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I see that the old debate on shackle length has come up again. Earlier this year I posted a rather long post to the old Toyota4x4 board. Below I have tried to recreate much of this post from memory (With some added) to try to clear some things up. If you disagree with any of this, let us all know. This is just my 'pinion and I am writing this on the fly. The education mentioned may be yours, and it may be mine, but we all benefit... Enjoy!

There are quite a few factors that come into play when considering shackle design for vehicles. I will run down a few here and how I feel they apply. I am not an engineer, but most of this is just commin sense I picked up on after thinking about it. Like I said, let me know what you think
BTW, this is not a vehicle specific description, so it is up to you to figure out where this fits your application.

1. To start off, shackles are obviously deisigned a to be a part of a complete suspenion package, and to work the most efficiently with that stock suspension system.
2. Obviously, changing the suspension in any way from stock will in some way (either large or miniscule) have an effect on how that suspeonsion works.

You will hear lots of peronal opinions on longer (lift) shackles used on vehicles, especially Jeeps, most of which I take with a grain... no, a bag of salt. If you sit down & think about this, you can form some opinions of your own.

First off, I think shackles made to a different length than stock have their place. They can be an excellent tool for fine tuning a custom suspension to get the best ride and travel characteristics. Those who discount longer than stock shackles for all applications are just plain ignoring the full picture.

To be clear on one thing, a longer than stock shackle will never give you the amount of lift equal to the increase in shackle length, asssoming a standard spring setup using a shackle at one end and the other end is a fixed eye mount to the frame. The actual amount of lift depends on length of the spring, as well as the location of the axle (or better measured at the spring center pin location). The difference in height location of the spring shackle eye mount in relation to the sprng solid eye mount also affects how much lift is gained. Assuming the spring is relatively flat, and the axle is located at the midpoint of the spring, and the actual lift gained from the longer shackle will be roughly half of the additional shackle length. Having the axle located some place other than at the center of the spring will change the actual lift gained somewhat. For an extreme example, if the axle was just a few inches from the shackle eye, you would gain almost the same lift as the added shackle lenght. If the axle was just a few inches from the spring solid eye mount, you would see ano change in lift height.

Another issue concerns claims that a longer than stock shackle will flatten a spring out. Again, several factors come into play here. The flatter the spring, and the longer the shackle, the more it will affect the spring.
Consider a suspension (for example, the rear of a CJ) where the lower shackle eye of the spring and the solid eye mount are roughly parallel. The spring has some arch to it, and is not flat. This would be a stock suspension. Now, imagine this vehicle traveling over a speed bump - where is the force traveling? The axle is forced up slightly, with the shackle doing it's job and moving through it's arc of movement, working in companion with the spring eye of the shackle as the spring compresses and lenghtens through it's arc.
Now, take this same suspension and change the stock (assume it was 3" long) shackle to a 20" long shackle. Now consider this vehicle traveling over that same speed bump. The arc the spring travels as it compresses/lengthens is no longer similar to the arc the shackle travels, so the force must go somewhere. Here, the spring is forced to take all of the force, and the shackle is not taking any of it. This is generally where the spring is overworked, and will usually fatigue(flatten) the spring faster than normal.

Also, as you lenghten the shackle, you change the suspensions ability to center itself over the axles. Although cross braces in the shackle combat this somewhat, it still affects the vehicle's drivability to some degree, again depending on the change in shackle length and the entire suspension as a whole.

Sorry if this is rambling, just trying to think through this as I write. I guess to sum it all up is that everything you do to a vehicle's supension will have an impact. Carefully considering these changes and how they affect the whole SYSTEM of your paritcular application is the best way to figure out what is right for you.

David
Davids 4x4 Page
 

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20" shackles for a CJ?! Where do I get those!!! /wwwthreads_images/icons/wink.gif Just kidding. Good informative post! It should make us all think about how a suspension "works" before changing things for a visual difference.

Mike H.
1983 CJ-7 Laredo
/wwwthreads_images/icons/cool.gifo[[[[o
 
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