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Leaf Spring Suspension Geometry Guru's- What causes a side to side rock?

11K views 21 replies 14 participants last post by  4rnr  
#1 · (Edited)
For the last couple of years, I have been experiencing a bad suspension rock, from the left rear to the right front. I want to attempt to get rid of this rock before I go about throwing more parts at the truck (85 Toyota 4runner)

Let me describe the suspension setup real quick. I am running 48" springs where 46" springs used to be. The springs are new (one trip) Old Man Emu's. The stock length 3.5" long shackles have been replaced with 6.5" shackles, with no change to the front spring hanger. The shocks are 15" Travel BBCS's (again, 1 trip old).
Side view of the suspension with the old leaf springs. New ones are the same length, just more arch.
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Idea of the current front shackle angle:
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I also recently installed a ford explorer sway bar, with little effect to the problem. I am a bit worried the angle of the links.
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Out back, I run 63" 1990 Chevy Springs, with a pro comp, and a rancho add a leaf (total of 5 leafs plus overload), with 6.5" shackles. Again, I installed the add a leafs to help combat this issue, with no return. For shocks, I run some 12" travel Monroe's these replaced the 5100 bilsteins).

The only other item I can think of that could be contributing to this issue is the welded rear differential.

If any more photos, or any measurements are needed, let me know.

Thanks for the help.
 
#6 · (Edited)
I should be at 105", although, I have never measured it.

Another oddity I should mention, when I strap the 4runner to a trailer (tie the 4 corners of the axles to the 4 corners of the trailer), I can pull the front axle forward. It will cause the leaf springs to gain more arch, move the shackles forward, and lift the truck.
 
#7 ·
check everything in the either axle for tight, and be sure bushings and steering links, and kingpins arent worn out. make sure ubolts are torqued properly on either end, and shock bushings are in good shape. (i know you said shocks were new, but you may have re-used old bushings :dunno:

if everything is tight i might wonder if the actual geometry is correct as far as where you welded on the new hangers for the chevies and the front hanger are placed.

if that is all proper, you might have a look at your hubs, maybe one isnt unlocking as it should, and be sure the transfer case is not stuck in 4wd.

run through everything, make sure its as it should be, and in good condition to run.
 
#8 ·
im curious why you didnt post this question on the 4runner section, since 100 others are running almost the exact same setup, and might not look here...
 
#9 ·
It almost sounds like when the top is on it's making the body and frame rigid and transferring the weight on the suspension to the other springs. It's got to go somewhere.

When the top comes off it is allowing the body and the frame to twist and not put the stress back on the suspension.

Are you running a cage or something besides the factory roll bar? Are you running just the front sway bar or do you have a rear also? By running just a front sway bar it's going to want to transfer the weight back to the front of the vehicle.

The BBCS are valved somewhat soft and that may be adding to the whole factor. Try calling Poly Performance and see what they have to say about the shock valving. I always wondered how the BBCS do on a daily driver.
 
#10 ·
Yes, I only run a front sway bar.

As far as the cage goes, I don't use the rear roll bar, although I do have this front cage which is tied into the frame.

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My old all pro cage was only bolted to the floor, and I had the same issue.

I should also mention the shock hoops are tied in with a hoop over the top of the motor.
 
#12 ·
I've got BBCS's on my SOA YJ as well. It acts very similar to what you describe. Basically feels like it has worn out shock and jerks around when you encounter irregularities in the road. Lots of body roll.

So far I've lived with it, but I'm probably going to change the shocks as it can be very scary.

My Jeep did not do this when I had DT3000s on it.
 
#17 · (Edited)
Bump-steer isn't the issue. The body rocking back and forth is.

I also don't see how 'soft springs' apply here. My rear pack is around 175 in lbs (stock was 85) and the front is 217 (I want to say stock was around 95). This is roughly twice the spring rate as stock.

I'll try swapping out the front shocks and see if it makes a difference.

I think it may have something to do with your drag link fighting your your leaf springs. Which is why it dips on the passenger side easier combined with the shock valving.
This comment adds to my thought that I should add a panhard rod to the suspension.
 
#15 ·
My truck felt like that years ago. I pulled the shocks and found one was much easier to compress than the rest were. Shock was doing almost nothing till partway through it travel then it would start damping. Shock had no internal pressure but did have some oil. It showed no signs of ever leaking either.

FWIW I've run rancho's, bilsteins, procomps and now Fox non coil 2.0 shocks and the fox's have felt the best in all situations. I dont DD mine but driving around down in Tellico I felt a huge difference between my bilstiens and my fox's. Less body roll was the biggest thing. I do run a shorter travel shock than most though at 11inch travel with a 1inch bumpstop on the shock.

My toy is probly close to runner weight and 110WB.
 
#16 · (Edited)
Love my BBCS. I thought they where valved to work on light rigs like Ftoys and lighter truggy's so they may not like the 4runners weight especially with the top on.

I think it may have something to do with your drag link fighting your your leaf springs. Which is why it dips on the passenger side easier combined with the shock valving.
 
#18 ·
Bring it up this weekend and I'll tell you whether or not you're being hypersensitive. My yota doesn't do this sort of thing and it's using Jacob's shitty old shocks on front and ES3000s on the rear.

Perhaps this is driving-style related?

If it is in fact as bad as you say, I would agree that it is a shock losing it's rebound dampning.

PS. Come up and get those door locks before I go back to Chico.
 
#20 · (Edited)
my old comanche had the sway of death pretty bad. i got used to "steering into it" to correct it. running a good shock cured it.

it is induced by bumpsteer whinch turns into a damping issue. a dip in the road or something causes the draglink to flatten steering the truck slightly. this induces a cyclic motion that a shitty shock cant control. and since every cycle the truck is steered by the bumpsteer, it just gets worse and worse until you steer the opposite way to cancel out the cyclic action. you just need enough damping to keep the cycle from starting.

before i put good shocks on the jeep i would just take my hands off the wheel when i hit a dip in a straight section of highway. around corners i'd just countersteer when i hit dips.

a panhard rod that matches the draglink would fix it, but to hell with running a panhard on a leafsprung rig.