Hot Rods Question about shackle length and how it relates to shackle angle

Good evening fellas,

This afternoon I swapped my 7 ('42ish) leaf rear spring with a 9 leaf spring of the same vintage in hopes of firming up the ride a bit. I also installed some NOS shackles which are longer than the ones I previously had on. The reason for switching is the new spring has larger diameter eyes. Anyways, the longer shackles dropped the car lower than I'd like and the suspension travel is unacceptable. My shackle angle is somewhat less than ideal at about 65 degrees. If I pop in shorter shackles, will. Y angle be more, less or unchanged? Sorry if this is a dumb question, I just am having a hard time visualizing it. I can get dimensions posted tomorrow evening if that helps. Thanks

 

shorter shackles should make them sit more horizontal, eh?

 

Hopefully this helps you visualize your problem.
Blue red and black represent 3 different length shackles, using the same length spring and the resultant shackle angles.
Visually move the penciled spring up into the different length shackles and you'll see the difference in ride height.

If you needed black length shackles to cure a spring eye to perch interference problem, you'd also need a shorter spring to achieve the same ride height and correct angle.

 

Thanks '31! This is very helpful. I definitely don't need longer shackles. I made a spacer between the spring and x-member and it bumped the ride height up about an inch. Still nd to rectify the angle a bit.


Posted using the Full Custom H.A.M.B. App!

 

'31 offers the sketch!
IF: the shackles were 45*, then any shackle length change, employs the geometry
using .707" per inch. Your spring eye (each side in a transverse) moves outward by ~.7 in. with each inch the shackle loses. (Cosine of the angle, for other than 45*) Height would change by the Sine. The sketch is easier to predict or Est. what you need to do.

 

Make your final spacer with a socket on the bottom to receive the spring bolt, and a pin on top of the spacer to go into the crossmember. Without the pin and socket in the spacer it will move around & the spring and the whole rear end will shift.

I suppose one might say to weld the spacer but at some point you may need to unweld it too.

 

Thanks. So a shorter shackle will increase my angle rather than bring it back to 45?


Posted using the Full Custom H.A.M.B. App!

 

Done!


Posted using the Full Custom H.A.M.B. App!

 

I guess what I meant to say was that with a shorter shackle, the angle should be reduced to 45 degrees however with the spring actually spreading under weight, will the angles actually be increased?


Posted using the Full Custom H.A.M.B. App!

 

Picture is worth a 1000 words.
You see the sketch, the red line shackles are at 45*
The blue line are shorter than the red and the black are longer than the red.
The shorter blue go more towards horizontal - is that your less or your more?
The longer black go more towards vertical- is that your less or your more?


Regardless, with no weight they should be in a straight line. With car weight you want 45* ish. If its not both theres a problem - your spring is the wrong length to match the perches or shackles are wrong length or the spring is too stiff. 3 variables to shake out. Shortest shackles that will work are better. As the suspension works and spring compresses it gets longer and the shackle angle will change moving towards vertical. As the spring rebounds they will move the other way.

 

Your picture makes sense. I got confused by not completely understanding pitman's post. I suppose the only way the angle could increase with shorter shackles is via insufficient spring rate?


Posted using the Full Custom H.A.M.B. App!

 

Lmao, I got confused by pitmans post too.


If your spring rate is too low, the spring will go flat increasing its length and dragging the shackles angle with it.

 

Got it!


Posted using the Full Custom H.A.M.B. App!

 

Alright you turkeys! actually, the angle just allows us to calculate something, like the height change.
In truth '31 gave us all the better tool, a sketch, in order to visualize what changes would happen if,
weight, height etc. I like his approach!

 

I just missed an A in trigonometry. That cheesehead wouldn't give me the A because I slept thru his class. He told me I either cheated or learned it thru osmosis. Lmao...

 

Lol. The help is appreciated!


Posted using the Full Custom H.A.M.B. App!

 

Devin, imagine setting up the rear spring, in place w/o shackles, and under a full live load. (one sandbag, or two?)
If the spring eyes were able to 'slide' they would be at their loaded width. For perch width, could aim for 45* and a std. shackle length.

 

I think I'm going to measure the width of the spring eyes as they currently sit then figure out how high they need to be raised to achieve a 45* angle and then measure the length the shackles need to be. Is mild steel plate suitable to make shackles out of or is it too weak? Thanks


Posted using the Full Custom H.A.M.B. App!

 

Mild is fine, I'd use CRS, make them thick (1/4") and deburr.
Fit the pin "bores" carefully, may weld the outer pin/bolt heads to the shackle,
then they won't rack. Look over the trad Ford product w/an eye to the quality, this is a critical assy. Strength is influenced by many features You select and build to! Check with Don's Hot Rods.

 

31 Vicky rarely appears to be wrong, but you should not weld on a spring, it alters temper and tends to break off where welded...(I'm not busting on you hemi !)

 

Bust away, cuz I'm wrong plenty.
I just refrain from say things I'm unsure about.
But I was referring to welding the spacer into the cross member not welding to the spring.
I should have been more clear.

 

Where did this mythical 45° shackle angle come from?
At FULL droop you need the shackles to be horizontal [ full droop is when the eye-eye measurement of the spring is shortest ]
If you want to limit the droop of the suspension, then make the shackles closer to horizontal.
The shackles will straighten up [vertical] under compression
The more horizontal the shackles are , the less probability of “shackle wobble “
If you’re running a panhard bar you’ll need the shackles to be more upright to prevent binding

 

I've always heard 45 degrees. That's not to say that there are other schools of thought. When you say "droop" are you talking about the suspension settled under the car's weight?


Posted using the Full Custom H.A.M.B. App!

 

45* is a clue that the spring rate is balanced with the preload tension and the spring Eye to eye is the right dimension.

 

Droop is the opposite of "Jounce" or compression.
When you jack the car up by the frame, the suspension droops.

Droop actually means to "hang downwards limply" in the dictionary.
[ think of "Brewers Droop" when you drink too much ]
It is a very common suspension term used in the "Rock Crawler" crowd

 

That is not totally correct [ but not incorrect either]
If the suspension has a soft spring load in it would settle more when weight is placed upon it ,It would have more droop than jounce [more extension than compression]
Whereas a stiffer spring would settle less when loaded.[ it would have more jounce than droop ]

It all boils down to intended usage.
Higher speeds require stiffer springs ,so the manufacturers use less Spring load [ which is actually the shape of the arch ]
That is why stiffer springs are usually flatter / shorter to maintain the correct ride height.

The shackles should be horizontal at full droop just short of the shocks being fully extended.

 

I'm more than reasonably certain you've contradicted yourself more than once.
So since I'm not familiar with your terms or thinking I'll just let it be.

 

Well I didn't contradict myself. At least you admit you're out of your depth.
I'll leave it at that [and get back to working on my race car tonight]

 

Just googled droop and jounce. Makes sense now that I understand the terminology. Once this storm passes I'm going to jack the frame up until the rear end dangles and see where my shackles are oriented. My perch centers are 47.5" apart and the spring eyes are 42.5" apart.

 

You mention 1942...is your whole rear that or what??
Earlier Fords used no panhard bar for anti-sway, and shackle angle is needed as thew will sway badly if shackles get too close to vertical. '42-48 Ford went to vertical shackles and turned sway control over to panhard bars front and rear.