Leaf spring shackle lift

[Joecar]

So after raising the front of the truck with the torsion bars, the rear is now lower than the front. I was told either blocks or a shackle lift would fix the problem. Has anyone found any good shackles that give the truck about 1” to 2”? 

[datzenmike]

Never use shackles. They distort the leaf spring and because the end is cantilevered a 4" shackle at best gets you 2 inches of actual body lift, maybe. The shocks are also extended. Put a spacer between the top of the axle and the bottom of the leaf spring pack. Has no effect of spring rate or arch and the shocks are not extended. Best is to have the leaf spring re-arched.

 

Any lift (or drop) of the front end will affect the toe in setting on a truck. Have that checked and corrected.

[Stoffregen Motorsports]

You might need to use angle shims after lifting the rear, to correct the pinion angle on the diff.

[Joecar]

1 hour ago, Stoffregen Motorsports said:

You might need to use angle shims after lifting the rear, to correct the pinion angle on the diff.

Yeah thought about that too but I think I’ll get my leads re arched instead. Just got to find a place that can do it and i don’t know many.

[frankendat]

23 hours ago, datzenmike said:

Never use shackles. They distort the leaf spring and because the end is cantilevered a 4" shackle at best gets you 2 inches of actual body lift, maybe. The shocks are also extended. Put a spacer between the top of the axle and the bottom of the leaf spring pack. Has no effect of spring rate or arch and the shocks are not extended. Best is to have the leaf spring re-arched.

 

Any lift (or drop) of the front end will affect the toe in setting on a truck. Have that checked and corrected.

What is considered the "right way" or "best way" to achieve more suspension travel? And I am thinking suspension travel as it equates with handling bumps in the road and not transferring the irregularities to those in the cabin. A friend of mine has one of those ungodly priced quad cab diesel trucks, the road to my house passes a school and there are speed bumps. In his lifted diesel he can roll over those bumps without slowing down while drinking coffee in a mug without a lid and not spill a drop. If I hit those speed bumps at anything less than a crawl in my pickup, the full soda can in my dash mounted soda can holder will launch out and land in my lap.  

What are tried and true improvements that could make a 720 ride more like the former and less like the latter?

[EDM620]

2 hours ago, frankendat said:

more suspension travel

Sounds like you're actually wanting more compliant suspension than range of travel. More compliant suspension gets you a "softer" ride and needs to be matched with softer shocks. KYB gas shocks for example would give you a harsher but more "sporting" ride. Coils are easier to make "progressive" - that is soft initial compression that ramps up as the suspension deflects vs leaf springs, but it can be done. Find yourself a spring shop and talk with them about what you want to achieve, it's what they do.

[datzenmike]

On 4/6/2024 at 12:09 PM, Joecar said:

So after raising the front of the truck with the torsion bars, the rear is now lower than the front. I was told either blocks or a shackle lift would fix the problem. Has anyone found any good shackles that give the truck about 1” to 2”? 

 

Looks like straight lift to level the ride height.

[Seven Two Oh No]

Quote

suspension travel as it equates with handling bumps

These trucks seem to suffer from a surplus of unsprung weight, particularly if you put heavy tires like KO2s on there, so when you hit a bump the (very light) body moves with the suspension. My truck rides much more smoothly when I throw a dirt bike in the back since it dampens the rear end travel.

[datzenmike]

They are trucks built to carry weight. My 521 and my later 620 always rode better with over 400 pounds in the back, did not help the performance and definitely not the braking but is was smooth on bumps. If wanting a softer ride, if just commuting, then remove a leaf or two, but the back ride height will drop. Again you can re-arch the springs. 

[Stoffregen Motorsports]

And you will still have to adjust the pinion angle once you adjust the ride height.

[frankendat]

21 hours ago, Seven Two Oh No said:

These trucks seem to suffer from a surplus of unsprung weight, particularly if you put heavy tires like KO2s on there, so when you hit a bump the (very light) body moves with the suspension. My truck rides much more smoothly when I throw a dirt bike in the back since it dampens the rear end travel.

I have noticed the same thing with my full size P/U, with 500lbs in the back it rides like a Cadillac 

[frankendat]

22 hours ago, EDM620 said:

Sounds like you're actually wanting more compliant suspension than range of travel. More compliant suspension gets you a "softer" ride and needs to be matched with softer shocks. KYB gas shocks for example would give you a harsher but more "sporting" ride. Coils are easier to make "progressive" - that is soft initial compression that ramps up as the suspension deflects vs leaf springs, but it can be done. Find yourself a spring shop and talk with them about what you want to achieve, it's what they do.

Perhaps, I am thinking that if I have 6" of up and 6" down travel on my suspension (front and rear) and I run across a 4" bump, the "bump" should be absorbed by the suspension and I should not feel it in the cab. Is this not true?

[Stoffregen Motorsports]

You will always feel a 4" bump, unless you're on aired down 42's.

 

I keep a 250# steel grate, a full size 35" spare tire and a steel basket with basic truck things in the back. It weighs roughly 500# combined. This is all there mainly for traction and comfort.

[frankendat]

6 hours ago, Stoffregen Motorsports said:

You will always feel a 4" bump, unless you're on aired down 42's.

It sounds as if the ride I seek is not possible, which is of some comfort, as it gives explanation to why I haven't been able to achieve it. (Other than I am just too stupid to figure it out) I rode in a sprung over FJ40 that could ride across washboard dirt roads and not transfer the bumps to the cab. It was no where near 4" washboard, but it was the best suspension I have encountered. 

