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Rear Weight Distribution and Shackle Length

Ol Blue

NAXJA Forum User
Arvada, CO
Hey guys, I have a theoretical question and hoping to get opinions and thoughts on it. I've always had a pull to the right under hard acceleration that I had thought was due to the lunchbox locker in the rear. Alignment is good. All bushings and joints in the suspension are good. Cruises straight and drives great while not accelerating hard. No DW or bumpsteer. Like you have to really punch it to make it pull. When you give it gas hard and it pulls, you can kind of feel the front left lift up more than the right side.

I was watching Roadkill and a light bulb came on when they were talking about weight distribution of drag cars. What if my Shittybilt XRC spare tire bumper (35" KM3 mounted) puts more weight on the passenger side of the bumper than the driver side? Out of curiosity I measured and sure enough the rear passenger side sits a little more than 0.5" lower than the rear drivers side. Makes sense to me since the hinge is all of the way passenger and the striker is about a third of the way in from the driver's side...which would put the tire weight slightly towards the passenger side of center assuming the hinge and striker take half the weight each. Seems like the hinge would take more than half the weight though. Also the gas can sits on the passenger side of the tire carrier. My adjustable shackles are the exact same length and the turns screwed setting the length were the same side to side.

A) So has anyone else noticed something like this with a spare tire bumper, specifically the XRC?

B) Would lengthening the passenger side shackle back to level help or hurt anything? At least it would be level but the weight would still be offset...
Well the locker does apply steering loads whether accel or decel. It's common for the lift not to be perfectly square. The XRC bumper is extremely heavy period. Having a difference in shackle length will impact the overall drivability way to much.
Besides the locker possibly having an effect, the thrust angle may be changing under accel. As the leaf springs compress, the axle moves up and back. If the degree of squat from side-to-side is not uniform, it may induce some thrust angle change.
Yea I was wondering about drivability getting messed up.

I'm thinking this thrust angle change has to be happening if the passenger side leaf is more compressed. If it is more compressed permanently then the shackle would be pointed further back at a shallower angle as the spring is longer and flatter. I also have adjustable shackle relocation brackets. What if I moved the upper bolt back one hole on the passenger side to keep a more similar shackle angle while keeping the same shackle length? This should also work to level it out.
You need to figure out the cause before moving forward. Again any differences in shackle angle or mounting position would make things worse, I would consider a shim under the leaf pack first. What springs are you running? The front springs also factor into this a the left front changes the right rear!
Well I can take the spare off and drive around. If that fixes it and it doesn't sit/squat unevenly then its pretty clear the weight imbalance is the cause.

Leafs are Rubicon Express RE1463 - 3.5" Lift. Last week I ordered a set of 4.5" lift leafs from Alcan Spring which should be stiffer than the RE ones to support the extra weight. They wont be here for a month or so. Hopefully those new leafs can solve the uneven squat issue.

No idea on the coils, they are unmarked and came on the Jeep. They are gold in color.
I have the same issue with my jeep which has a 1st generation JCR carrier. I haven't measured it in a while but I seem to recall my difference being 2" or so.

My guess is the torque twist going that way on top of the extra weight of the spare hanging way over the passenger side thus weakening the spring when getting into the hatch area.

My plan is to just swap leaves side to side and try to even it out some. Seems like most xjs have a lean that direction tho.
How hard of a pull are you talking about? Like, need to slightly put weight on the steering wheel to counter it and maybe hard to even notice, or need to change your underwear after an unscheduled lane change?

A changing thrust angle is probably more likely than weight transfer. Before you go trying to make any changes to compensate (which I don't recommend):

Are your tire pressures all identical, and tires worn evenly?

Are all of your suspension bushings in good condition (front too)?

Are your u-bolts torqued properly?
If the top of the steering wheel is at 12 when cruising, Ill have to compensate quickly to 11 to keep it in the lane. If you don't correct it will definitely be in the next lane or shoulder.

Fronts are 27 psi and rears are 25 psi since that's what the chalk test liked. Tires have 500-600 miles on them and are were balanced when new, 2 months ago.

All bushings on the Jeep are less than 2 years old. And the bolts through them are torqued to spec.

Dont know about the u-bolts but they will be torqued properly once the Alcans are swapped in. The current u-bolts are severely rusted and nasty so I can't image them backing off on their own.
Was the jeep level and given a chance to settle before torquing all the bolts? Also when aligned did you get a thrust angle?
FWIW, mine are perfectly aligned but can scare the crap out of you when at high speed making a quick throttle change!
It was sitting on the ground when the bolts were torqued but I guess it was not level if the passenger rear is sitting more than 0.5" below the driver rear. And as maxbraketorque said "As the leaf springs compress, the axle moves up and back." If its back then my thrust angle is all wrong when it was torqued. I'll check the thrust angle from the alignment when I get home.
So the initial thrust angle was -0.22 deg and the final was +0.3 deg...yet they didn't touch the trackbar or rear at all. Tie rod and draglink were the only adjustments they made. Not sure how that works.

Anyways took the spare tire off and the rear leveled out perfectly. Drove much better, still pulled some but much less. While the spare was off, I retorqued the shackle bolts with the jeep level. Seems better than it was but still not as good as without the spare.
It sounds like you have the solution, move the spare over!
Unfortunately there's no side to side adjustment, so it would be a hack and weld job. I think the only real solution is to get a better bumper or mount the tire inside
Changing the shackle angle with affect both the static ride height as well as the effective spring rate. The effect is pretty mild for small changes. If you make the spare-side shackle one step more vertical, that may resolve the issue because it would raise the spare side slightly, and it would make it more resistant to additional load on that spring. I would definitely try it because its super easy to do and un-do.
I agree, its very easy to change and go back. Think I'll wait until the new leafs get here and hopefully the higher quality and stiffer leaf makes a difference. If there are still issues I'll experiment with the shackle angle on the passenger side. Can't hurt to try
So, after finishing a front axle swap a few weeks ago, my XJ was pulling to the right. Actually, its pretty much always pulled to the right. I had been taking my XJ to a shop that uses a laser-based alignment system, but this time, I decided to buy some toe plates (bleepinjeep on Amazon), and set toe myself.

I half expected the toe to be pretty close because it seems reasonable to assume that variations in axle dimensions from one D30 to another should be pretty small, and because I didn't change the toe setting on the steering. However, I did lower the front a bit which causes the stock steering linkage to widen. Anyhow, it had a fair bit of toe-out (more than 0.5 degrees), so I set it to 0.08 deg toe-in, and much to my surprise, the pull to the right is pretty much completely gone. So now I'm wondering whether my XJ had been pulling to the right the entire time because toe was way out of spec. Thought I'd mention this.
What a waste of money!

Sounds like money well spent, instead of handing it to an alignment shop and still having the rig pull to the right. :doh:

I've done driveway alignments for years and recently purchased a set of Synergy toe plates and they are slick to use.