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How's this shackle angle?

H

HeavyMetal72

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Just wondering if this shackle angle is ok. I have read so much on it the last few days that I don't know what to think anymore. These are attached to 6" lift springs. I think it is pretty close and should kick back a little when there is weight on them. Still need to shim the axle, but I am waiting to get the engine, trans and t-case back inbetween the frame rails.

P1010036.JPG
 
Its very hard to tell before there is any weight on them, It can change a lot once you start adding stuff on, who knows, maybe you will end up with to much?
 
I can't see your picture, but I can tell you how to do the math to figure out where you want your angles. I'll use my '76 K5 as an example.

You need to know your eye-to-eye length of your main leaf when it's flat. For kicks let's say you're using stock 2nd gen. rear K5 springs which are 52" when flat.

Now, you need to measure the eye-to-eye length of your hangers (ignore the shackle) on the truck. My '76 K5 is 51.5" if I remember correctly.

Now use some subtraction to get the difference here...in this case the spring is .5" longer (52"-51.5") than the hangers are apart.

Now you need to know your shackle length. Stock GM shackles are 4" (at least every one I've ever seen).

Use the sin function of a calculator to calculate the shackle angle based on your shackle length and the length difference of the spring and hangers that we calculated above. If you've never learned basic Trig, or simply forgotten it, it's easy to grasp how to use sin/cos/tan and their inverses through the use of Google. Or, you can use a nifty trig tool such as this one: http://www.carbidedepot.com/formulas-trigright.asp.

You need to make sure that when your spring is completely flat (when it is longest), your shackle angle is no more than 45 degrees. When designing suspension, I give myself about a .25" of forgiveness with those calculations too, as spring bushings are capable of deflecting quite a bit. With my 6" shackle, 41-42 degrees came out to about .25" of forgiveness I believe.

If your shackle angle is greater than 45 degrees ever, you run a high risk of inverting your shackle. This happens because at 45 degrees, the bottom of the shackle is at its greatest horizontal distance from the front hanger (the shackle moves in an arc). Beyond 45 degrees, the bottom of the shackle actually comes closer to the front hanger, forcing the spring to arc again.

Inverting the shackle sucks, and is usually a big pain to fix on the trail. It's also sort of like injuring a knee, as things deform during the occurrence, and it becomes easier and easier to do again. However, if you calculate hanger placement and shackle length like this, you will ensure it will never happen. It will also give you the greatest amount of travel possible and the best ride with the given shackle & spring length.
 
Yep, I'm going to have to brush up on my trig, it has been a few years. I think I still have that scientific calculator somewhere. Thanks for the info. It will definitely come in handy.
 
owenst7 said:
If your shackle angle is greater than 45 degrees ever, you run a high risk of inverting your shackle. This happens because at 45 degrees, the bottom of the shackle is at its greatest horizontal distance from the front hanger (the shackle moves in an arc). Beyond 45 degrees, the bottom of the shackle actually comes closer to the front hanger, forcing the spring to arc again.
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I think you mean 90 degrees here.

But since this is a front shackle, you never risk inverting it when the spring is compressed.
All you need to worry about is keeping the spring-eye from touching the frame.

There is really no need to do any math to solve this.
First measure the spring along the arch.
Loosen the shackles, so you can swing them back and forth.
Swing the shackle so that the distance from it's lower mount to the front spring-mount (in a straight line) is the same as in the first measurement.
Then compare the distance from the shackles lower mount to the frame, to the radius of the spring-eye, to see if it clears.

Judgeing from the picture I doubt that would be a problem.
But as earlier stated, don't read to much into it without any weight resting on the axle.

You might experience a lack of down-travel instead, but that is common with regular lift-springs.
 
Fred_M1010 said:
I think you mean 90 degrees here.
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No, it's definitely 45 degrees. If you graph it out it makes more sense than trying to explain it over a forum.

But if it's the front shackle you're right about inverting not being important since the frame is in the way. I can't see the attached picture :).
 
Its not easy to tell shackle angle without the weight but if thats in the stock location then I can tell you right away its not gonna be any good. Apparently noone was ingenious enough to make up for the pos vs neg arch problem between stock and lift springs and simple just went for it.

That being said your shackle angle should be as far back as possible at ride height, so that when you compress the shackle/spring doesnt hit the frame. Generally a rough guestimate would be a 30-45* angle. Its very hard to guage with no weight especially since I see none of these math dealys taking spring rate into account.
 