Where is the fail in my logic: suspension travel 6" up/6" down, bump in road, either 4" speed bump or 4" dip, suspension travel will accommodate bump/dip with 2" to spare before bump stops. Why can't shocks and springs be adjusted to negate transfer of the movement to the cab?

[Stoffregen Motorsports]

Why? Because of physics.

 

Case in point - Racing 4x4 trucks, ie, King of the Hammers. Up until about ten years ago, 90% of the race trucks had solid axles, and IFS was coming onto the scene. If you don't know anything about KOH, it's hardcore rock racing combined with 100 mph desert runs, so the vehicle has to be good at both.  The saying goes, you can win the race in the rocks, but you can lose the race in the desert. So guys were building solid axle rock crawlers and trying to modify them to hit 3 foot whoops at 100 mph. When you have a 600 pound front axle and 500 pounds of front tires, there's no amount of shock and spring that can keep that axle from bouncing back up at you on the far side of a whoop. So while solid axle trucks are much better in the rocks, the IFS trucks will crush them in the desert, and win the race.

 

Back to reality, it is possible to make a leaf spring setup more comfortable. In the case of your buddy's FJ40, simply doing a spring over makes them more supple. Why? Because you've increased your center of gravity which puts more leverage on the springs, which essentially makes the spring softer. So you could do that - make the spring softer. Remove a leaf or two.

 

Second mod you could do is to lay back the shackles. In stock form, shackles have roughly 5 degrees of inclination. Increasing that angle to 15 or 20 degrees can make them react more smoothly. Problem is, if the shackle is too short, laying it back like that can cause the spring to run out of travel at the shackle because when a leaf spring goes through its arc, the length of the spring changes. Flatter springs don't do this as much, but a spring with a heavy arc will change a lot. So lay the shackle back by moving the shackle mount on the frame and install longer shackles.

 

Third thing you can do is install longer leafs. I always look at leaf springs in two parts, the part connected to the frame with a single bushing is the control arm, while the part that connects via shackle is the spring. Lengthen the part of the leaf that connects to the shackle and that can make it more comfy. Using springs that are longer on both sides of the center pin would be better, but sometimes you don't have the room to move the spring hanger. I'm sure you've seen crawlers with shackles hanging off the back of the frame. That's the only downside. The springs become so long that they can decrease your departure angle and drag on the rocks.

[frankendat]

6 hours ago, Stoffregen Motorsports said:

Why? Because of physics.

 

Case in point - Racing 4x4 trucks, ie, King of the Hammers. Up until about ten years ago, 90% of the race trucks had solid axles, and IFS was coming onto the scene. If you don't know anything about KOH, it's hardcore rock racing combined with 100 mph desert runs, so the vehicle has to be good at both.  The saying goes, you can win the race in the rocks, but you can lose the race in the desert. So guys were building solid axle rock crawlers and trying to modify them to hit 3 foot whoops at 100 mph. When you have a 600 pound front axle and 500 pounds of front tires, there's no amount of shock and spring that can keep that axle from bouncing back up at you on the far side of a whoop. So while solid axle trucks are much better in the rocks, the IFS trucks will crush them in the desert, and win the race.

 

Back to reality, it is possible to make a leaf spring setup more comfortable. In the case of your buddy's FJ40, simply doing a spring over makes them more supple. Why? Because you've increased your center of gravity which puts more leverage on the springs, which essentially makes the spring softer. So you could do that - make the spring softer. Remove a leaf or two.

 

Second mod you could do is to lay back the shackles. In stock form, shackles have roughly 5 degrees of inclination. Increasing that angle to 15 or 20 degrees can make them react more smoothly. Problem is, if the shackle is too short, laying it back like that can cause the spring to run out of travel at the shackle because when a leaf spring goes through its arc, the length of the spring changes. Flatter springs don't do this as much, but a spring with a heavy arc will change a lot. So lay the shackle back by moving the shackle mount on the frame and install longer shackles.

 

Third thing you can do is install longer leafs. I always look at leaf springs in two parts, the part connected to the frame with a single bushing is the control arm, while the part that connects via shackle is the spring. Lengthen the part of the leaf that connects to the shackle and that can make it more comfy. Using springs that are longer on both sides of the center pin would be better, but sometimes you don't have the room to move the spring hanger. I'm sure you've seen crawlers with shackles hanging off the back of the frame. That's the only downside. The springs become so long that they can decrease your departure angle and drag on the rocks.

Thank you for the excellent explanation, it answers many questions. There are kits for Suzuki Samurai's to install YJ leafs for extra length and it seems most of the bugs have been worked out of that swap. Is there a go-to common swap or improvement for 720's? 

[Stoffregen Motorsports]

I don't know, but you can use that Stengel Brothers link I put somewhere else a couple days ago. Click around the site and find replacement springs. Each page of springs shows dimensions, number of leafs, spring rate, etc.

 

Or head to the wrecking yard with a tape measure and go crazy.

 

Are the 720 leafs centered? How long are they?

[EDM620]

20 hours ago, Stoffregen Motorsports said:

When you have a 600 pound front axle and 500 pounds of front tires

Unsprung weight will always work against your want for a supple ride. There isn't a lot you can do about that except switch to alloy wheels if you have steel wheels, or change tire size, for example going to a +2 set-up will reduce the tire weight marginally over the full set. The advantage of steel wheels is simply the abuse they can take.