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  #1  
Old September 18th, 2020, 07:58
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splitz splitz is offline
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Please Double Check My Rear Lift Math

My king of all bastard packs (something like 3 XJ mains and 2 secondaries) are pretty much shot, so I need to replace them.

The front has Rubicon Express RM60010/RE1310 4.5" coils. The rear lift has a HD Offroad shackle relocation bracket and shackles that measure 5" eye to eye. I am guessing that these give me about 3.5" of lift (2.5" + 1") and the aforementioned bastard packs give me about 1" because the Jeep is level. The shocks are ProComp ES3000s, which seem to work fine up front.

Here is a pic measuring from the stock shackle bolt to the shackle eye on the spring. Do you think my estimate is about right? What options would provide the best ride quality at 4.5" to 5"?

Other related tidbits about the Jeep:
  • 4.56 gears
  • SYE
  • Current wheels & tires: 15x? w/33x12.5 MTRs that measure less than 31.5"
  • Future wheels & tires: 17s w/35" AT/MT hybrid tires
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  #2  
Old September 18th, 2020, 08:21
Evan03 Evan03 is offline
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Re: Please Double Check My Rear Lift Math

Shackle relocation boxes plus 6" long shackles gets about 3" then bsterd pack to level with front
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  #3  
Old September 18th, 2020, 08:58
maxbraketorque maxbraketorque is offline
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Re: Please Double Check My Rear Lift Math

Your shackle setup is giving right about 2" of lift. If you want 4.5-5" lift, you'll want springs that give 2.5-3" lift. If you are looking to go cheap, then build another bastard pack or get a longer shackle as Evan03 suggested, although at that shackle angle, every inch of additional shackle length will give only about 0.3" of lift.

Otherwise, look for 2.5-3" lift leaf. Your shackle angle is pretty aggressive, so the effective spring rate will be lower than the spring rate advertised by the leaf spring manufacturer. OME has a spring in this lift range.
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Old September 18th, 2020, 15:39
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WAZZUXJ WAZZUXJ is offline
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Re: Please Double Check My Rear Lift Math

Shackle relocation boxes ~1" lift
5" shackles (stock are 3") ~1" lift. Shackles are 2 inches longer than stock, but are only adding length to one side of the leaf spring, so you only gain 1/2 the added length.

Your picture confirms about 2" to 2.25" of total shackle lift.

To measure your current total rear lift height; measure from the top of the axle tube to the bottom of the unibody frame. Stock is 6". Anything over that is your total lift.

Last edited by WAZZUXJ; September 18th, 2020 at 15:59.
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  #5  
Old September 18th, 2020, 17:21
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splitz splitz is offline
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Re: Please Double Check My Rear Lift Math

Quote:
Originally Posted by WAZZUXJ View Post
...are only adding length to one side of the leaf spring, so you only gain 1/2 the added length.
This is one of those "duh" moments. The relocation box and shackle do only move the mounting point of one side of the leaf. While there is probably a more precise formula to calculate how much lift the combination adds to the entire lift, half is probably about right.

Quote:
Originally Posted by WAZZUXJ View Post
Your picture confirms about 2" to 2.25" of total shackle lift.
Quote:
Originally Posted by maxbraketorque View Post
Your shackle setup is giving right about 2" of lift.
It sounds like the consensus so far is that the relocation bracket and shackle account for about 2" of the lift, which means the bastard pack is adding about 2.5". I will confirm the total rear lift height using the method WAZZUZJ provided. If it is truly around 4.5" total, then I will look for a 2.5" to 3" leaf.

I am okay with reducing the spring rate because I need to soften the ride. My girlfriend was not too pleased after our 1,200 mile trip.
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  #6  
Old September 18th, 2020, 17:27
maxbraketorque maxbraketorque is offline
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Re: Please Double Check My Rear Lift Math

Quote:
Originally Posted by splitz View Post
...

I am okay with reducing the spring rate because I need to soften the ride. My girlfriend was not too pleased after our 1,200 mile trip.
lol. My wife requires a plush ride too. If you buy a leaf pack, one option is to remove a leaf or two. Worked great for me in setting ride comfort.
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  #7  
Old September 18th, 2020, 17:33
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splitz splitz is offline
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Re: Please Double Check My Rear Lift Math

Quote:
Originally Posted by maxbraketorque View Post
lol. My wife requires a plush ride too. If you buy a leaf pack, one option is to remove a leaf or two. Worked great for me in setting ride comfort.
That brings up a good point. I may look for leafs that give me more than I think I need. If it is too much, then I can just remove a leaf or two. But if I buy a pack that is too small, then I'm going to be bastardizing all over again!
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  #8  
Old September 19th, 2020, 05:40
89Mock 89Mock is offline
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Re: Please Double Check My Rear Lift Math

Why not buy 4.5 inch lift leaf springs if. Your busted packs are shot? You’re going to build another used set out of used parts and they’re going to be shot again. You could get what you want if you spend the money now instead of on a third set when the “new” ones crap out.
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Old September 19th, 2020, 17:39
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splitz splitz is offline
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Re: Please Double Check My Rear Lift Math

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Originally Posted by 89Mock View Post
Why not buy 4.5 inch lift leaf springs if. Your busted packs are shot? You’re going to build another used set out of used parts and they’re going to be shot again. You could get what you want if you spend the money now instead of on a third set when the “new” ones crap out.
That's the plan. An old used set was never on the table. With maxbraketorque's suggestion of removing a leaf, I'll probably spring (ha, spring) for 4.5s and remove leaf(s) as needed.
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  #10  
Old September 19th, 2020, 19:52
maxbraketorque maxbraketorque is offline
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Re: Please Double Check My Rear Lift Math

One thing that's I'm not sure about with your setup is how much your shackle angle is going to affect spring rate. I was thinking of recommending the IRO 3.5" leaf pack, but already has a fairly light spring rate of 185 lbs/in. With your shackle angle, the effective spring rate will be somewhat reduced, possibly making it too soft.
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  #11  
Old September 20th, 2020, 06:37
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splitz splitz is offline
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Re: Please Double Check My Rear Lift Math

Quote:
Originally Posted by maxbraketorque View Post
One thing that's I'm not sure about with your setup is how much your shackle angle is going to affect spring rate. I was thinking of recommending the IRO 3.5" leaf pack, but already has a fairly light spring rate of 185 lbs/in. With your shackle angle, the effective spring rate will be somewhat reduced, possibly making it too soft.
The IROs are longer than stock, so I would move my shackle one bolt to the rear. Ideally, that would put the angle right at about 45 degrees, which I think is the desired angle.
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  #12  
Old September 20th, 2020, 20:36
maxbraketorque maxbraketorque is offline
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Re: Please Double Check My Rear Lift Math

Quote:
Originally Posted by splitz View Post
The IROs are longer than stock, so I would move my shackle one bolt to the rear. Ideally, that would put the angle right at about 45 degrees, which I think is the desired angle.
If the current shackle mounting hole is more forward than stock, moving back one hole may be warranted, although IRO says these work fine in the stock hole with a longer shackle.

As long as the shackle is long enough to allow full flex of the leaf pack, all the angle does is adjust the effective spring rate. This is sometimes called the wheel rate. Having the shackle pointed 45 deg back will just make the effective spring rate softer compared to having the shackle vertical at ride height. No other magic, at least if a good length shackle is used, which yours is. Well, I guess it goes without saying that a more vertical shackle gives more ride height.
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