounted pivot lock forward and back along the spring, and you can adjust the ride height by blocking out between the frame and the spring. with that in mind, you can probably get away with really soft springs, or just cit dowm junkyard stockers to experiment with and give you a direction to follow.
 
#11 ·
My swivle shackles will be built with a Heim or similar attachment to the axle, which will allow a little height adjustment, maybe up to 1" or so, and the adjustable solid mount will be built with a left and right hand threaded link similar to the suspension links to allow adjustment of the static ride height and spring preload. I never really thought about making the solid mounting point adjustable fore and aft, I like it. I know this thinkg will probably get built and all the adjustments will probably never change, but it would be nice to be able to change some settings for running with full trail load or light playing or competing. I was just wondering how the mounting would effect the calculation of spring rate so I could get a good starting point since I don't plan on brackets that will accept anywhere from 5 to 15 leaves, just maybe one or two leaves more less depending on performance.
 
#12 ·
Okay, let me re-word the question, how does 1/4 elliptic mounting effect spring rate? I have 140lb/in springs mounted in traditional semi elliptic fashion, if I were to use these same springs in 1/4 elliptic (nope, I'll be building new ones), would they still be able to hold the Jeep up? With the effective spring length shortened, you would think that less leaves would be required to acheive the same rate, yet the increased leverage of the 1/4 elliptic mount would seem to require more spring rate than conventional mounting. The only post I've seen on this subject went like this...


quote:
--------------------------------------------------------------------------------
Originally posted by Donovan
Does anyone know the formula for a 1/4 Elliptic Spring or is it one half that of a regular spring?
--------------------------------------------------------------------------------

FatCity said:


Technically,
it should be twice the average spring rate of a full elliptic,

Eric Filar
I'm not questioning the knowledge here, just wondering what the tech behind it is
 
#13 ·
1/4 elip is something that's been on my mind for a while. But, I've still got lots of things to play with before starting THAT project. :D I've also got some of the same goals as you when/if I do this. I would really like adjustable ride height and rate to deal with different loads too. My rig is non-comp and may have only me plus minimal gear, or 4 people and gear, so weight covers a rather large range. Hmm, considering how I toasted my reversed F150 rear leafs in only 3 days at Moab, maybe I'll be doing it sooner than later?

Anyway, in my mind, I had thought to use air bags under a fairly healthy set of packs where the pacs are setup to provide a fairly minimal (but functional) height and firm ride with no air in the bags and a light load. Then, air could be added to the bags providing a softer rate and more height as needed for fording, break-over, whatever. It could also take up the slack from excess weight. If the bag system (driven by OBA) blows, the ellips are still there to get you by... Just thought I would throw that out “for what it’s worth” (not much since I have no experience with that sort of thing).

Now for my stupid custom spring newbie question. It’s obvious how you are going to get the eye near a pin when ordering a custom pack. But, one of the pics seems to show a “factory pack”(?) with an eye near the pin. How is that done? Take the main leaf to a spring shop and have it rolled in?
 
#14 ·
I'm not questioning the knowledge here, just wondering what the tech behind it is
the moment of a simple span beam (your leafspring, L inches long) with a load in the middle (your axle, P) is PL/4

the moment for a cantilever beam with the load at the end is PL

so if you cut the spring in half, use it as a 1/4 eliptic, you should be doubling the spring rate.

I could have missed something, that's a gross simpification, but I think it'll get you in the ballpark.
 
#15 ·
Sundowner, that's the kind of simple logic I was trying for, thanks, I'll try to adapt my spring rate calculation for cantilever beam loading.

BadDog, I like the airbag idea. The spring packs in that pic are K5 Blazer springs cut in half, with a spring eye double pass welded to the top and bottom leaves by a very good welder. I say scary, especially with no other solid mount to the frame, if that weld breaks, that side of the Jeep drops about two feet and that spring becomes a pole vault for the rear axle. Although it does work for testing out spring length before building a custom leaf pack. I plan on using a full main leaf, with the center pin re-drilled about six inches from one eye, the rest will be leaves from a longer pack, cut off about six inches from the center pin and bolted up.
 
#17 ·
Don't get me wrong, that guy can weld like nobodies bidnez, and knows way more about welding than I ever will, but I just don't like welding to spring steel, and I like to design my stuff with more mechanical advantage, so my shitty welds are not the only thing separating me from happy wheeling or a firey crash.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top