Shackles: A Study in Leaf Spring Setup

While researching for my upcoming Chevy 63 swap I was disappointed by the lack of good information on the internet about shackle setup. Yes there is information, but much of it is incorrect, vague, and misleading. So down the rabbit hole I went, falling deep into a search for mathematically backed answers to my various questions.

Today I will be touching on one that I have seen often debated but never fully explained: how do I get the most dirty hooker flex out of my leaf springs?

Let's start with a simple drawing of a compression shackle leaf spring setup.



This setup is common, though not stock on our late model Tacoma's. This is mostly because of its digressive dampening characteristics. In other words the more you compress the spring, the softer it gets. I won't dive further into this because there are actually a lot of good videos about it on YouTube. What I want to point out though is the triangle formed by the 3 points formed by each end of the leaf spring and the frame mount of the shackle. If we simplify the drawing to show these points we end up with this:



Here you can see the 3 sides formed by the frame, shackle, and effective spring length. The latter is the distance between the 2 spring eyes, not to be confused with the length of the spring along the arch.

So now you might be thinking great we made a triangle. Why do we give a shit?

This triangle is important because it's what we will use to place the mounts on our frame when installing longer leaf springs. This same triangle can be used in a tension shackle setup too, except the shackle mount will be below the datum line. The relationship between these 3 points can and will have a huge effect on whether your truck has droop for days or leaves you stacking rocks to make it through a trail.

Now the fun part: the math.
Using a simple online triangle calculator and some values relative to our setup we can decide, before install, our shackle length and shackle angle, and get a rough idea of how much travel we will get. In the following 6 examples we can see that very slight changes in angle or length can have substantial impacts on our overall suspension travel.



This probably looks like a chicken scratch mess, so I'll explain further.
Each triangle is the same as the ones shown above, but drawn when the spring is completely flat. This is when the leaf is at its longest. If it compresses further, the shackle will reverse it's direction of travel and the leaf will begin to shorten. This is a good place to start from because it puts the shackle at its most extreme angle. We will also consider this position to be full bump, so we do not end up over bending our leaf spring, causing it to fatigue quicker and be at a higher risk of breaking.

The base(bottom) of our triangle is our flat leaf spring length. In my case with my Chevy springs this length is 64 inches. This length will remain constant throughout all these examples.

The right side of the triangle is our shackle length. I used 5 and 6 inch shackles in these examples as that's what most people seem to be running with Chevy springs.

The shackle angle is the angle of the shackle in relation to the datum line.

From these measurements we can easily calculate the other side of the triangle which represents the distance between our frame mounts. The other 2 numbers you see are the height of the triangle and the effective spring length when the shackle is at 90 degrees(full droop). The shackle length - the height of our triangle gives us the vertical distance that our rear spring eye moves during its travel from bump to droop. We get the effective spring length by making our one big triangle into 2 right angle triangles, separated by the total height measurement of the original triangle.

Still with me?

The parabola below the triangle takes the effective spring length at full droop and combines it with the total spring length, and from that we can calculate the height of the arch of the leaf springs. I understand that leaf springs do not form a perfect arc, and there are multiple factors that go into how a spring is shaped under droop, but since the leaf springs are the same in all our examples and behave in a constant manner, we can negate these factors.

If you actually read all this then good on ya! I'm willing to bet I have lost most of you by now. In the next post I will summarize my findings, but for those who are interested, all the data is right there for you to see in the last picture.

 

Taking this info and making it useful:

Through these examples we can see a few different things. In general, the less of an angle we have been the flat spring datum line and the shackle, the more travel we will get with a consistent shackle length. Be warned though, if this angle is too small then you risk stretching your main spring or prematurely wearing out your bushings because the vertical force acting on the leaf is redirected towards the rear of the vehicle. Leverage also plays a role here, and can exacerbate the problem. This is also why our effective spring rate goes down as the shackle gets closer to being parallel with the datum line.

Something else to note is the further away from parallel your shackle is to the datum line when your leaf is totally flat, the more your leaf spring length has to change to get the same amount of wheel travel. This is because as the shackle travels in an arc, and the closer it gets to bring perpendicular to the datum line, the less vertical movement and the more horizontal movement it makes. The lack of vertical shackle movement means the leaf has to make up for this to get the same travel numbers.

I've added some percentages to the bottom right corner of each example. The ∆ is the change in effective leaf length represented as a %, and the ER is the effective spring rate represented as a %.



The change in effective spring length does not matter much, as long as your shackle length accommodates for this change and does not let the spring bind.

