HA LOVE IT!!! so we were totally wrong, kinda. The shackle length and separation distance controls the spring rate and travel. The softer you make the setup the more travel you get, the stiffer the less travel you get. I would have to say the 52.5" number must have come from F67s on a Tacoma with very little weight in the back (just bed/cage and a spare or two).
We can also say that the SOFTEST you can make a setup is where the shackle length plus separation is equal to the leaf length. Increasing the shackle length or separation will stiffen the suspension while decreasing either will soften it. So for people who say their truck is too stiff simply drop a shorter shackle in there and people whose truck is too soft drop a longer shackle in there.
How does the truck ride without being loaded down with all the weight in the back?
with no weight it is riding pretty nice. i have a bed cage that is a bit on the heavy side for someone who would be racing, and with that, the springs seem to be working great (hmmp, race springs working good under conditions similar to racing and not all loaded down...that should have been obvious
what you're saying with the shackle's length creating a softer/harder ride is exactly what im seeing. but, if the shackle length changes are matched with the changing of the frame mount spread then travel should still be maximized (i think). you'll have to prove that
let us go over what we know:
frame mount distance (spread) -----------B
If A=B+C then up travel will be maximized. This is because B and C will be able to flatten out to equal the length of A creating two parallel lines with no space in-between them (aka: Full Jounce or Full up travel)
So, with the F67 and 50t leaf packs we have a 61.5" leaf spring, "A"
this puts our equation to 61.5=B+C
most seem to be running a shackle between 9 and 12 inches. so lets throw that in, "C"
I know middle school was long ago, but lets do some algebra!....I came up with, 52.5=B
so, 61.5(A) = 52.5(B) + 9(C) will result in a truck that maximizes travel. but this is the easy part
now weight, ride height, and shackle angle are the next problems to solve. all of these are inner-twined. ei, add more weight and your ride height and shackle angle change.
why does the angle of the shackle matter?? (remember this is not in reference to the ground or to the frame. the angle is taken from the "datum line" of the leaf spring. the datum line is the imaginary line from the front mounting eye of the leaf spring to the rear mounting of the shackle to the frame)
the reason the shackle's angle is important is because it does have an effect on the spring's rate. this changes through the movement of the shackle. at 90 degrees off the datum line, the shackle has zero effect on the spring's rate. the moment it moves back the shackle begins to increase the rate of the spring. this effect is some kind of bell curve parabola BS (I think. FYI, I have my degree in English, not math hehe)
because the shackle increases the rate of the spring, this is very useful to assist in these leaf springs in being more progressive. the downside, if the shackle is taken too far, it starts down the other side of the bell curve. this actually makes the leaf become less stiff. (very bad. causes truck to bottom out hard. I know, because this is the life story of my truck)
so, the angle of a shackle with 61.5=52.5+9
red line = datum line
blue line = shackle angle
the way this is sitting with no weight in the truck has been show to be effective. adding a longer shackle without accommodating the rest of the equation will stiffen up the ride, add a little height, and limit the travel.
from what I have figured (not tested) is if i decrease my shackle length to maybe 7 inches and increase my spread "B" my shackle will ride closer to 90 degrees off the datum line. this should initially make my ride softer through the first part of my travel and stiffer through the last part.
so the new equation im going to run on my setup will be
61.5(A) = 54.5(B) + 7(C)
I will not remove my current mount and i may make one more in addition. at this point, i wish the rear of my frame was bolt on! I would just make many options until I pinned it down.
, where are you with your truck as of today?? are the pictures the most recent??