Effects on spring rate from a shackle flip

Let me preface this by stating I am a carpenter. No schooling in physics, engineering, etc. After listening to rumors and conjecture for a long time, I decided my curiosity needed to be satisfied. Lacking the math background to do calculations, I figured to build a full size model from scrap steel, the top two leaves from my original leaf pack, my removed factory shackle hanger, and a fabbed up shackle flip. Factory shackle was used in both instances.
I built a frame section to hold my weights, in this case a bundle of shingles and 80# bags of concrete mix, made a pivot for the front that could bolt to my workbench, and a stand to simulate the axle and wheel.
Next step was to mount the factory shackle hanger, front leaf mount, and bolt on the leaves.
Using the bundle of shingles to preload the leaves, I took my first measurement as my baseline.
I then added one bag of concrete mix at a time, measuring from the bottom of the frame rail to the top leaf.
Next up was the shackle flip. I made one with five sets of holes so I could vary the shackle angle and see any effect shackle had on the spring rate.
Starting with the front hole, I repeated the experiment as with the factory shackle set up. Each measurement was taken from the bottom of the frame rail to the top of the top leaf. Each baseline was measured for each set up.
I then repeated the experiment for each hole in the shackle flip, measuring starting and ending shackle angle as well as the frame to leaf measurements.
Unfortunately the last(fifth) run through ended prematurely as the shackle flattened out against the frame after adding only 160# to the frame as the shackle did not allow enough movement for the spring eye distance elongation. Iirc the starting shackle angle was around 52 degrees, and with the leaves having a decent arch to them with the weight on, there simply was not enough shackle movement left. I essentially discovered a maximum starting shackle angle for that amount of spring arch.
As I did this experiment over 2 1/2 years ago, the individual measurements have all been lost. As have some of the pictures.
Full disclosure dictates me stating that I do not typically take a ton of pics, don't do write ups, and don't even have a build page. I was mostly interested in the results as I wanted to prove my theory.
The short and skinny is that regardless of either tension or compression shackle, and regardless of shackle angle, adding 80# moved the frame down 1/2'. I measured a full two inch drop in all phases after adding 320#, which establishes that the two top factory leaves have a spring rate of 160# per inch, and that did not vary in any configuration.
Takeaways from this experiment:
1. Shackle flip does not change spring rate.
2. Shackle angle does not change spring rate.
3. Shackle flip will not flatten your springs faster. How can it if it does not effect spring rate?
4. Shackle flip will not change ride quality or alter the dynamics of the rear suspension. Again, no change to spring rate.

The flip does typically raise the rear of the truck. That moves COG and will effect handling.
Assuming the shocks allow full articulation of the axle, and the bump stops are not moved down the same amount as the flip lowered the axle, then the leaves will have more travel, and will fatigue sooner. It is an unfortunate fact that spring steel can and will fatigue after a certain number of cycles. Too many people have stated that allowing the springs to go into negative arch is the culprit. It is in fact the amount of change in state that causes the fatigue.
The front springs on my dually are negative arch and came that way from the factory. After 18 years and over 400k miles they are still serviceable. Why? limited suspension travel, i.e. small
change in state through full cycle.
Hopefully someone finds this helpful, or I just wasted a ton of time typing this.

 

I had more pics but they went walkabout

 

Sharp eyes will pick up that I only had four holes in the flip. And that the shackle flattened out on the third hole. And I had to plamsa part of the flip off as I went because the shackle was hitting it.
And no I am not going to bother to edit the first post. Too damn tired.

 

what a sweet experiment! If i was better at FEA i'd love to model this stuff and run simulations. maybe @Sub-Zero could? haha not to add to your workload Ryan

 

^^^™This is the real issue.

Nice write-up and experiment.

 

The angle of shackle and tension vs compression is going to change spring rate, based on the vectors, but I don't think it's going to be significant. I'll see if I can draw it up later. Someone else is going to have to do that math thought.

 

I don't think I have the proper software as I can only do linear static studies, but for this experiment a nonlinear static study would prove more realistic i believe. It would be so awesome to simulate this result and verify the results with computer models. That being said, I def. appreciate all the time spent on the physical tests. Awesome work @stairgod !!!!!!!!!!

 

So does that mean if you limited uptravel a little bit via bumps, and overall limited the amount the springs articulate, you won't fatigue them a quickly?

 

Short answer is yes.

Long answer is that your leaf pack should be designed around the range of motion, intended shape, allowed travel and weight being supported. In @stairgod 's example of his dually above, part of the reason that pack is fine in negative arch is that it was designed to live there. Putting it into positive arch like our packs would probably be bad for it's life span.

 

Of the aftermarket leaf packs available, I wonder which ones fare better at the shackle flip life (if any) vs stock shackle configuration.

 

Only way to know is either test or a meta-analysis of people's builds based on thier form posts.

 

Deaver make an expo pack that has many more and thinner leaves. They will fatigue more slowly.

 

If you limited up travel to essentially recreate factory travel numbers then yes. Because the leaves would end up with the same exact motions as stock.
However, even with stock shackles and leaves, a street truck will probably not have spring issues where an offroaded truck that regularly has the springs fully cycled will fatigue much faster.

 

It was not significant enough to show up in 2" of travel. If I had measured the slightest amount of change between the various set ups I would agree. Perhaps a long shackle would show some difference. I do not plan on testing that. I also stuck with the top two leaves as if made spring rate linear so that measurements could be more easily compared. A full leaf pack could have caused some weird measurements if the pack was progressive.

 

I like tests!

I thought the difference comes into play because a leaf spring isn't linear (not to mention packs themselves are progressive as more leafs engage) and the rate does change as they transition from compression and tension.

You get a rising rate with a tension shackle that you don't get with a compression shackle, which is due to the spring's datum and the relationship of the front and rear eyes.

In the stock configuration the rear eye is further above the centerline through the spring and axle so the effective spring rate is different than when the rear eye is lower than the front eye as happens in a flip. It worked on 1st gen Tacoma because the frame was designed to locate the spring and shackle properly for the geometry. In a 2nd gen the frame must not curve up as much and flipping the shackle is forcing the spring even further down the soft side of the rate curve.

If I'm not mistaken, I think the term for all of this is roll steer. You're changing how the wheel arcs through its travel as the weight shifts, so just flipping the shackle and changing nothing else can make a big difference in how the rear suspension cycles and feels even if the absolute spring rate of the leaf hasn't changed.

But I would also be very interested in hearing a mechanical engineering explanation.

 

Yeah, it's going going to be a small % that it matters (I think). Does it matter, yes, if you're an engineer at Toyota. Not to us.

 

So many words.

I just need to know if it’s safe to tow stuff now that the shackle flip has caused my springs to flatten?

 

My current set up is allpro expo leafs, archive garage shackle flip with sumo springs basically in fulltime contact with the frame. I can load ALOT of weight in the bed but I believe that the sumo springs are the only thing saving saving the leafs/enabling this. I figured a large amount of thin leaves was best for ride quality. It was until they started to go flat without any offroad driving. When I actually got offroad I snapped the main leaf in the pack from what I believe was too much tension on the leaf from a 5" shackle. Could have also been shitty quality control on allpros part. Could have been I was heavily loaded. Either way this combination of thin leaves and a shackle flip does not sit right with me. I'll be doing the 63 swap in the future as these are over sized and tested leafs that I can somewhat trust. My 2 cents.

 

I would suggest air bags or sumo springs if your towing often. I believe our trucks aren't meant to tow too heavy or too often.

 

I was just messing with Mohawk