The effective spring rate is very important though, and will directly affect how your vehicle handles and how much weight you can carry. This rate is proportional to the shackle angle; the closer to perpendicular to datum line your shackle angle is, the lower your effective spring rate. Below are some examples of where you would want different effective spring rates during the last few inches of travel before full bump:

Rock crawling - low ER
General trail running - low to medium ER
Overlanding, general truck use, towing - medium to high ER
Prerunning, high speed - high ER

 

Reserved for install of the 63s

 

I've religiously ran 5" shackles for around 10 years with 63s, and always set them up so when the leaf is flat, the shackle is just near the point of collapsing/bottoming out on the frame. Back off a hair from that point, and then let the leaf invert a good 3" at full bump. Currently using 15" of shock stroke at articulation, which nets to who knows what at tire centerline.
Vertical travel is at around 12", still running the oem chevy bushings to those have quite a bit of resistance on the pack drooping.

Heres 37s tucking/falling out of the wheel well on my setup



Edit: You can only lay the shackle so flat, this was a 4" shackle and with the pack flat, the main leaf would contact the shackle hanger itself. Switche to a 5", and fixed that clearance issue and gained .5" of droop.

 

Nice flex! It's not surprising that a 4" shackle was not long enough. In most of the examples I've done the leaf spring effective length changes by about 5". For a 4" shackle to allow for this without binding it would have to rotate 88°. For a 5" shackle it would only have to rotate 73°.

 

Sorry to bother, but I made a study with a an extended research, and have a different perspective on this setup. I believe setting shackles near the point of collapse could compromise stability. I have been learnt based on my mistakes (while still being a pupil of a great tutor) and prefer maintaining a balance between suspension travel, articulation, and tire clearance. By allowing more room for leaf inversion and using longer shock stroke, I achieve better wheel travel and improved off-road performance.
Moreover, upgrading bushings can reduce resistance and enhance suspension operation.

 

Vanillion,

Thanks for going to the trouble to math this up for everyone.

I found your post yesterday and have been using it to nail down dimensions using your guide. I built an Excel calculator and am resurrecting this post because I need some help with the dimensions circled in Example F, attached.

Let me walk through it and I bet you will immediately know where I am screwing up.



First, when I break the right triangles apart and I calc for B using Pythagorean theorem (so SQRT((6^2)-(3.82^2)) I get 4.62 for B, which then I believe I subtract from 64" to get to section A? But when I do that, my answer (59.38) does not line up with your answer (59.20), so I am missing something. That answer then leads us to the parabola portion of the math, and if you would not mind taking a minute to guide me through how you got to 10.45, that would be VERY much appreciated.

Once I have these pieces, I will be in good shape to complete my spreadsheet and I can post that if it will be helpful to others. The inputs required will be simple....all that we really need are shackle length and spring length, and the formulas will run the values from 0 degrees out to 50 degrees.

Thanks! RB in Colorado

 

I don't have the notes anymore, so I'm not sure how or why I got to the numbers that I did. I was going to follow up the original post with more but due to a lack of interest at the time I didn't see much of a point so I pretty much forgot about this topic all together. What I can tell you is I am running a 5" shackle and the truck has very good flex, and feels very stable as well.

 

Thanks for the reply! Any guidance or link to a math resource would be really appreciated so that I can figure out the parabola math. Given that there is so little quality information on figuring out leaf spring math out there, I REALLY appreciated that you went to the trouble here.

Based on what you provided, I built an Excel tool that allows anyone to plug in the shackle length and flat length of leaf spring, and it will run the math and provide eye to eye dimensions for any desired angle, along with other helpful calculations, but I really want to provide effective spring rate and build charting to show sweet spots for effective rate by desired use. For example, if the user toggles that they are going to set up the truck for overland use, the color coding would shift to show best angles based on effective rate appropriate to that use.

The only piece I am missing is the height of parabola math (pink below).

 

Wow you went all out! And here I was doing this by hand like a sucker haha! There are several parabola calculators available online, as well as formulas for them. However I have no idea which one I used as it was long ago. I think it may be difficult to accurately decide which one to use, as not all leaf springs are the same thickness the whole way through, so won't arc the same. The number of leafs and their respective thickness and length will also effect this calculation

 

Hey Vanillion,

Your math makes the setup of the shackle and shackle placement seem so simple, and I appreciate it. I recently got my hands on some 16 inch shocks and I'd love to long travel my truck, but I'm having a hard time finding more budget leaf springs. Yall mentioned the 64 inch chevy ones and I was just curious if the ones you had were any good on hard prerunning or if they are more for overlanding and rock crawling. Id like to do a very similar 5 inch shackle setup for high speed stuff. Thanks for your help!

 

I'm debating Chevy 63s or SUA. Seems like the Chevy's are great for droop but don't gain much bump...so less of a benefit for prerunning. I like go fast ish plushness...but SUA is so $$$$ ha.

 

Do you gain any lift with the 63's? How are you clearing 37's?

 

Yeah probably around 3” of lift or something, depending on the weight I had in the bed. I cleared 37s by cutting

 

Had 4” shackles and made contact with a bunch of weight. Changed to 5” and all nice and dandy. Running 3 leafs no overload on 14” shocks.