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Shackle Angle Math

Babaganoosh

Babaganoosh

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I have this posted in my build thread, but I think I will get more eyes to proof read my thinking...


Ok guys I need some math help...

I'm looking at shackle angles and I would like to run a 45 degree angle. I'm using this calculator here. http://www.carbidedepot.com/formulas-trigright.asp

Here is my inputs.
Side C. 4.5 for shackle length
Side B 45.

With that I get 3 inches, so I would subtract 3 from 47(spring length). Now I measure out 44 inches from the center of the bushing to the center of the other bushing and those are my marks?

For reference here is Ryoken's Mutt factory measurement.
bedaf04b.jpg
I might just use this measurement and run with it, but I had one of those what if moments. :doah:
 
My suggestion is dont over think it with measurements. Set the shackle angle to between 0 and 10* back with no weight on it and it will come out fine.
 
Stomis said:
My suggestion is dont over think it with measurements. Set the shackle angle to between 0 and 10* back with no weight on it and it will come out fine.
Click to expand...

I don't think that is possible for me. :haha: I'm over thinking everything because well I want it right the first time.

My question now is how would I do that? I don't want to miss that and end up with a 2 inch hole in my frame.
 
Remove the leafs from the axle so theres no unsprung weight. Bolt the leafs in from the front. Position shackle hanger to mount at 0*. Bolt everything up and watch the angle sink 30-40* back with weight.
 
Now why didn't I think of that one. :doah: Thanks for the suggestion, that seems like a no brainer.
 
Set it so that your shackle is at 45* when the spring is longest (completely flat). That will give you the most droop without getting excessively progressive compression. You're right on with using trig to get it right the first time. I did my 64"s that way and got it exactly where I wanted it, and the suspension works awesome.

IMG_2028.JPG
 
45 degrees is too much to let the spring compress past flat. With that steep of a shackle angle the rate skyrockets when the leaf pack approaches flat. To put this in some kind of perspective, my SAE spring manual doesn't even have a rate chart past a 30 degree angle and it's not pretty at 30 degrees for longer shackle lengths.

I'd aim for something along the lines of 30 degrees or a little less and run with it. Also, it might be easier for you to figure a triangle with one leg being 52", one leg being 4" (or your shackle length) with the shackle to spring line at 60 degrees. Then find your other leg length which in this case is 50 1/8". This is the measurement from your main eye to the eye of your shackle hanger. Maybe this is the method you were using already?

All my referenced angle measurements are between the eye to eye CL on the leaf spring when it's flat (as long as possible) and the eye to eye line on the shackle.
 
I did what was suggested before, I don't remember what the measurement is from the center of each spring eye is.
 
And my numbers were assuming a stock rear spring on the rear where you're working on the front. You may be lucky in that a 10 degree angle when the front spring is at free arch may only translate to 20-30 degrees when it's flat and will stilll work out pretty well. Not as good in compression as a shallower angle but it should be OK.
 
Thanks, I ordered the springs from you made by alcan. When I get a final angle I will post a update.

I appreciate the advice.
 
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