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Leaf Spring Options


NAXJA Forum User
For context I am set up with a more overland build with 33x12.5s and I don’t really intend to do any crazy rock crawling. I have good pinion angle and have no rear vibrations, I sit right around 4-5 degrees as per Adam’s DS recommendation. I currently have IRO 3.5” leafs with their 1.5” boomerang shackles in the rear. I have a rear bumper with tire carrier and I am often loading the rear with lots of weight: bikes and bike rack, roof top tent, gear, fridge, etc. and it seems to be sagging a bit. I picked up some used RE 3.5” leafs. Is it worth switching them out? Should I keep the IROs and get shackle relocation brackets to add more lift in the rear? Should I swap to the used RE springs AND get SRBs? Has anyone had experience with IRO springs and lots of weight in the rear? Will I need axle shims to fix angles?
I have IRO springs and tend to carry a large payload, although maybe not quite as much as you describe.

I found the IRO springs are very soft. Really good for flex and wheeling, but they don't take the weight well. I contacted IRO with some questions, and they recommended I get the add-a-leaf kit. They specifically said I was likely overloading the springs and when springs bend into a "W" shape they are being abused and will not last. I added the extra leaf into the pack, and I'm happy with the performance now. That is what I would recommend trying first.

The IRO springs have some great features that other springs don't at their price point. Notably the Berlin eyes and the military wrap, but also the tapered ends, anti-friction pads, and nice spring clamps.

What's your shackle angle look like? I found the IRO springs are an excellent length to use with the factory shackle location. However, I'm not using the factory location because my SRB's don't allow for it. I am at the next position forward, which puts my shackle angle around 60-70°. I'd recommend not getting SRB's that move your shackle mounting position lower than the factory bolt hole. Keeping the upper shackle mount at the stock height keeps your roll center closer to stock.

I installed these springs after coming from an XJ/S10 bastard pack spring setup. Those springs were super stiff and hardly flexed at all. The S10 leaves were like a 5/16" thick. Great load carrying capacity, great cornering stability, awful flex, awful ride.

Even if you install the IRO add a leaf, you might find they're still too soft for the weight you carry. You could add a sway bar, add airbags to adjust your spring rate for loaded and unloaded conditions, or just try the RE springs to see if they work better for you.
My shackle angle is ok. I’ve got the boomerang shackles and bolt hole to bolt hole they are maybe 70 ish degrees. Should I consider doing an add a leaf and SRBs? Like I said too, I have the used RE springs, I want to prevent digging in there as much as possible. I don’t have a good area to work in an apartment complex.