Stomis said:
Its not easy to tell shackle angle without the weight but if thats in the stock location then I can tell you right away its not gonna be any good. Apparently noone was ingenious enough to make up for the pos vs neg arch problem between stock and lift springs and simple just went for it.

That being said your shackle angle should be as far back as possible at ride height, so that when you compress the shackle/spring doesnt hit the frame. Generally a rough guestimate would be a 30-45* angle. Its very hard to guage with no weight especially since I see none of these math dealys taking spring rate into account.
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Spring rate is irrelevant. The length of the spring and the length of the shackle dictates how far apart things should be on the frame. Spring rate is relative, as the infinitely variable force applied to the springs (weight of rig can change, how you are loading the springs, etc) is a variable. To ensure that things work properly under all circumstances, it's necessary to place things on the frame based on the spring length.

People say 35-40* because that's the ball park that the math I posted will put you at at ride height (my 64"s are about 35.7* w/ a 6" shackle at ride height, and about 41.5* completely flat so I have room for the bushings to deflect). 35-40* only applies to stock (read almost flat) springs. A lift spring (more arc) would require you to run less shackle angle (more perpendicular to the ground), otherwise in the event that you were able to (not that they flex at all) compress them they would be likely to invert. The same ideas apply to lift springs as apply to springs with a high spring rate. You need to set your angle based on the length, not ride height or spring rate. Let's say you're jumping the truck, carrying a heavy load, towing, going around a corner fast, twisted up funny on a trail, etc.. In a situation like those, you are exerting a quantity of force on the spring that you aren't going to be able to calculate for all circumstances.

On the front, you guys are right about just making things so they don't hit the frame. The way these GM frames are shaped, the rail acts as a bumpstop on the spring eye before you get any bind issues (shackle inversion).

There is a solution to the pos/neg arc issues that GMs have. Move the frame eye forward with the ORD or DIY4x parts :), or use a longer shackle (depending on what springs you're using).

Or you could just lower your bumpstops a ton. But if you're going to sacrifice ride quality and travel, why mess with the stock stuff in the first place?
 
Shackle angle is something that gets debated alot.

The longer the shackle, and greater arch of the spring, the longer the possible travel.

Springs with more arch should have longer shackles. This lets you utilize the maximum droop/ compression of the spring.

When I set my angle, I use this method. Total spring length minus shackle length. I then add a desired amount (fudge room) and set my hangers that far apart. This is because when the spring is flat, the shackle is flat. This will give MAXIMUM droop/ compression for your shackle/ spring setup.

This is how I do it for any front/ rear flipped suspension. You have 90* of shackle travel, between 6 o'clock and 9 o'clock, to keep the shackle from flipping around.
 
I am sooooo not looking forward to settingup my 64's lol. I have read and read and read all the threads till my brain hurt. Still dont undersand the angles and Im no math guy so Im doubly boned lol.
 
doonjumper said:
I am sooooo not looking forward to settingup my 64's lol. I have read and read and read all the threads till my brain hurt. Still dont undersand the angles and Im no math guy so Im doubly boned lol.
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I just did it on mine. Was really simple. I'd be glad to help you if you'd like.
 
owenst7 said:
Spring rate is irrelevant. The length of the spring and the length of the shackle dictates how far apart things should be on the frame. Spring rate is relative, as the infinitely variable force applied to the springs (weight of rig can change, how you are loading the springs, etc) is a variable. To ensure that things work properly under all circumstances, it's necessary to place things on the frame based on the spring length.

People say 35-40* because that's the ball park that the math I posted will put you at at ride height (my 64"s are about 35.7* w/ a 6" shackle at ride height, and about 41.5* completely flat so I have room for the bushings to deflect). 35-40* only applies to stock (read almost flat) springs. A lift spring (more arc) would require you to run less shackle angle (more perpendicular to the ground), otherwise in the event that you were able to (not that they flex at all) compress them they would be likely to invert. The same ideas apply to lift springs as apply to springs with a high spring rate. You need to set your angle based on the length, not ride height or spring rate. Let's say you're jumping the truck, carrying a heavy load, towing, going around a corner fast, twisted up funny on a trail, etc.. In a situation like those, you are exerting a quantity of force on the spring that you aren't going to be able to calculate for all circumstances.

On the front, you guys are right about just making things so they don't hit the frame. The way these GM frames are shaped, the rail acts as a bumpstop on the spring eye before you get any bind issues (shackle inversion).

