Effective Spring Rate: Shackle angle vs. Spring Rate (1 Viewer)

[ntsqd]

This thread: https://forum.ih8mud.com/60-series-wagons/496481-chevy-63-questions.html got me to wondering about something I've known intuitively, but had no idea what the magnitude of it might be.

Shackle angle plays too much of a role in actual effective spring rate to make a blanket statement about spring rate when only the rate of the spring itself is known. Say the rate of the spring is 500 lbs per inch, and for simplicity's sake let's assume that the eye to eye distance increases by 1/4" with the first inch of spring compression. (I'm using the 1/4" number as a guess and to have at least something to work with. It may be more and it may be less.) In the below I'm using a 6" long shackle and considering one side only.

If the shackle were not having any effect then putting 500 lbs on a 500 lbs/in rated spring would compress it 1 inch.

With an at-rest shackle angle of 30* from vertical we put 500 lbs on the frame. The frame will move down 1.160" due to the swing of the shackle with the 1/4" spring lengthening assumption. That makes the effective spring rate 431 lbs/in.

With an at-rest shackle angle of 45* from vertical we put 500 lbs on the frame. The frame will move down 1.284" due to the swing of the shackle with the 1/4" spring lengthening assumption. That makes the effective spring rate 389 lbs/in.

With an at-rest shackle angle of 60* from vertical we put 500 lbs on the frame. The frame will move down 1.557" due to the swing of the shackle with the 1/4" spring lengthening assumption. That makes the effective spring rate 321 lbs/in.

 

[nikos8]

This is some interesting reading. It doesn't really go into detail regarding shackle angle and how it changes the spring rate but interesting none the less.

RPM Net Tech Articles: Leaf Springs - Powered by: AFCO

 

[TobyB]

considering one side only.

I think I see what you're thinking, but with one fixed end and one shackled end,
the effective rate of the axle (assuming the axle is centered)
will be the average of the shackled end and the fixed end...
since the 500 lb 1/2 elliptic spring is really 2 250lb quarter elliptics.

hmmm...

t

 

[GLTHFJ60]

Very cool to see how the shackle angle affects the suspension capacity. You are a wealth of information, ntsqd, thank you

 

[phishincruisin]

so.. the combination of shackle angle and spring rate could allow you to tune the effective spring rate to be variable, in a variety of ranges… I had always thought about what shackle angle does, but had never looked at the numbers like that. pretty cool.

 

[ntsqd]

I think I see what you're thinking, but with one fixed end and one shackled end,
the effective rate of the axle (assuming the axle is centered)
will be the average of the shackled end and the fixed end...
since the 500 lb 1/2 elliptic spring is really 2 250lb quarter elliptics.

hmmm...

t

One side only, as in one leaf set, not both sets of leaves. You're making it more difficult than it needs to be, and I've left out some things that would alter any real example's real world results for the sake of keeping it simple.

 

[GLTHFJ60]

One side only, as in one leaf set, not both sets of leaves. You're making it more difficult than it needs to be, and I've left out some things that would alter any real example's real world results for the sake of keeping it simple.

As in friction between the leaves as crud builds up between them, or how long the perch is, or how it changes when your spring hits the overload leaf...

Still, you're giving us a pretty good understanding.

 

[lehiguy]

The real-world difference, what you feel, with different shackle lengths and angles is remarkable. I played with that geometry quite a bit before I got what I was looking for. With the same set of springs you can lose or gain sustantial amounts of articulation depending on shackle length and static angle.