There is a solution to the pos/neg arc issues that GMs have. Move the frame eye forward with the ORD or DIY4x parts :), or use a longer shackle (depending on what springs you're using).

Or you could just lower your bumpstops a ton. But if you're going to sacrifice ride quality and travel, why mess with the stock stuff in the first place?
Click to expand...

The reason I argue that spring rate is relevant is because of this type of scenario with lift springs. If your springs arent gonna flatten for **** because of how high the rate is then clearly thats going to affect your shackle angle because the spring isnt going to get any longer at ride weight.

I understand what your saying but thats some tricky math for something that can generally be eyeballed.
 
Stomis said:
The reason I argue that spring rate is relevant is because of this type of scenario with lift springs. If your springs arent gonna flatten for **** because of how high the rate is then clearly thats going to affect your shackle angle because the spring isnt going to get any longer at ride weight.

I understand what your saying but thats some tricky math for something that can generally be eyeballed.
Click to expand...

If we're going to say the springs can't flex at all, then why even worry about the angle? I wouldn't do it on my rig even if I ran 3 add-a-leaves because you could still load the spring enough under certain conditions. It's not complicated math, you just have to type something into a calculator and it spits out the number you need. You need to be measuring everything really exact when you're moving hangers anyways or you're going to end up with the axle not square. Maybe being a mechanical engineer is encouraging me to look into this more than most people, but I don't see where overlooking some simple stuff punched into a calculator is going to save you any more time.

But I guess if a guy wanted a rig that doesn't track straight, has no suspension travel, and is dangerous to drive, he could go ahead and ignore everything. Even the old guys that eyeball things and don't do literal calculations are working these things out in their head based on their own experience. But I guess one could just run a bunch of rebel flags and a huge body lift with the loudest exhaust known to man and the girls would still think he's cool.
 
owenst7 said:
If we're going to say the springs can't flex at all, then why even worry about the angle? I wouldn't do it on my rig even if I ran 3 add-a-leaves because you could still load the spring enough under certain conditions. It's not complicated math, you just have to type something into a calculator and it spits out the number you need. You need to be measuring everything really exact when you're moving hangers anyways or you're going to end up with the axle not square. Maybe being a mechanical engineer is encouraging me to look into this more than most people, but I don't see where overlooking some simple stuff punched into a calculator is going to save you any more time.

But I guess if a guy wanted a rig that doesn't track straight, has no suspension travel, and is dangerous to drive, he could go ahead and ignore everything. Even the old guys that eyeball things and don't do literal calculations are working these things out in their head based on their own experience. But I guess one could just run a bunch of rebel flags and a huge body lift with the loudest exhaust known to man and the girls would still think he's cool.
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O no I'm not saying dont measure them equally or anything like that dont get me wrong man. I'm just saying theres no reason to get an exact measurement on how far back to move hangers for the exact perfect shackle angle.
 
One thing I see missed quite a bit when setting up shackle angles, Alot of guys get that 30* spot and are quite happy with how it rides and that's great. Then you load it up with spare parts and tools and such. Now your shackle angle is well past what you had planned.

Don't forget to figure in the weight you'll add in as you prep for the trail. With the softer longer springs, this can be a huge change.
 
cybrfire said:
One thing I see missed quite a bit when setting up shackle angles, Alot of guys get that 30* spot and are quite happy with how it rides and that's great. Then you load it up with spare parts and tools and such. Now your shackle angle is well past what you had planned.

Don't forget to figure in the weight you'll add in as you prep for the trail. With the softer longer springs, this can be a huge change.
Click to expand...

I thought the next step was to have the proper spring rate around what you carry?
 
Stomis said:
I thought the next step was to have the proper spring rate around what you carry?
Click to expand...


Yes, you should have the spring rate correct for your vehicles "trail weight."

I'm just saying you should set the angle with it loaded for the trail. Or, figure a little less angle if you don't know for sure what you'll be carrying.
 
owenst7 said:
But I guess if a guy wanted a rig that doesn't track straight, has no suspension travel, and is dangerous to drive, he could go ahead and ignore everything. Even the old guys that eyeball things and don't do literal calculations are working these things out in their head based on their own experience. But I guess one could just run a bunch of rebel flags and a huge body lift with the loudest exhaust known to man and the girls would still think he's cool.
Click to expand...
You sure you're name isn't really CDA455?
 
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